WEARABLE AMBULATORY MEDICAL DEVICE WITH MULTIPLE SENSING
ELECTRODES

CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional
Application Serial No. 61/345,914 entitled "WEARABLE AMBULATORY MEDICAL
DEVICE WITH MULTIPLE SENSING ELECTRODES," filed May 18, 2010, and to U.S.
Provisional Application Serial No. 61/424,344 entitled "WEARABLE AMBULATORY
MEDICAL DEVICE WITH MULTIPLE SENSING ELECTRODES," FILED December 17,
2010, each of which is incorporated herein by reference in its entirety.

BACKGROUND OF INVENTION
1. Field of Invention
The present invention is generally directed to the detection of cardiac function in a
patient, and more particularly to the detection of cardiac function and the treatment of cardiac
conditions in an ambulatory medical device, such as a wearable defibrillator.
2. Discussion of Related Art
With a portable medical device, such as a wearable defibrillator worn by an ambulatory
patient, the patient's electrocardiogram (ECG) signal is obtained from body surface electrodes.
When the ECG signal is obtained in this manner, electrical noise or electrode fall-off
frequently degrades the quality of the ECG signal. The challenge becomes one of extracting a
clean ECG signal from the sometimes noisy signals derived from the body-surface electrodes.
Electrode noise can be caused by electrodes sliding on the patient's body due to extreme
patient movement, such as vigorous exercise. Noise can also be caused by a poorly fit
electrode belt or garment allowing the electrodes to slide on the patient's body with minor
patient movement. Electrode fall-off can be caused by the electrodes flipping over and losing
contact with the body, or lifting from the body and losing contact. Even where the electrodes
are properly positioned on the patient' s body, excessively dry skin can also cause noise.
Known ambulatory wearable defibrillators, such as the LifeVest® Wearable
Cardioverter Defibrillator available from Zoll Medical Corporation of Chelmsford,
Massachusetts, use four ECG sensing electrodes in a dual-channel configuration. That is, an
electrical signal provided by one of the four ECG sensing electrodes is paired with the
electrical signal provided by another of the four ECG sensing electrodes to form a channel.
This arrangement of ECG sensing electrodes is usually suitable because in most cases it is rare
that noise or electrode movement affects the entire body circumference. The dual-channel
configuration provides redundancy and allows the system to operate on a single channel if
necessary, when one of the channels is declared unusable due to ECG sensing electrode falloff,
or to an inferior signal-to-noise ratio. Because signal quality also varies from patient to
patient, having two channels provides the opportunity to have improved signal pickup, since
the ECG sensing electrodes are located in different body positions. The two channel system
also allows analysis of the ECG signal to determine cardiac conditions as described in U.S.
Patent No. 5,944,669.
A problem with existing electrode systems used in ambulatory medical treatment
devices, such as a wearable defibrillator, is that there are still instances where there is noise on
both channels. When there is noise or fall-off, the device issues alarms so that the patient can
take action to correct the problem. With a noisy ECG signal, the arrhythmia detection
algorithm in the wearable defibrillator device can be "fooled" into detecting the noise as an
arrhythmia, thereby causing the device to issue a treatment sequence that, if not terminated by
the patient, could deliver an unnecessary shock.
SUMMARY OF INVENTION
Embodiments of the present invention are directed to a wearable medical monitoring
device and/or to a wearable medical monitoring and treatment device that incorporates multiple
ECG sensing electrodes disposed at different axial positions around the body of a patient and
that can choose from multiple channels corresponding to different pairings of those multiple
ECG sensing electrodes to vastly improve the quality of the ECG signal obtained. This
improved ECG sensor design can be used to reduce noise, to reduce the number of fall-off
alarms, to reduce the number of cardiac arrhythmia false detections, or all of the above. The
multiple channels provide different views of the heart' s electrical activity and can be used to
improve the detection sensitivity and specificity.
In accordance with one aspect of the present invention, an ambulatory medical device is
provided. In one embodiment, the ambulatory medical device comprises a plurality of
electrodes configured to be disposed at spaced apart positions about a body of a patient, an
electrode signal acquisition circuit, and a monitoring circuit. The electrode signal acquisition
circuit has a plurality of inputs, each respective input of the plurality of inputs being
electrically coupled to a respective electrode of the plurality of electrodes. The electrode
signal acquisition circuit is configured to sense a respective signal provided by a plurality of
different pairings of the plurality of electrodes. The monitoring circuit is electrically coupled
to an output of the electrode signal acquisition circuit. The monitoring circuit is configured to
analyze the respective signal provided by each of the plurality of different pairings and to
instruct the electrode signal acquisition circuit to select at least one of the plurality of different
pairings to monitor based upon at least one of a quality of the respective signal provided by the
selected at least one of the plurality of different pairings, a phase difference between the
respective signal provided by the selected at least one of the plurality of different pairings and
the respective signal provided by another selected at least one of the plurality of different
pairings, a position of the respective electrodes of the selected at least one of the plurality of
different pairings relative to the body of the patient, a plane defined by the respective
electrodes of the selected at least one of the plurality of different pairings, and a cardiac cycle
of a heart of the patient.
In accordance with one embodiment, the ambulatory medical device further comprises
a garment that is configured to be worn about the body of the patient, and the plurality of
electrodes are integrated into the garment. In accordance with another embodiment, the
plurality of electrodes are ECG sensing electrodes, and the monitoring circuit is a cardiac
monitoring and arrhythmia detection circuit.
In one embodiment, the plurality of ECG sensing electrodes includes at least three ECG
sensing electrodes. In another embodiment, the plurality of ECG sensing electrodes are not all
located in a common plane.
In one embodiment, the cardiac monitoring and arrhythmia detection circuit is
configured to analyze the respective signal provided by each of the plurality of different
pairings and to instruct the electrode signal acquisition circuit to select the at least one of the
plurality of different pairings to monitor based upon the quality of the respective signal
provided by the selected at least one of the plurality of different pairings and the phase
difference between the respective signal provided by the selected at least one of the plurality of
different pairings and the respective signal provided by the other selected at least one of the
plurality of different pairings. In accordance with a further aspect of this embodiment, the
ambulatory medical device further comprises a plurality of therapy electrodes integrated into
the garment and configured to deliver a defibrillating shock to the body of the patient in
response to detection of a treatable cardiac arrhythmia by the cardiac monitoring and
arrhythmia detection circuit.
In accordance with another embodiment, the cardiac monitoring and arrhythmia
detection circuit is configured to analyze the respective signal provided by each of the plurality
of different pairings and to instruct the electrode signal acquisition circuit to select the at least
one of the plurality of different pairings to monitor based upon the quality of the respective
signal provided by the selected at least one of the plurality of different pairings and the plane
defined by the respective electrodes of the selected at least one of the plurality of different
pairings.
In another embodiment, the cardiac monitoring and arrhythmia detection circuit is
configured to analyze the respective signal provided by each of the plurality of different
pairings and to instruct the electrode signal acquisition circuit to select the at least one of the
plurality of different pairings to monitor based upon the position of the respective electrodes of
the selected at least one of the plurality of different pairings relative to the body of the patient
and the cardiac cycle of the heart of the patient.
In an alternative embodiment, the cardiac monitoring and arrhythmia detection circuit
is configured to select at least three of the plurality of different pairings to monitor based upon
the at least one of the quality of the respective signal provided by each of the selected at least
three of the plurality of different pairings, the phase difference between the respective signal
provided by each of the selected at least three of the plurality of different pairings, the position
of the respective electrodes of the selected at three of the plurality of different pairings relative
to the body of the patient, the plane defined by the respective electrodes of the selected at least
three of the plurality of different pairings, and the cardiac cycle of the heart of the patient.
In one embodiment, the plurality of electrodes includes at least four ECG sensing
electrode that are not all located in a common plane.
In accordance with one embodiment, the monitoring circuit is configured to analyze the
respective signal provided by each of the plurality of different pairings and to instruct the
electrode signal acquisition circuit to select the at least one of the plurality different pairings to
monitor based upon the quality of the respective signal provided by the selected at least one of
the plurality of different pairings and the phase difference between the respective signal
provided by the selected at least one of the plurality of different pairings and the respective
signal provided by the other selected at least one of the plurality of different pairings.
In accordance with another embodiment, the monitoring circuit is configured to analyze
the respective signal provided by each of the plurality of different pairings and to instruct the
electrode signal acquisition circuit to select the at least one of the plurality of different pairings
to monitor based upon the quality of the respective signal provided by the selected at least one
of the plurality of different pairings and the plane defined by the respective electrodes of the
selected at least one of the plurality of different pairings.
In accordance with yet another embodiment, the monitoring circuit is configured to
analyze the respective signal provided by each of the plurality of different pairings and to
instruct the electrode signal acquisition circuit to select the at least one of the plurality of
different pairings to monitor based upon the position of the respective electrodes of the
selected at least one of the plurality of different pairings relative to the body of the patient and
the cardiac cycle of the heart of the patient.
In accordance with one embodiment, the monitoring circuit is configured to select at
least three of the plurality of different pairings to monitor based upon the at least one of the
quality of the respective signal provided by each of the selected at least three of the plurality of
different pairings, the phase difference between the respective signal provided by each of the
selected at least three of the plurality of different pairings, the position of the respective
electrodes of the selected at three of the plurality of different pairings relative to the body of
the patient, the plane defined by the respective electrodes of the selected at least three of the
plurality of different pairings, and the cardiac cycle of the heart of the patient. In accordance
with a further embodiment, the monitoring circuit is configured to select at least two of the
selected at least three of the plurality of different pairings to monitor during a first time interval
and to select a different at least two of the selected at least three of the plurality of different
pairings to monitor during a second time interval.
In one embodiment, the plurality of ECG sensing electrodes includes at least three ECG
sensing electrodes.
In accordance with one embodiment, the electrode signal acquisition circuit includes a
selection circuit and a plurality of differential amplifiers. The selection circuit has a plurality
of inputs and a plurality of outputs, each respective input of the plurality of inputs of the
selection circuit being electrically coupled to a respective one of the plurality of electrodes.
Each respective differential amplifier of the plurality of differential amplifiers has a pair of
inputs and an output, each respective input of the pair of inputs being electrically coupled to a
respective one of the plurality of outputs of the selection circuit, each respective output of the
plurality of differential amplifiers providing an output signal corresponding to a difference
between the pair of inputs of the respective differential amplifier.
In accordance with another embodiment in which the electrode signal acquisition
circuit includes a plurality of differential amplifiers and a selection circuit, the plurality of
differential amplifiers includes a respective differential amplifier for each unique pairing of the
plurality of electrodes. In this embodiment, the selection circuit is configured to select at least
one output of the plurality of differential amplifiers to provide to the monitoring circuit.
In accordance with another embodiment, the electrode signal acquisition circuit
includes an analog multiplexer and an analog-to-digital converter. The analog multiplexor has
a plurality of inputs and an output, each of the plurality of inputs being electrically coupled to a
respective one of the plurality of electrodes, and the analog-to-digital converter has an input
electrically coupled to the output of the analog multiplexer. In accordance with an aspect of
this embodiment, the analog-to digital converter has a sampling rate that is at least N times a
desired sampling rate of a signal provided by each of the plurality of electrodes, where N is the
number of the plurality of electrodes that are to be monitored. In accordance with another
aspect of this embodiment, the monitoring circuit includes at least one processor configured to
receive a first digital signal corresponding to a first electrode of the plurality of electrodes and
a second digital signal corresponding to a second electrode of the plurality of electrodes, to
invert one of the first and second digital signals and sum the inverted one of the first and
second digital signals with the other of the first and second digital signals to analyze the
respective signal provided by a pairing of the first electrode and the second electrode.
In accordance with another embodiment, the electrode signal acquisition circuit
includes a plurality of analog-to-digital converters, each respective analog-to-digital converter
having a respective input that is electrically coupled to a respective one of the plurality of
electrodes. In one embodiment, each of the plurality of analog-to-digital converters is
connected to another of the plurality of analog-to-digital converters by a serial bus.
In accordance with another aspect of the present invention, a method of monitoring
ECG signals is provided. In one embodiment, the method comprises selecting, from among a
plurality of ECG sensors, a plurality of different pairings of ECG sensors; analyzing a
respective ECG signal provided by each of the plurality of different pairings; identifying at
least one of the plurality of different pairings to monitor based upon at least one of a quality of
the respective ECG signal provided by the identified at least one of the plurality of different
pairings, a phase difference between the respective ECG signal provided by the identified at
least one of the plurality of different pairings and the respective ECG signal provided by
another identified at least one of the plurality of different pairings, a position of respective
ECG sensors of the identified at least one of the plurality of different pairings relative to a
body of a patient, a plane defined by the respective ECG sensors of the identified at least one
of the plurality of different pairings, and a cardiac cycle of a heart of the patient; and
monitoring the identified at least one of the plurality of different pairings.
In accordance with one embodiment, the act of identifying at least one of the plurality
of different pairings to monitor is based upon the quality of the respective ECG signal provided
by the identified at least one of the plurality of different pairings and the phase difference
between the respective ECG signal provided by the identified at least one of the plurality of
different pairings and the respective ECG signal provided by the other identified at least one of
the plurality of different pairings.
In one embodiment, the act of selecting the plurality of different pairings of ECG
sensors from among the plurality of ECG sensors includes an act of selecting, from among the
plurality of ECG sensors, each unique pairing of ECG sensors, and the act of analyzing the
respective ECG signal provided by each of the plurality of different pairings includes
analyzing the respective ECG signal provided by each unique pairing of ECG sensors.
In a further embodiment, the act of monitoring includes monitoring the identified at
least one of the plurality of different pairings to detect a cardiac arrhythmia. In accordance
with one embodiment, the method further comprises acts of detecting the cardiac arrhythmia
responsive to the act of monitoring; determining that the detected cardiac arrhythmia is a type
of cardiac arrhythmia that can be treated by applying defibrillation to the body of the patient;
and applying at least one defibrillation pulse to the body of the patient.
In another embodiment, the method further comprises acts of detecting the cardiac
arrhythmia responsive to the act of monitoring; selecting at least one additional pairing of ECG
sensors in response to detecting the cardiac arrhythmia and analyzing the respective ECG
signal provided by the at least one additional pairing; determining that the detected cardiac
arrhythmia is also present on the respective ECG signal of the at least one additional pairing;
determining that the detected cardiac arrhythmia is a type of cardiac arrhythmia that can be
treated by applying defibrillation to the body of the patient; and applying at least one
defibrillation pulse to the body of the patient.
In an alternative embodiment, the method further comprises acts of detecting the
cardiac arrhythmia responsive to the act of monitoring; selecting at least one additional pairing
of ECG sensors in response to detecting the cardiac arrhythmia and analyzing the respective
ECG signal provided by the at least one additional pairing; determining that the detected
cardiac arrhythmia is also present on the respective ECG signal of the at least one additional
pairing; and increasing a confidence level that the cardiac arrhythmia has been detected.
In another embodiment, the method further comprises acts of detecting the cardiac
arrhythmia responsive to the act of monitoring; selecting at least one additional pairing of ECG
sensors in response to detecting the cardiac arrhythmia and analyzing the respective ECG
signal provided by the at least one additional pairing; determining that the detected cardiac
arrhythmia is not present on the respective ECG signal of the at least one additional pairing;
and decreasing a confidence level that the cardiac arrhythmia has been detected.
In accordance with one embodiment, the acts of selecting, analyzing, and identifying
are repeated at periodic intervals.
In accordance with another embodiment, the plurality of ECG sensors are integrated in
a garment that is worn about the body of the patient, and the acts of selecting, analyzing, and
identifying are performed each time the garment is placed about the body of the patient.
In accordance with another embodiment in which the plurality of ECG sensors are
integrated in a garment that is worn about the body of the patient, the method further comprises
an act of detecting strenuous physical activity of the patient, and repeating the acts of selecting,
analyzing, and identifying in response to the act of detecting the strenuous activity of the
patient.
In accordance with another embodiment, the method further comprises acts of
determining that the quality of the respective ECG signal provided by a first pairing of ECG
sensors of the identified at least one of the plurality of different pairings is below a determined
threshold; selecting another paring of ECG sensors to replace the first pairing of ECG sensors;
and monitoring the other pairing of ECG sensors.
In accordance with yet another embodiment, the method further comprises acts of
determining, from the quality of the respective ECG signal provided by a first pairing of ECG
sensors of the identified at least one of the plurality of different pairings, that one or more of
the ECG sensors of the first pairing may have at least partially lost contact with the body of the
patient; selecting another paring of ECG sensors to replace the first pairing of ECG sensors;
and monitoring the other pairing of ECG sensors.
In accordance with one embodiment, the act of identifying at least one of the plurality
of different pairings to monitor is based upon the quality of the respective ECG signal provided
by the identified at least one of the plurality of different pairings and the plane defined by the
respective ECG sensors of the identified at least one of the plurality of different pairings.
In accordance with another embodiment, the act of identifying at least one of the
plurality of different pairings to monitor is based upon the position of respective ECG sensors
of the identified at least one of the plurality of different pairings relative to the body of the
patient and the cardiac cycle of the heart of the patient.
Still other aspects, embodiments, and advantages of these exemplary aspects and
embodiments, are discussed in detail below. Moreover, it is to be understood that both the
foregoing information and the following detailed description are merely illustrative examples
of various aspects and embodiments of the present invention, and are intended to provide an
overview or framework for understanding the nature and character of the claimed aspects and
embodiments. Any embodiment disclosed herein may be combined with any other
embodiment in any manner consistent with at least one aspect of the invention disclosed
herein, and references to "an embodiment," "some embodiments," "an alternate embodiment,"
"various embodiments," "one embodiment," "at least one embodiment," "this and other
embodiments" or the like are not necessarily mutually exclusive and are intended to indicate
that a particular feature, structure, or characteristic described in connection with the
embodiment may be included in at least one embodiment. The appearances of such terms
herein are not necessarily all referring to the same embodiment. Furthermore, in the event of
inconsistent usages of terms between this document and documents incorporate herein by
reference, the term usage in the incorporated references is supplementary to that of this
document; for irreconcilable inconsistencies, the term usage in this document controls.
BRIEF DESCRIPTION OF DRAWINGS
The accompanying drawings are not intended to be drawn to scale. In the drawings,
each identical or nearly identical component that is illustrated in various figures is represented
by a like numeral. For purposes of clarity, not every component may be labeled in every
drawing. In the drawings:
Fig. 1A illustrates an electrode system of a portable medical device in which a plurality
of ECG sensing electrodes are integrated into a garment, such as a shirt or vest that can be
worn on the body of the patient, and in which the electrodes are generally disposed in a plane
of the patient's heart;
Fig. IB illustrates an alternative electrode system of a portable medical device in which
the plurality of ECG sensing electrodes are integrated into a garment such as that depicted in
Fig. 1A, but in which the electrodes are distributed about the torso of the patient;
Fig. 1C illustrates an alternative electrode system of a portable medical device in which
the plurality of ECG sensing electrodes are integrated into a garment, such as a belt that can be
worn on the body of the patient, and in which the electrodes are generally disposed in a plane
of the patient' s heart;
Fig. ID illustrates an alternative electrode system of a portable medical device in which
the plurality of ECG sensing electrodes are integrated into a garment, such as a belt that can be
worn on the body of the patient, and in which the electrodes are distributed about a portion of
the torso of the patient;
Fig. IE illustrates a further alternative electrode system of a portable medical device in
which the plurality of ECG sensing electrodes are integrated into a garment, such as a harness,
and in which the electrodes are disposed about the torso of the patient;
Fig. IF illustrates an alternative electrode system of a portable medical device in which
the plurality of ECG sensing electrodes are directly attached to the patient's torso;
Fig. 1G illustrates a plan view of the electrode systems of Figs. 1A-F;
Fig. 2A illustrates an electrode signal acquisition circuit that may be used with
embodiments of the present invention to select, from among a plurality of different pairings of
ECG sensing electrodes, those pairings of electrodes that maximize the signal-to-noise ratio
and maximize the phase discrimination provided by the electrodes;
Fig. 2B illustrates an electrode signal acquisition circuit according to another
embodiment of the present invention that is similar to that of Fig. 2A but includes the ability to
permit one or more the ECG sensing electrodes to be used as a driven ground electrode;
Fig. 2C illustrates an electrode signal acquisition circuit according to another
embodiment of the present invention that is similar to that of Fig. 2B and permits one or more
the ECG sensing electrodes to be used as a driven ground electrode;
Fig. 3 illustrates an alternative electrode signal acquisition circuit that may be used with
embodiments of the present invention;
Fig. 4 is a functional block diagram of a control unit that may be used with
embodiments of present invention;
Fig. 5 is a flow diagram of an electrode selection process that may be performed by at
least one processor of the control unit of Fig. 4;
Fig. 6 is a flow diagram of a noise/fall-off detection process that may be executed by
the at least one processor of the control unit of Fig. 4;
Fig. 7 is a flow diagram of a monitoring and analysis routine that may be executed by
the at least one processor of the control unit of Fig. 4;
Fig. 8 is a flow diagram of an alternative monitoring and analysis routine that may be
executed by the at least one processor of the control unit of Fig. 4;
Fig. 9 illustrates a further alternative signal acquisition circuit that may be used with
embodiments of the present invention;
Fig. 10 illustrates yet another alternative signal acquisition circuit that may be used
with embodiments of the present invention; and
Fig. 11 illustrates another alternative signal acquisition circuit that may be used with
embodiments of the present invention.
DETAILED DESCRIPTION
This invention is not limited in its application to the details of construction and the
arrangement of components set forth in the following description or illustrated in the drawings.
The invention is capable of other embodiments and of being practiced or of being carried out in
various ways. Also, the phraseology and terminology used herein is for the purpose of
description and should not be regarded as limiting. The use of "including," "comprising,"
"having," "containing," "involving," and variations thereof herein is meant to encompass the
items listed thereafter and equivalents thereof as well as additional items.
U.S. Patent No. 5,944,669, which is incorporated herein by reference in its entirety,
describes a method and apparatus for sensing cardiac function in a patient that may be used to
initiate treatment of a detected cardiac condition. ECG sensing electrodes are used to obtain
ECG signals from the heart of the patient and those ECG signals are analyzed using various
techniques to provide information indicative of the operation of the patient' s heart, and
whether a treatable cardiac condition is present for which treatment, such as defibrillation,
should be initiated. As described therein, a plurality of pairs of ECG sensing electrodes are
used, such that signals received from the different pairs of ECG sensing electrodes may be
compared to one another to improve reliability or detection, so that noise present on one or
more of the electrodes can be identified, so that monitoring may be provided even in the event
that one or more of the sensing electrodes falls off, etc.
Embodiments of the present invention are directed to an electrode system that may be
used in a wearable medical device, such as that described in U.S. Patent No. 5,944,669
(hereinafter "the '669 patent"), to monitor cardiac function, to initiate treatment of a detected
cardiac condition, or both. As described in more detail below, although embodiments of the
present invention are primarily described in terms of monitoring signals from a plurality of
ECG sensing electrodes, it should be appreciated that the techniques described herein may
readily be extended for use with other types of sensors, other than ECG sensing electrodes.
For example, other types of sensors may include activity sensors, such as multiple axis
accelerometers, pulse oxygen sensors, temperature sensors, respiratory rate sensors, thoracic
impedance sensors, blood pressure sensors, acoustic sensors, etc.
As shown in Fig. 1A, in one embodiment of the present invention, the electrode system
100 includes a plurality of ECG sensing electrodes 10 that are disposed at different axial
positions around the body of a patient and integrated into a garment 20a, such as a shirt or vest
which is worn on the torso of the patient. As depicted in Fig. 1A (as well as in Figs. 1B-1F),
those ECG electrodes shown in solid line form are disposed on the front of the patient' s body,
while those ECG electrodes shown in dotted line form are disposed on the back of the patient's
body. It should be appreciated that although not depicted in the figures, the plurality of ECG
sensing electrodes 10 will generally include electrodes disposed on the sides of the patient's
body, as well as electrodes disposed on the front and back of the patient' s body.
The plurality of ECG sensing electrodes 10 may be discrete, dry-sensing capacitive or
conductive electrodes that are, for example, attached to the garment 20a by an adhesive or
hook and loop fastener, magnetically attached to the garment 20a, or alternatively, sewn into
the garment 20a. Alternatively still, some or all of the ECG sensing electrodes may be formed
from electrically conductive threads sewn into the garment 20a, such as described in U.S.
Patent Application Serial No. 13/109,079, entitled "WEARABLE THERAPEUTIC DEVICE,"
filed on May 17, 201 1 under attorney docket number Z201 1-700510, which is incorporated
herein by reference in its entirety. It should be appreciated that the present invention is not
limited to a particular type of ECG sensing electrode or method of attachment, as various types
of ECG sensing electrodes, including wet ECG sensing electrodes, and various methods of
attachment, including adhesive attachment to the patient' s body may be used. Moreover,
although embodiments of the present invention are primarily described with respect to ECG
sensing electrodes that are electrically coupled to a control unit by wires, the present invention
is not so limited, as embodiments of the present invention may also be used with ECG sensing
electrodes that communicate with a control unit using a wireless communication interface and
protocol, such as Bluetooth, Wireless USB, ZigBee, Wireless Ethernet, GSM, etc. as discussed
further below.
As shown in Fig. 1A, the plurality of ECG sensing electrodes 10 are deployed about the
body of the patient at spaced apart axial positions generally located in a plane of the heart of
the patient. In accordance with one embodiment of the present invention, the ECG sensing
electrodes 10 are deployed about the body of the patient in pairs of generally opposed
electrodes (e.g., ECG sensing electrodes 10a, 10b) that are integrated into a garment 20a, such
as a shirt or vest, although the present invention is not so limited. It should be appreciated that
the location of the plurality of ECG sensing electrodes 10 may be varied to avoid placing an
electrode in a location where it could promote discomfort for the patient, such as directly on
the spine of the patient. Insulated lead wires 15 electrically couple each ECG sensing electrode
of the plurality of ECG sensing electrodes 10 to a control unit 30 that may include a signal
acquisition circuit, such as that described in more detail with respect to Figs. 2A-C, 3, 9, 10,
and 11 below. Although not shown, each of the insulated lead wires 15 may be electrically
connected to a connector that is received in a receptacle of the control unit 30. The control unit
30 may be attached to the garment 20a, attached to a belt, received in a holster, or attached to a
clip so that it may be easily worn by the patient, or the control unit 30 may be carried with the
patient in any other convenient manner. As shown, the electrode system 100 also includes at
least one driven ground electrode 12 that is attached to the garment 20a and is electrically
coupled to the control unit 30 by an insulated lead wire 18. The at least one driven ground
electrode 12 may be used in the manner described in the '669 patent to reduce the effects of
noise and/or detect if an ECG sensing electrode has fallen off. Although the use of a driven
ground electrode is preferred to cancel the effects of noise, it should be appreciated that the
electrode 12 need not be actively driven, and could simply be a passive circuit ground.
Fig. IB illustrates an electrode system 100 in accordance with an alternative
embodiment of the present invention in which the plurality of ECG sensing electrodes 10 are
again integrated into a garment 20a, such as a shirt or vest that is worn over the torso of a
patient. However, in the embodiment depicted in Fig. IB, the plurality of ECG sensing
electrodes 10 are distributed about the torso of the patient, rather than being generally located
in a plane of the patient's heart. As in the embodiment of Fig. 1A, the plurality of ECG
sensing electrodes 10 are electrically coupled to a control unit 30 by a respective electrically
insulated lead wire 15 (not all of which are shown for ease of illustration), although wireless
ECG sensing electrodes could alternatively be used. The electrode system 100 also includes at
least one driven ground electrode 12 that is attached to the garment 20a and electrically
coupled to the control unit by an insulated lead wire 18. Although only one driven ground
electrode 12 is illustrated in the figures, it should be appreciated that multiple driven ground
electrodes may be used. For example, multiple driven ground electrodes may be provided,
with one of the driven ground electrodes being used with certain pairings of ECG sensing
electrodes, and another of the driven ground electrodes being used with other pairings of ECG
sensing electrodes. Moreover in certain embodiments, the at least one driven ground electrode
may be switched to be used with different pairings of ECG sensing electrodes. For example, if
it were determined that one of the driven ground electrodes had fallen off or had poor contact
with the body of the patient, another of the driven ground electrodes could be used instead.
The plurality of ECG sensing electrodes 10 may be deployed in pairs (e.g., ECG
electrodes 10a, 10b) of generally opposed electrodes, or simply spaced apart about the torso of
the patient. Although not shown in Fig. 1 B, the plurality of ECG sensing electrodes 10 may
include a first grouping of electrodes that are located at spaced apart axial positions generally
located in the plane of the patient' s heart, and a second grouping of electrodes that are located
at varying position about the torso of the patient. As discussed in more detail further below,
the presence of ECG sensing electrodes that are not all co-located in the plane of the patient's
heart permits the selection of different pairings of ECG sensing electrodes that can correspond
to different planes intersecting the patient's heart. Moreover, although not depicted in Fig. IB,
the plurality of ECG sensing electrodes 10 may include ECG sensing electrodes positioned on
side of the patient's torso, as well as the patient's front and back. As the embodiment of Fig.
IB is substantially similar to that of Fig. 1A, further discussion of those elements common to
both embodiments is omitted herein.
Fig. 1C illustrates an electrode system 100 in accordance with a further embodiment of
the present invention in which the plurality of ECG sensing electrodes 10 are again disposed at
different axial positions around the body of a patient and generally located in a plane of the
heart of the patient as in Fig. 1A. However, in the embodiment depicted in Fig. 1C, the
plurality of ECG sensing electrodes 10 are integrated into a garment 20b, such as a belt, that is
worn about the torso of the patient. As with the electrode system of Figs. 1A and IB, the
electrode system 100 of Fig. 1C may include any type of ECG sensing electrodes, such as
discrete, dry-sensing capacitive or conductive electrodes that are attached to the garment 20b
by an adhesive or fastener, magnetically attached to the garment 20b, or sewn into the garment
20b. As with the embodiment of Figs. 1A and IB, the plurality of ECG sensing electrodes 10
may be deployed about the patient's body in pairs of generally opposed electrodes (e.g., ECG
sensing electrodes 10a, 10b), or may simply be axially spaced about the torso of the patient.
Lead wires 15 (not all of which are shown for ease of illustration) electrically couple each ECG
sensing electrode of the plurality of ECG sensing electrodes 10 to the control unit 30, which
may be worn on or carried with the body of the patient. It should be appreciated that wireless
ECG sensing electrodes could alternatively be used. As in the embodiments described
previously with respect to Figs. 1A and IB, the electrode system 100 of Fig. 1C also includes
at least one driven ground electrode 12 that is attached to the garment 20b and may be used to
reduce the effects of noise and/or detect if an ECG sensing electrode has fallen off. The at
least one driven ground electrode 12 may be disposed in the same plane as the plurality of
ECG sensing electrodes 10, or may be located in a different plane. In some embodiments, the
at least one driven ground electrode 12 may also be used as a therapy electrode to administer
an electrical shock to the heart of the patient, as discussed further with respect to Fig. IE
below. Although not depicted in Fig. 1C, the plurality of ECG sensing electrodes 10 may
include ECG sensing electrodes positioned on side of the patient's torso, as well as the
patient' s front and back.
Fig. ID illustrates an electrode system 100 in accordance with yet another embodiment
of the present invention in which the plurality of ECG sensing electrodes 10 are again
integrated into a garment 20b, such as a belt that is worn over the torso of a patient, but in
which the plurality of ECG sensing electrodes 10 are distributed more widely about the torso
of the patient. As in the embodiment of Figs. 1A-C, the plurality of ECG sensing electrodes 10
are electrically coupled to a control unit 30 by a respective electrically insulated lead wire 15
(not all of which are shown for ease of illustration), and the electrode system 100 also includes
at least one driven ground electrode 12. The at least one driven ground electrode 12 may
include a first driven ground electrode that is generally disposed in a same plane as a plurality
of the ECG sensing electrodes, and a second driven ground electrode disposed in a different
plane. The plurality of ECG sensing electrodes 10 may be deployed in pairs (e.g., ECG
electrodes 10a, 10b), or simply spaced apart about the torso of the patient. As the embodiment
of Fig. ID is substantially similar to that of Figs. 1A-C, further discussion of those elements
that are in common with Figs. 1A-C is omitted herein.
Fig. IE illustrates an electrode system 100 in accordance with an alternative
embodiment of the present invention in which the plurality of ECG sensing electrodes 10 are
again integrated into a garment 20c. However, in this embodiment, the garment 20c is
constructed in the form of a harness, such as that used in the LifeVest® Wearable Cardioverter
Defibrillator. The harness includes an adjustable belt 110 and adjustable shoulder straps 120
that permit the harness be easily adjusted to fit different body types and sizes.
As shown in Fig. IE, the plurality of ECG sensing electrodes 10 are disposed about the
body of the patient at varying locations along the belt 110 and shoulder straps 120 of the
garment 20c. As with the previously described embodiments, the plurality of ECG sensing
electrodes 10 may be discrete, dry-sensing capacitive or conductive electrodes that are attached
to the garment 20c, for example, by an adhesive, by hook and loop fasteners, or by sewing, or
alternatively, the electrodes may be formed from electrically conductive threads sewn into the
garment 20c. As with the previously described embodiments, the plurality of ECG sensing
electrodes 10 may be deployed in pairs of generally opposed electrodes (e.g., ECG sensing
electrodes 10a and 10b), or alternatively, may not be deployed in pairs, but simply disposed at
various locations about the body of the patient. Although not depicted in Fig. IE, the plurality
of ECG sensing electrodes 10 will generally include ECG sensing electrodes disposed on the
sides of the patient' s body, as well as on the front and back of the patient' s body.
As with the embodiments of Figs. 1A-D, the electrode system 100 of Fig. IE may
include a control unit 30 that is electrically coupled to each of the plurality of ECG sensing
electrodes 10 by a respective insulated lead wire 15, although wireless ECG sensing electrodes
could alternatively be used. The control unit 30 may include a signal acquisition circuit, such
as that described in more detail with respect to Figs. 2A-C, 3, 9, 10, and 11, and the control
unit may also include a controller that may not only monitor the ECG signals from the patient,
but may analyze those ECG signals and initiate electrical shock therapy to the patient in the
event that such treatment is warranted. The control unit 30 may be integrated into the garment
20c, attached to the belt 110, received in a holster (not shown), or attached to a clip so that it
may be easily worn by the patient, or the control unit 30 may be carried with the patient in any
other convenient manner.
As with the previously described embodiments, the electrode system 100 also includes
at least one driven ground electrode 12. As illustrated in Fig. IE, in at least one embodiment,
the at least one driven ground electrode includes three driven ground electrodes 12a, 12b, and
12c. The driven ground electrodes 12a-c may be used in the manner described in the '669
patent to reduce the effects of noise and/or detect if an ECG sensing electrode has fallen off.
In one embodiment of the present invention, the driven ground electrodes 12a-c can also be
used as a therapy electrodes to deliver a defibrillating shock to the body of the patient, where
such treatment is warranted. In this embodiment, the electrodes 12a and 12b are electrically
coupled together and act as a first therapy electrode, with electrode 12c acting as a second
therapy electrode. The use of two therapy electrodes permits a biphasic shock to be delivered
to the body of the patient, such that a first of the two therapy electrodes delivers a first phase of
the biphasic shock with the other therapy electrode acting as a return, and the other therapy
electrode delivers the second phase of the biphasic shock with the first therapy electrode acting
as the return. It should be appreciated that in some embodiments, a monophasic shock or other
type of defibrillating pulse of energy may be used.
Fig. IF illustrates an electrode system 100 in accordance with another embodiment of
the present invention that includes a plurality of ECG sensing electrodes 10. In contrast to the
embodiments described with respect to Figs. 1A-E, rather than being attached or integrated
into a garment 20a, 20b, or 20c, each of the plurality of ECG sensing electrodes 10 and the at
least one driven ground electrode 12 is directly attached to the body of the patient. As with the
previously described embodiments, each of the plurality of ECG sensing electrodes 10 is
electrically coupled to the control unit 30 by a respective insulating wire 15 (not all of which
are shown for ease of illustration), and the electrode system 100 includes at least one driven
ground electrode 12 that is electrically coupled to the control unit 30 by an insulated lead wire
18. In accordance with an aspect of the present invention, the control unit 30 may include a
signal acquisition circuit that is capable of selecting, from among the plurality of ECG sensing
electrodes 10, those pairs of electrodes that provide the best ECG signals, in terms of signal
level, noise, phase discrimination, or any other criteria. As with the previously described
embodiments of Figs 1A-E, the plurality of ECG sensing electrodes 10 may be deployed about
the body of the patient in pairs of generally opposed electrodes (e.g., ECG sensing electrodes
10a, 10b), or simply spaced about the torso of the patient. Although not depicted in Fig. IF,
the plurality of ECG sensing electrodes 10 will typically include ECG sensing electrodes
positioned on side of the patient's torso, as well as the patient's front and back. It should be
appreciated that the embodiments of Figs. 1A-E may additionally include one or more ECG
sensing electrodes (or driven ground electrodes) that are directly attached to the body of the
patient, in addition to those that are integrated into the garment 20a-20c.
Fig. 1G illustrates a plan view of electrode system 100 of Figs. 1A -IE. As shown, the
plurality of ECG sensing electrodes 10 are disposed at different axial positions about the body
of the patient, although they need not be deployed in a single plane. Indeed, in at least one
embodiment, the plurality of ECG sensing electrodes are not all co-located in a single plane so
that pairs of electrodes corresponding to different planes may be selected. Further, although
the plurality of ECG sensing electrodes 10 are shown as being deployed in generally
diametrically opposed pairs of electrodes (e.g., electrodes 10a and 10b, electrodes 10c and
lOd), it should be appreciated that the present invention is not so limited. In the embodiment
depicted in Fig. 1G, the plurality of ECG sensing electrodes 10 includes 16 ECG sensing
electrodes, with each ECG sensing electrode being spaced apart from an adjacent ECG sensing
electrode by approximately 22.5°. In another embodiment, the plurality of ECG sensing
electrodes 10 includes 12 sensing electrodes, with each ECG sensing electrode being spaced
apart from an adjacent ECG electrode by approximately 30°, and in a further embodiment, the
plurality of ECG sensing electrodes includes 18 sensing electrodes spaced approximately 20°
apart. It should be appreciated that more or fewer ECG sensing electrodes may be provided,
for example, as few as three, and that some or all of the ECG sensing electrodes may be
located outside of a horizontal plane intersecting the heart of the patient.
Advantageously, the use of multiple electrodes permits different pairings of ECG
sensing electrodes to be selected, where that selection provides a better or more desirable ECG
signal, in terms of signal level, noise immunity, phase difference, cardiac arrhythmia detection
specificity, or any other criteria. For example, ECG sensing electrode 10a could be paired with
either of ECG sensing electrodes 10k or lOj, rather than with ECG sensing electrode 10b,
where such a pairing resulted in a better ECG signal level, better noise immunity, or a
maximum phase discrimination, or where it was determined that ECG sensing electrode 10b
had fallen off or has poor contact with the body of the patient. Different pairings of ECG
sensing electrodes having a similar phase difference, or representing different phase
differences may be selected and compared to one another. For example, ECG sensing
electrodes lOg and lOh that are spaced approximately 180° apart may be paired and the ECG
signal compared to that from ECG sensing electrodes 10c and lOd (also spaced 180° apart), or
alternatively, ECG sensing electrodes lOg and lOh may be paired and the ECG signal
compared to that from ECG sensing electrodes 10b and lOd that are spaced approximately 90°
apart in order to screen out noise or derive additional information. Where the plurality of ECG
sensing electrodes 10 are not all located in a single plane, the pairings of ECG sensing
electrodes may be selected to correspond to different planes. It should be appreciated that the
different pairings of ECG sensing electrodes need not be disjoint. For example, ECG sensing
electrode 10a may be paired with ECG sensing electrode 10b and the ECG signal compared to
that from ECG sensing electrodes 10a and 10c and/or to that from ECG sensing electrodes 10a
and lOd.
Fig. 2A illustrates a signal acquisition circuit that may be used with embodiments of the
present invention to select, from among a plurality of ECG sensing electrodes, those pairings
of electrodes that provide a desired ECG signal, in terms of signal-to-noise ratio, phase
discrimination, or any other criteria, and provide those ECG signals to downstream circuitry
for further signal conditioning, processing, analysis, and/or monitoring. Advantageously, the
signal acquisition circuit 200 depicted in Fig. 2A may be used as a front end to the analog to
digital conversion and signal conditioning block 14 described with respect to the arrhythmia
detection system of Figs 2a-2c of the ' 669 patent.
As shown, the signal acquisition circuit 200 includes a selection circuit 210 that is
electrically coupled to a differential circuit 220. Signals from each of the plurality of ECG
sensing electrodes lOa- are provided to a respective input 212 of the selection circuit 210.
Signals from one or more of the driven ground electrodes 12 may also be provided to an input
212 of the selection circuit 210, such that a signal may be transmitted on the driven ground
electrode 12, and that signal compared to the signals received on each of the plurality of ECG
sensing electrodes to identify whether a particular ECG sensing electrode may have fallen off,
or to identify noise issues relating to a particular ECG sensing electrode. The selection circuit
210 has a plurality of outputs 216 that are electrically coupled to respective inputs 222 of the
differential circuit 220. In operation, the selection circuit 210 operates in a manner similar to a
multiple output multiplexer, and includes a plurality of control inputs 214 to select signals
from different ECG sensing electrodes and/or the driven ground electrode and provide those
selected signals to the inputs 222 of the differential circuit 220. It should be appreciated that
rather than a single selection circuit, a plurality of conventional single output multiplexers may
be used to achieve the same functionality.
The differential circuit 220 includes a plurality of analog differential instrumentation
amplifiers 220a, 220b, . . . 220n, to receive the signals provided by different pairings of the
ECG sensing electrodes and/or different pairings of a respective ECG sensing electrode and a
driven ground electrode and provide a respective differential output signal 226 corresponding
to the difference therebetween. Where the signals provided to a respective amplifier 220a,
220b, . ..220n correspond to signals provided by different ECG sensing electrodes, a
differential ECG signal is provided. This differential analog ECG signal may then be digitally
converted and conditioned by an analog-to-digital conversion and signal conditioning block of
an arrhythmia detection system, such as that described with respect to Figs. 2a-2c of the '669
patent, prior to further analysis and/or monitoring by an arrhythmia monitoring and/or
treatment system, such as a wearable defibrillator.
Fig. 2B illustrates an alternative signal acquisition circuit that may be used with
embodiments of the present invention to select, from among a plurality of ECG sensing
electrodes, those pairings of electrodes that provide a desired ECG signal, in terms of signal-tonoise
ratio, phase discrimination, or any other criteria, and provide those ECG signals to
downstream circuitry for further signal conditioning, processing, analysis, and/or monitoring.
The signal acquisition circuit 200b is substantially similar to that of signal acquisition circuit
200a described immediately above with respect to Fig. 2A, and thus, only the selection circuit
210 is shown in Fig. 2B, and the other portions of circuit 200a, such as the differential circuit
220, are not depicted. However, in addition to being able to select any pairings of ECG
sensing electrodes, the signal acquisition circuit 200b additionally allows any one of the
plurality of ECG sensing electrodes lOa- to be used as a driven ground electrode.
As known to those skilled in the art of signal processing, a driven ground electrode is
frequently used to eliminate noise that may be common to many or all sensors, such as ECG
sensing electrodes lOa- . Noise signals present on some or all of the sensors, such as the
ECG sensing electrodes are summed, then inverted, and then injected into the driven ground
circuitry. Where the sensors are ECG sensing electrodes that are attached to the body of a
patient, the inverted signal may be actively driven onto the body of the patient where it is
picked up by the ECG sensing electrodes, effectively cancelling out the noise that would
normally be detected.
However, in a wearable medical device, such as the wearable medical device described
with respect to Figs. 1A-F, there may be instances where the driven ground electrode 12 that is
used to transmit the driven ground signal to the body of the patient may have fallen off, lost
contact with the body of the patient, or simply not be working appropriately. Indeed, even
where the driven ground electrode 12 is in good contact with the body of the patient and
working properly, the driven ground electrode may simply be located in a sub-optimal
position. Where any of these conditions exist, one or more of the plurality of ECG sensing
electrodes 10a-10p may be used as a driven ground electrode. This aspect of the present
invention in now described in more detail with respect to Fig. 2B.
As shown in Fig. 2B, a plurality of signal pads 230 can be provided with each
respective signal pad 230a-230p being electrically coupled to a driven ground circuit (not
shown). A plurality of switches 232 are provided, with each respective switch 232a-p being
electrically coupled between a respective ECG sensing electrode lOa-p and a respective input
212a-p of the selection circuit 210. Each switch 232a-p is capable of being in one of two
positions. In a first position, the switch 232 electrically couples a respective ECG sensing
electrode lOa-p to a respective input 212a-p of the selection circuit. In the second position, the
switch 232 electrically couples the respective electrode lOa-p to a respective signal pad 230a-p
that is electrically coupled to the driven ground circuit. For example, as illustrated in Fig. 2B,
switch 232a is in a position such that the signal pad 230a is electrically coupled to ECG
sensing electrode 10a, whereas each of switches 232b, 232c, . . . 232p are in a position such that
ECG sensing electrodes lOb- are respectively electrically coupled to a respective input
212b-212p of the selection circuit 210. In this configuration, ECG sensing electrode 10a may
be used as a driven ground electrode, where that use provides a better signal on others of the
plurality of ECG sensing electrodes, where another driven ground electrode has lost contact or
has poor contact with the body of the patient, or for any other reason.
Fig. 2C illustrates a further alternative signal acquisition circuit that may be used with
embodiments of the present invention to select pairings of ECG sensing electrodes in a manner
similar to that described above with respect to Figs. 2A and 2B, and to allow any one of the
plurality of ECG sensing electrodes to be used as a driven ground electrode in a manner similar
to that described above with respect to Fig. 2B. The signal acquisition circuit 200c is
substantially similar to that of signal acquisition circuit 200b described immediately above, and
therefore, only the differences are described.
As in the signal acquisition circuit 200b, a plurality of signal pads 230 are provided
with each respective signal pad 230a-230p being electrically coupled to a driven ground circuit
(not shown). A plurality of switches 232 are also provided. Each respective switch 232a-p of
the plurality of switches 232 is electrically coupled to a respective signal pad 230a-p of the
plurality of signal pad 230, which in turn, are electrically coupled to a driven ground circuit
(not shown). Each switch 232a-p is capable of being in one of two positions, opened and
closed. In the open position, the driven ground signal on a respective signal pad 230a-p is an
open circuit, and in the closed position, the switch 232 electrically couples a respective ECG
sensing electrode lOa-p to a respective signal pad 230a-p. For example, as illustrated in Fig.
2C, switch 232a and each of switches 232c-232p is in an open position, and switch 232b is in a
closed position, such that the signal pad 230b is electrically coupled to ECG sensing electrode
10b. This embodiment relies on the fact that the selection circuit 210 generally will have a
relatively high input impedance, such that each of the inputs 212 may remain connected to an
ECG sensing electrode 10 while that ECG sensing electrode is electrically coupled to the
driven ground circuit, as the driven ground circuit will typically have a relative low output
impedance. In this configuration, ECG sensing electrode 10b may be used as a driven ground
electrode, where that use provides a better signal on others of the plurality of ECG sensing
electrodes, where another driven ground electrode has lost contact or has poor contact with the
body of the patient, or for any other reason.
It should be appreciated that in the embodiments of Figs. 2B and 2C described above,
more than one driven ground circuit may be provided. For example, ECG sensing electrode
10a could be used as a driven ground electrode for use with ECG sensing electrodes 10c, lOd,
lOo, 10, lOe, lOh, lOi, and 101 (see Fig. 1G), and ECG sensing electrode 10b could be used as
a driven ground electrode for use with ECG sensing electrodes 10m, lOp, lOg, lOf, 10k, and
lOj.
Fig. 3 illustrates a signal acquisition circuit according to another embodiment of the
present invention that may be used to acquire signals from different pairings of ECG sensing
electrodes, or from different pairings of ECG sensing electrodes and a driven ground electrode,
and provide those signals to downstream circuitry, such as an A/D conversion and signal
conditioning block 330 of an arrhythmia monitoring and/or treatment system. As shown in
Fig. 3, the signal acquisition circuit 300 includes a plurality of analog differential
instrumentation amplifiers 320a-320n, 321a-321n-l, . . . 329a that are each configured to
receive signals from different pairings of ECG sensing electrodes. For example, a first
grouping of amplifiers 320a-320n may be configured to respectively pair each of ECG sensing
electrodes 10b- lOp (Fig. 1G) with ECG sensing electrode 10a, a second grouping of amplifiers
321a-321n-l may be configured to respectively pair each of ECG sensing electrodes lOc-lOp
with ECG sensing electrode 10b, etc. Although not shown, each grouping of amplifiers may
also compare a signal from a respective ECG sensing electrode with a driven ground electrode,
or with each of a number of driven ground electrodes. In contrast to the signal acquisition
circuit 200 of Figs. 2A-C, individual signals from each of the different pairings of ECG
sensing electrodes are provided directly to the inputs of a respective amplifier. As such, this
embodiment may avoid noise or signal degradation caused by the selection circuit 210 being
located prior to the differential circuit 220. Such selection circuitry may instead be provided
after an analog-to-digital conversion and signal conditioning block, such as the analog-todigital
conversion and signal conditioning block 14 described with respect to Figs 2a-2c of the
'669 patent. For example, as shown in Fig. 3, the analog differential output signals provided
by each respective amplifier 320a-320n, 321a-321n-l, 329a may be provided to a respective
input 332 of an A/D conversion and signal conditioning block 330 that digitizes and conditions
the analog differential output signals. The digitized and conditioned signals provided on a
respective output 334 of the A/D conversion and signal conditioning block 330 may then be
provided to a respective input 342 of an output selection circuit 340 that operates in a manner
similar to a multiple output multiplexer. Responsive to control signals provided to control
inputs 344 of the selection circuit 340, the selection circuit selects, from among the plurality of
digitized and conditioned signals, which of those digitized and conditioned signals to provide
to a respective output 346 of the selection circuit for monitoring and/or analysis.
In accordance with an aspect of the present invention, each of the different pairings of
ECG sensing electrodes 10 may be selected and their signals analyzed to identify those
pairings of ECG sensing electrodes that provide a desired ECG signal, in terms of signal-tonoise
ratio, phase discrimination, or any other criteria. Those pairings of ECG sensing
electrodes providing the highest signal-to-noise ratio, a particular phase discrimination or a
maximum phase discrimination, or those pairings of electrodes corresponding to particular
planes may then be selected to provide those signals to a cardiac monitor, or to an arrhythmia
detection system, such as that illustrated in Figs. 2a-c of the '669 patent. For example,
referring to Fig. 1G, if it were determined that the pairing of ECG sensing electrodes 10a and
lOj, and 10c and lOd provided the highest quality signal (in terms of a high signal-to-noise
ratio and maximum phase discrimination), but the pairing of ECG sensing electrodes 10a and
10b did not, the signal from ECG sensing electrode lOj would be paired with ECG sensing
electrode 10a and the signals from these ECG sensing electrodes could be analyzed with
respect to the signals from ECG sensing electrodes 10c and lOd.
It should be appreciated that embodiments of the present invention provide a cardiac
monitoring system and/or a cardiac monitoring and arrhythmia detection system with the
ability to select, from among a plurality of electrodes, those pairings of electrodes that provide
the highest quality signal, a particular phase difference or a maximum phase discrimination, or
any other criteria. With this ability to choose ECG sensing electrodes, the analyzer of the
cardiac monitoring and arrhythmia detection system can, for example, be tuned to give the best
orthogonal view and can provide more cardiac information than a single or dual channel
sensing system. The analyzer can select multiple templates representing different phase angles
between ECG sensing electrode leads, or templates representing different planes of view of the
patient's heart. Each electrode channel can be auto correlated (compared to itself) or cross
correlated (compared with other channels) in order to screen out noise and derive additional
information.
Embodiments of the present invention can also return to the best axis positions if the
overall electrode system was shifted at a later time, such as when the electrodes are configured
as part of a wearable electrode belt or garment system. Because this multiple electrode
configuration can select the electrodes with the best quality signal, the number of alarms due to
ECG noise and fall-off can be reduced. Another byproduct of a cleaner ECG signal is a
reduction in false detections. By checking multiple electrodes, and finding that the majority
are sensing the same thing, embodiments of the present invention can increase the confidence
level of the detection algorithm. In addition, each time the electrode belt or garment is worn,
the electrodes may move to a slightly different location, resulting in a change to the ECG
signal. With multiple electrode configurations, the detection system can scan the multiple
paths and select the highest quality signals. Furthermore, by providing redundancy to the
sensing system, this multiple electrode configuration helps to improve the overall system
reliability. A fault in one or more channels can be tolerated because there are other working
channels. These and other aspects of the present invention are now described with respect to
Figs. 4-8.
Fig. 4 functionally illustrates a control unit, such as the control unit 30 depicted in Figs.
1A-F that may be used by a portable medical device, such as a cardiac monitor or a wearable
defibrillator, in accordance with the present invention. As shown, the control unit 30 includes
at least one processor 410, a battery 420, a data storage 412, a sensor interface 414, a therapy
delivery interface 416, and a user interface 418. The battery 420 may be a rechargeable three
cell 2200mAh lithium ion battery pack that provides electrical power to the other device
components. The data storage 412, the sensor interface 414, the therapy delivery interface 416,
and the user interface 418 are coupled to the at least one processor 410. The data storage 412
includes a computer readable and writeable data storage medium configured to store nontransitory
instructions and other data, and can include both nonvolatile storage media, such as
optical or magnetic disk, ROM or flash memory, as well as volatile memory, such as RAM.
The instructions may include executable programs or other code that can be executed by the at
least one processor 410 to perform any of the functions described here below.
The therapy delivery interface 416 couples one or more therapy delivery devices, such as
defibrillator therapy electrodes 12a-c (Fig. IE), to the at least one processor 410. Where the
control unit is used solely for monitoring a patient's cardiac condition, the therapy interface
416 and associated defibrillation therapy electrodes may be omitted. The user interface 418
includes a combination of hardware and software components that allow the control unit 30 to
communicate with an external entity, such as a user. These components are configured to
receive information from actions such as physical movement, verbal intonation or thought
processes. In addition, the components of the user interface 418 can provide information to
external entities, for example, in a manner such as described in U.S. Patent No. 6,681,003,
which is incorporated herein by reference. Examples of the components that may be employed
within the user interface 418 include keyboards, mouse devices, trackballs, microphones,
electrodes, touch screens, printing devices, display screens and speakers.
The sensor interface 414 couples the at least one processor 410 to a plurality of
physiological sensors, such as the plurality of ECG sensing electrodes 10. In some
embodiments, the sensor interface 414 may also couple the at least one processor 410 to other
physiological sensors, such as activity sensors, pulse oxygen sensors, temperature sensors,
respiratory rate sensors, thoracic impedance sensors, blood pressure sensors, acoustic sensors,
etc. The sensor interface 414 can include a signal acquisition circuit, such as the signal
acquisitions circuits 200 and 300 described above with respect to Figs. 2A-C and 3, or the
signal acquisition circuits 900, 1000, and 1100 described further below with respect to Figs. 9-
11, to select, from among the plurality of ECG sensing electrodes and/or other physiological
sensors, those that provide a desired signal, in terms signal-to-noise ratio, phase discrimination,
or any other criteria.
Although not illustrated in Fig. 4, the control unit 30 may include additional components
and or interfaces, such as a communication network interface (wired and/or wireless), and the
at least one processor 410 may include a power conserving processor arrangement such as
described in co-pending Application Serial No. 12/833,096, entitled SYSTEM AND
METHOD FOR CONSERVING POWER IN A MEDICAL DEVICE, filed July 9, 2010
(hereinafter the "'096 application"), and incorporated by reference herein in its entirety. For
example, as described in the Ό96 application, the at least one processor 410 may include a
general purpose processor, such as an Intel® PXA270 processor that is coupled to a critical
purpose processor, such as a Freescale™ DSP56311 Digital Signal Processor (DSP). The
general purpose processor can be configured to perform non-critical functions that do not
require real time processing, such as interfacing with the communication network interface and
the user interface, while the critical purpose processor is configured to perform critical
functions that require real time processing, such as the sampling and analysis of ECG
information, the charging of the capacitors to a particular voltage, and the generation and/or
delivery of therapeutic defibrillating pulses. It should be appreciated that in some
embodiments, the functionality of the at least one processor may be implemented in a Field
Programmable Gate Array (FPGA), one or more Programmable Logic Devices (PLDs), a
Complex PLD (CPLD), or custom Application Specific Integrated Circuit (ASIC).
Figures 5-8 illustrate a number of different processes that may be performed by the at
least one processor 410 of the control unit 30 to improve the monitoring and analysis of cardiac
activity, to improve the detection of cardiac abnormalities, and to reduce the number of false
detections and fall-off alarms in accordance with embodiments of the present invention.
Fig. 5 illustrates a selection process that may be executed by the at least one processor
410 of the control unit 30 to select, from among a plurality of ECG sensing electrodes 10,
those providing a highest quality ECG signal, in terms of signal-to-noise ratio and maximum
phase discrimination, in accordance with one embodiment of the present invention. In act 510,
the at least one processor 410 selects a pair of ECG sensing electrodes to monitor. As
discussed previously, this may be performed by the at least one processor sending appropriate
control signals to selection circuit 210, 340.
In act 520, the at least one processor analyzes the ECG signal obtained from the
selected pair of ECG sensing electrodes and records information identifying the selected pair
of ECG sensing electrodes and a metric indicative of the quality of the ECG signal provided
therefrom. Although a number of different criteria may be used to identify the quality of the
ECG signal, in one embodiment, those ECG signals having a highest signal-to-noise ratio and a
maximum phase discrimination are assigned a higher quality metric than those pairings that do
not.
In act 530, the at least one processor determines whether each of the possible pairings
of ECG sensing electrodes have been selected and analyzed. Where it is determined that all
the possible pairings of ECG sensing electrodes have been selected and analyzed, the process
proceeds to act 540. Alternatively, where it is determined that fewer than all of the possible
pairs of ECG sensing electrodes have been selected and analyzed, the process returns to act
510, where a next sensor pairing is selected. Acts 510 through 530 are then performed for each
of the possible pairings of ECG sensing electrodes.
In act 540, the at least one processor selects, from among the plurality of different
pairings of ECG sensing electrodes, those pairs of ECG sensing electrodes having the highest
quality metric. It should be appreciated that the number of different pairings of ECG sensing
electrodes that are selected in act 540 will depend on the number of different channels
provided at the output 226 of the differential circuit 220 (Fig. 2A) or at the output 346 of the
selection circuit 340 of Fig. 3. In general, a minimum of two channels would be selected in act
540, and in most implementations, at least four different channels would be selected. In some
embodiments, the number of channels provided may correspond to each unique pairing of
electrodes.
In act 550, the at least one processor monitors and analyzes the ECG signals provided
by the selected pairings of ECG sensing electrodes. The act of monitoring and analyzing the
ECG signals provided by the selected ECG sensor pairs (i.e., act 550) may continue until the
terminated by removal and/or power down of the wearable medical device.
In accordance with one embodiment of the present invention, the selection process
described with respect to Fig. 5 may be performed each time the electrode system 100 is
powered on to account for any potential repositioning of the plurality of electrodes 10 on the
body of the patient. Thus, for example, when a garment 20a-20c incorporating an electrode
system 100 is removed from the body of the patient to allow the patient to shower, for service,
or for any other reason, and then returned to a position on the patient' s body, the positioning of
some or all of the ECG sensing electrodes may change from their prior position. By reexecuting
the selection process of Fig. 5, the electrode system may select those pairings of
ECG sensing electrodes that provide the highest quality ECG signals, irrespective of whether
those pairings of ECG electrodes are the same, or different from those selected previously. In
certain implementations, an initial pairing of ECG sensing electrodes may be based upon those
that were previously selected in act 540. For example, in response to a garment incorporating
an electrode system 100 being removed from and returned to the body of a patient, the
electrode system may initially select pairings of ECG sensing electrodes based upon those that
were selected prior to removal of the garment in act 540. That initial selection may then be
confirmed by re-executing the selection process of Fig. 5.
It should be appreciated that the selection process described with respect to Fig. 5 may
be re-executed, either at periodic intervals (e.g., every half hour), or in response to another
sensor, such as an activity sensor, indicating strenuous physical activity, to ensure the optimal
pairings of ECG sensing electrodes are selected. By re-executing the selection process, either
periodically, or in response to detected physical activity, embodiments of the present invention
can ensure that those pairings of ECG sensing electrodes providing the highest quality ECG are
identified and used for monitoring and analysis.
Although the selection process of Fig. 5 was described as selecting those pairs of ECG
sensing electrodes providing the highest quality ECG signal, in terms of signal-to-noise ratio
and maximum phase discrimination, it should be appreciated that other criteria may be used.
For example, the process described with respect to Fig. 5 may be modified to include an act of
selecting a desired template prior to act 510. The desired template may, for example, reflect
different phase angles between ECG sensing electrodes that are desired to be monitored. The
acts 510 and 520 of selecting and analyzing different ECG sensing electrode pairings could
thus select, from among the plurality of ECG sensing electrodes, those pairs of ECG sensing
electrodes that provide the highest signal-to-noise ratio from among those pairings that meet
the desired phase angle(s) of the template. It should be appreciated that other criterion, other
than phase angle, may be reflected in a template, and that multiple templates may be provided
and/or selected. For example, one template may correspond to different pairings of ECG
sensing electrodes that correspond to different planes intersecting the patient' s heart, while
another template may correspond to different pairing of ECG sensing electrodes that are all colocated
in the same plane.
Fig. 6 illustrates a noise/fall-off detection process that may be executed by the at least
one processor 410 of the control unit 30 (Fig. 4) in accordance with an aspect of the present
invention to improve the quality of monitoring and analysis of ECG signals and/or to reduce
the number of fall-off alarms. In act 610 the at least one processor monitors and analyzes
selected ECG signals from different pairings of ECG sensing electrodes. The pairings of ECG
sensing electrodes that are monitored and analyzed in act 610 may have been previously
selected based upon a selection process such as that described with respect to Fig. 5. In act
620, the at least one processor makes a determination as to whether there is noise in the ECG
signal of a selected pairing of ECG sensing electrodes, or whether there has been a fall-off or at
least partial loss of contact with the body of the patient by a selected pairing of ECG sensing
electrodes. Where it is determined in act 620 that there is no appreciable noise or a diminished
signal or a lack of signal on any of the selected pairings of ECG sensing electrodes, the at least
one processor returns to act 610 and continues monitoring the selected ECG signals.
Alternatively, where it is determined that there is appreciable noise or a diminished signal or
lack of signal from one of the selected pairings of ECG sensing electrodes, the process
proceeds to act 630.
In act 630 the at least one processor 410 selects a new pairing of ECG sensing
electrodes to replace the pairing in which increased noise, or a diminished ECG signal was
detected. Act 630 may be performed in a manner similar to the selection process described
with respect to Fig. 5. In act 630, each of the possible pairings of ECG sensing electrodes may
be re-evaluated to select those pairings of ECG electrodes to be monitored. Alternatively,
those selected pairings of ECG sensing electrodes in which noise or fall-off was not detected
may be retained as selected pairings, and the remaining ECG sensing electrodes evaluated to
identify and select a pairing of ECG sensing electrodes to replace the pairing in which noise or
fall-off was detected. In response to the selection of a new pairing of ECG sensing electrodes,
or a number of new pairings, the process returns to monitoring an analyzing ECG signals in act
610.
Although not shown in Fig. 6, in response to the detection of noise or fall-off in a
selected pairing of ECG sensing electrodes, the at least one processor 410 may conduct
additional tests on the selected pairing. For example, the at least one processor may pair each
ECG sensing electrode of the selected pair with a driven ground electrode to identify which of
the ECG sensing electrodes of the selected pair may have a noise issue or may have at least
partially lost contact with the body of the patient. The at least one processor 410 may also
send a message to the user of portable medical device (or a bystander) via the user interface
418 to notify the user that one or more of the ECG sensing electrodes of the selected pairing
may have a noise issue or may have at least partially lost contact with the body of the patient,
and may further request the user to reposition the ECG sensing electrodes of the selected
pairing.
Fig. 7 illustrates monitoring and analysis routine that may be executed by the at least
one processor 410 of the control unit 30 to improve the detection of cardiac arrhythmias and
reduce the number of false detections. In act 710 the at least one processor monitors and
analyzes selected ECG signals from different pairings of ECG sensing electrodes. The pairings
of ECG sensing electrodes that are monitored and analyzed in act 710 may have been
previously selected based upon a selection process such as that described with respect to Fig. 5.
In act 720 a determination is made as to whether a cardiac arrhythmia has been detected.
Where it is determined in act 720 that a cardiac arrhythmia, such as ventricular tachycardia or
ventricular fibrillation, has not been detected, the process returns to act 710 and continues to
monitor and analyze the selected ECG signals. Alternatively, where it is determined in act 720
that a cardiac arrhythmia has been detected, the at least one processor proceeds to act 730
wherein the at least one processor sets a flag or indicator identifying that a cardiac arrhythmia
has been detected, with the at least one processor proceeding to act 740.
In act 740, the at least one processor 410 selects a different or additional pairing of
ECG sensing electrodes to monitor, to identify whether the determined arrhythmia is also
present in the ECG signals from other pairings of ECG sensing electrodes. The additional or
different pairings of ECG sensing electrodes may be based upon the selection process
described previously with respect to Fig. 5. For example, the additional or different pairings of
ECG sensing electrodes that are selected in act 740 may be one or more of those pairings that
provides the next highest signal quality level other than those that were selected in act 540 of
Fig. 5. In act 750, the at least one processor continues to monitor and analyze the selected
ECG signals, including those from additional or different pairings of ECG sensing electrodes
selected in act 740.
In act 760, the at least one processor 410 again determines whether a cardiac
arrhythmia has been detected, based upon the ECG signals monitored in act 750. Where it is
determined that a cardiac arrhythmia has not been detected in the different or additional
pairings, the at least one processor may simply return to act 750 and continue to monitor the
selected ECG signals. However, where it is determined in act 760 that a cardiac arrhythmia,
such as ventricular tachycardia or ventricular fibrillation has been detected, the at least one
processor may proceed to act 770. In act 770, in response to detecting that the cardiac
arrhythmia is still present, or is also present on the selected additional or different pairings of
ECG sensing electrodes, the at least one processor increases a confidence level of the indicator
or flag set in act 730. Although not depicted in Fig. 7, in response to the confidence level
being above a certain threshold, and the cardiac arrhythmia being a type of cardiac arrhythmia
for which defibrillation is an appropriate treatment, the at least one processor 410 may execute
one or more instructions that result in defibrillation being applied to the body of the patient via
the therapy delivery interface 416. In accordance with this aspect of the present invention, by
examining other pairings of ECG sensing electrodes in response to a detected cardiac
arrhythmia, the detection specificity of cardiac arrhythmias may be increased and the number
of false detections of cardiac malfunction may be reduced.
Fig. 8 illustrates a monitoring and analysis routine in accordance with another
embodiment of the present invention that may be executed by the at least one processor 410 of
the control unit 30 to improve the monitoring and analysis of cardiac activity. In act 810 the at
least one processor monitors and analyzes selected ECG signals from different pairings of
ECG sensing electrodes. The pairings of ECG sensing electrodes that are monitored and
analyzed in act 810 may have been previously selected based upon a selection process such as
that described with respect to Fig. 5, or they may have been selected for other reasons. For
example, the pairings of ECG electrodes may not provide the highest quality ECG signal of all
of the ECG sensor pairs, but may correspond to a particular plane or planes, or to a particular
position relative to the heart.
In act 820, the at least one processor 410 monitors and analyzes the ECG signals
provided by the selected pairings of ECG sensing electrodes. In act 830 a determination is
made as to whether to select new pairs of ECG sensing electrodes to monitor. The
determination as to whether to select new pairs of ECG sensing electrodes may be based upon
a number of different criteria, including the number of channels that are capable of being
monitored and analyzed at a time, the type of information that is sought, the stage of the
cardiac cycle (e.g., the diastolic stage, or the systolic stage), the position of the ECG sensing
electrodes relative to the heart and/or the stage of depolarization or repolarization of the heart
(e.g., as indicated by PQRST waveform of the ECG signals), etc. For example, where the
control unit 30 is capable of simultaneously monitoring three different channels and the
plurality of ECG sensing electrodes 10 includes 12 ECG sensing electrodes, three pairings of
ECG sensing electrodes (including six distinct ECG sensing electrodes) may be monitored and
analyzed during a first time interval, and the remaining three pairings of ECG sensing
electrodes that were not monitored and analyzed during the first interval may be monitored and
analyzed during a second and subsequent time interval. Alternatively, where the control unit is
capable of simultaneously monitoring three different channels and the plurality of ECG sensing
electrodes 10 includes 16 ECG sensing electrodes (as shown in Fig. 1G), three different
pairings of ECG sensing electrodes including ECG sensing electrode pairs -, lOc-lOd,
10m- 1On may be monitored and analyzed during a first time interval, three different pairings of
ECG sensing electrodes including ECG sensing electrode pairs lOg-lOh, lOk-101, and lOb-lOa
may be monitored during a second interval, and three different pairs of ECG sensing electrodes
including ECG sensing electrodes pairs lOj-lOi, lOf-lOe, and -may be monitored and
analyzed during a third time interval. In this manner, the selected pairings of ECG electrodes
may sweep about the circumference of the heart. It should be appreciated that where the
number of channels that can be simultaneously monitored by the control unit 30 are sufficient
to monitor all pairings of ECG sensing electrodes, or all unique pairings of ECG sensing
electrodes, then all such pairings may be monitored simultaneously.
Accordingly, in act 830, where it is determined that a new or different pairing of ECG
sensing electrodes are to be monitored and analyzed, the monitoring and analysis routine
returns to act 810 wherein those new or different pairings of ECG sensing electrodes are
selected (act 810) and monitored and analyzed (act 820). Alternatively, where it is determined
in act 830 that a new or different pairing of ECG sensing electrodes is not desired, the routine
returns to act 820 and continues monitoring the pairings of previously selected ECG sensing
electrodes.
Fig. 9 illustrates an alternative signal acquisition circuit that may be used with
embodiments of the present invention to select, from among a plurality of ECG sensing
electrodes, those pairing of electrodes that provide a desired ECG signal, in terms of signal-tonoise
ratio, phase discrimination, or any other criteria, and provide those ECG signals to
downstream circuitry for further signal conditioning, processing, analysis, and/or monitoring.
In contrast to the embodiments described previously with respect to Figs. 2A-C and 3, the
signal acquisition circuit 900 does not include any differential amplifiers but instead generates
differential ECG signals corresponding to selected pairings of ECG sensing electrodes in
software executed by a processor, such as the at least one processor 410 described previously
with respect to Fig. 4.
As shown, the signal acquisition circuit 900 includes an analog multiplexor 910 and an
analog-to-digital (A/D) converter 920. Signals from each of the plurality of ECG sensing
electrodes lOa- are provided to a respective input of a plurality of inputs 912 of the analog
multiplexor 910. The analog multiplexor has an output 916 that is electrically coupled to an
input 922 of the A/D converter 920. The analog multiplexor 910 includes a plurality of control
inputs 914 to select which one of the plurality of signals received from a respective ECG
sensing electrode 10a- lOp is provided to the input 922 of the of the A/D converter 920. The
A/D converter 920 receives the selected signal from the selected one of the plurality of ECG
sensing electrodes and converts that analog ECG sensor signal to a digital signal. To ensure
adequate resolution for the processing of the digitized signals that is performed by the at least
one processor 410, the A/D converter 920 may be a 24 bit A/D converter, although an A/D
converter with fewer bits may be used. In general, the sampling rate of the A/D converter 920
should be at least N times the desired sampling rate of the ECG signal, where N is the number
of ECG sensing electrodes that are desired to be monitored. For example, where it is desired to
monitor ECG signals provided by each of three pairs of ECG sensing electrodes at a sampling
rate of 400 Hz, the A/D converter 920 should have a sampling rate in excess of 2.4 KHz. It
should be appreciated that higher sampling rates may of course be used.
Although not shown in Fig. 9, each of the signals from a respective ECG sensing
electrode 10-lOp may first be buffered, filtered, and/or amplified prior to being received at a
respective input of the analog multiplexor 910. For example, each of the signals received from
a respective one of the plurality of ECG sensing electrodes lOa-p may be provided to the input
of a high impedance buffer so that the analog multiplexor and the A/D converter to do not load
down the respective ECG sensing electrode. The output of a respective buffer may be lowpass
filtered (i.e., anti-aliased) to ensure that any frequency components of the signal are below
the Nyquist frequency of the A/D converter 920, and the filtered signal provided to a low-noise
and low to moderate gain amplifier to amplify the signal before that signal is provided to a
respective input of the analog multiplexor 910. As would be appreciated by one skilled in the
art, the combination of buffering, filtering, and/or amplifying the signal received from each of
the plurality of ECG sensing electrodes may be performed in multiple and distinct stages (e.g.,
a high impedance buffer stage followed by a filtering stage and one or more amplification
stages), or some of the stages, such as the buffering and amplification stages may be performed
in a single stage (e.g., a high impedance low-noise amplifier with low to moderate gain). In
some embodiments, the amplification stage may be programmable by the at least one processor
410.
In accordance with one embodiment, the analog multiplexer 910 may be a conventional
analog multiplexer, available from companies such as Analog Devices, Inc. of Norwood
Massachusetts, in which control signals received on the control inputs of the analog
multiplexer select which one of the signals received on a respective input of the multiplexer is
provided to the output. The A/D converter 910 converts the received signal to a digital signal
and provides the converted digital signal to the at least one processor 410. The at least one
processor is configured to control the multiplexor 910 and the A/D converter 920 to sample
and convert each of the signals received from a respective ECG sensing electrode over a
different time interval and provide the converted signals to the at least one processor 410.
Dependent upon which of the plurality of ECG sensing electrodes lOa-p are selected to be
paired with one another, the at least one processor 410 takes the two selected digital signals,
inverts one of them, and digitally sums the signals, effectively performing the same
functionality as the differential instrumentation amplifiers described with respect to Figs. 2A-C
and 3 above. The selection, inversion, and summing of selected pairs of digital signals may be
performed for any pairing of ECG sensing electrodes. The digitally summed signals may then
be processed to monitor the patient's ECG signals, to detect any arrhythmic cardiac condition,
or both. It should be appreciated that which of the pairs of ECG sensing electrodes to pair and
monitor may be performed in software by the at least one processor in a manner similar to that
shown in Fig. 5. Each of the digitized signals may be compared to one another for maximum
phase difference, or a specific phase difference, or for any other criterion. Those pairings of
ECG sensing electrodes may then be selected and monitored and analyzed in the manner
described above.
In accordance with an alternate embodiment, the analog multiplexer 910 may be an
analog sample-and-hold multiplexer that is capable of simultaneously sampling signals
received from each of the plurality of ECG sensing electrodes over a first time period, and then
providing each of the plurality of sampled signals to the A/D converter 920 during subsequent
time periods. In this embodiment, the at least one processor 410 is configured to control the
analog multiplexer 910 and the A/D converter 920 to sample and hold the signals received
from each of the plurality of ECG sensing electrodes lOa-p over a first time period, and
provide each, or selected ones, of the sampled signals to the A/D converter 920 to be converted
to digital signals and provided to the at least one processor over subsequent time periods. As
in the embodiment described above, dependent upon which of the plurality of ECG sensing
electrodes lOa-p are selected to be paired with one another, the at least one processor 410 takes
the two selected digital signals, inverts one of them, and digitally sums the signals, effectively
performing the same functionality as the differential instrumentation amplifiers described with
respect to Figs. 2A-C and 3 above. The selection, inversion, and summing of selected pairs of
digital signals may be performed for any pairing of ECG sensing electrodes. The digitally
summed signals may then be processed to monitor the patient' s ECG signals and/or to detect
any arrhythmic cardiac condition.
Fig. 10 illustrates a further alternative signal acquisition circuit that may be used with
embodiments of the present invention to select, from among a plurality of ECG sensing
electrodes, those pairing of electrodes that provide a desired ECG signal, in terms of signal-tonoise
ratio, phase discrimination, or any other criteria, and provide those ECG signals to
downstream circuitry for further signal conditioning, processing, analysis, and/or monitoring.
In contrast to the embodiments described previously with respect to Figs. 2A-C and 3, and in a
manner similar to the embodiment of Fig. 9, the signal acquisition circuit 1000 does not
include any differential amplifiers, but instead generates differential ECG signals
corresponding to selected pairings of ECG sensing electrodes in software executed by a
processor, such as the at least one processor 410 described previously with respect to Fig. 4.
As shown, the signal acquisition circuit 1000 includes a plurality of analog-to-digital
(A/D) converters 1010a-p. Each of the plurality of A/D converters 1010a-p is configured to
receive a signal from a respective one of the plurality of ECG sensing electrodes lOa-p, for
example, with a first A/D converter 1010a receiving a signal from ECG sensing electrode 10a,
A/D converter 1010b receiving a signal from ECG sensing electrode 10b, etc. Each respective
A/D converter 1010a-p converts the signal to a digital signal and provides the converted digital
signal to the at least one processor 410 over a communication link 1020, such as a serial or
parallel bus. Although not shown in Fig. 10, each of the signals from a respective ECG
sensing electrode 10-lOp may first be buffered, filtered, and/or amplified prior to being
received at a respective input of a respective A/D converter 1010a-p in a manner similar to that
described above with respect to Fig. 9 so that the A/D converter does not load down the
respective ECG sensing electrode, and to ensure that any frequency components of the
received signals are below the Nyquist frequency of a respective A/D converter 1010a-p.
To ensure adequate resolution for the processing performed by the at least one
processor 410, each of the plurality of A/D converters 1010a-p may be a 24 bit A/D converter,
although an A/D converter with fewer bits may be used. In contrast to the embodiment
described above with respect to Fig. 9, each of plurality of A/D converters 1010a-p of this
embodiment need not have a sampling rate that is N times the desired sampling rate of the
ECG signal, where N is the number of ECG sensing electrodes that are desired to be
monitored, because each of the signals received from a respective ECG sensing electrode may
be sampled in parallel. For example, where it is desired to monitor ECG signals provided by
each of three pairs of ECG sensing electrodes at a sampling rate of 400Hz, each of the plurality
of A/D converters may have a sampling rate of 400Hz, thereby allowing the use of less costly
A/D converters. Of course, it should be appreciated that higher sampling rates may be used.
In accordance with this embodiment, the at least one processor 410 may send a control signal
to each of the plurality of A/D converters 1010a-p to sample a respective signal at substantially
the same period of time, and then send the sampled and converted digital signal to the
processor at a subsequent time. Dependent upon which of the plurality of ECG sensing
electrodes are selected to be paired with one another, the at least one processor takes the two
selected digital signals, inverts one of them, and digitally sums the signals, effectively
performing the same functionality as the differential instrumentation amplifiers described with
respect to Figs. 2A-C and 3 above. The selection, inversion, and summing of selected pairs of
digital signals may again be performed for any pairing of ECG sensing electrodes. The
digitally summed signals may then be processed to monitor the patient's ECG signals and/or to
detect any arrhythmic cardiac condition.
Fig. 11 illustrates yet a further alternative signal acquisition circuit that may be used
with embodiments of the present invention to select, from among a plurality of ECG sensing
electrodes, those pairing of electrodes that provide a desired ECG signal, in terms of signal-tonoise
ratio, phase discrimination, or any other criteria, and provide those ECG signals to
downstream circuitry for further signal conditioning, processing, analysis, and/or monitoring.
This embodiment is similar to the embodiment described above with respect to Figs. 9 and 10
in that it again does not include any differential amplifiers, but instead generates differential
ECG signals corresponding to selected pairings of ECG sensing electrodes in software
executed by a processor, such as the at least one processor 410 described previously with
respect to Fig. 4.
As in the embodiment described above with respect to Fig. 10, the signal acquisition
circuit 1100 shown in Fig. 11 again includes a plurality of analog-to-digital (A/D) converters
11lOa-p, each of which is configured to receive a signal from a respective one of the plurality
of ECG sensing electrodes lOa-p. Although not shown in Fig. 11, each of the signals from a
respective ECG sensing electrode 10-1Op may first be buffered, filtered, and/or amplified prior
to being received at a respective input of a respective A/D converter 11lOa-p in a manner
similar to that described above with respect to Figs. 9 and 10 so that the A/D converter does
not load down the respective ECG sensing electrode, and to ensure that any frequency
components of the received signal are below the Nyquist frequency of a respective A/D
converter 1110a-p.
In contrast to the embodiment of Fig. 10 in which the plurality of A/D converters
1010a-p are arranged in parallel, each of the plurality of A/D converters 1110a-p of this
embodiment are daisy chained (or cascaded) to one another, for example via a serial bus, such
as a SPI™ serial bus, a QSPI™ serial bus, or Microwire™ serial bus. Each respective A/D
converter 11lOa-p is arranged to sample a signal from a respective one of the plurality of ECG
sensing electrodes lOa-p during a first time interval, and convert the signal to a digital signal
and provide the converted digital signal to the next A/D converter in the chain during a
subsequent time interval. The output of the last A/D converter 11lOp in the chain is
communicatively coupled to the at least one processor via a communication link 1120, such as
a serial bus. The output of the last A/D converter in the chain (e.g., A/D converter 11lOp)
therefore provides a multi-bit signal to the at least one processor 410 with different bits in the
multi-bit signal corresponding to different ECG sensing electrodes, for example, with a first
series of bits corresponding to the converted digital signal obtained from ECG sensing
electrode lOp, the second series of bits corresponding to the converted digital signal obtained
from ECG sensing electrode 10, and the last series of bits corresponding to the converted
digital signal obtained from ECG sensing electrode 10a.
To ensure adequate resolution, each of the plurality of A/D converters 1110a-p may be
a 24 bit A/D converter, although an A/D converter with fewer bits, such as 16 bits may
alternatively be used. In contrast to the embodiment described above with respect to Fig. 9,
and similar to the embodiment described above with respect to Fig. 10, each of plurality of
A/D converters 11lOa-p of this embodiment need not have a sampling rate that is N times the
desired sampling rate of the ECG signal, where N is the number of ECG sensing electrodes
that are desired to be monitored, because each of the signals received from a respective ECG
sensing electrode may be sampled in parallel. Thus, where it is desired to monitor ECG signals
provided by each of three pairs of ECG sensing electrodes at a particular sampling rate, each of
the plurality of A/D converters may operate at that same sampling rate, thereby allowing the
use of less costly A/D converters. However, because the plurality of A/D converters 1110a-p
are daisy chained together, the rate at which the converted digital signals are communicated
from one A/D converter to the next, and then to the at least one processor 410 should be at
least N times the desired sampling rate of the ECG signal, where N corresponds to the number
of ECG sensing electrodes that are desired to be monitored. A suitable type of A/D converter
that can be cascaded or daisy chained in the manner described above is a MAX 11040K (24
bit) or Maxll060 (16 bit) ADC available from Maxim Integrated Products of Sunnyvale
California, although other analog-to digital converters available from other companies may
alternatively be used.
In accordance with this embodiment, the at least one processor 410 may send a control
signal to each of the plurality of A/D converters 1110a-p to sample a respective signal at
substantially the same period of time, and send the sampled and converted digital signal to the
next A/D converter in the chain, at a subsequent time. Ultimately, the last A/D converter
11provides the serial bitstream to the at least one processor 410. Dependent upon which
of the plurality of ECG sensing electrodes are selected to be paired with one another, the at
least one processor 410 extracts the digital signals corresponding to the two selected digital
signals (typically corresponding to the same time period), inverts one of them, and digitally
sums the signals, effectively performing the same functionality as the differential
instrumentation amplifiers described with respect to Figs. 2A-C and 3 above. The selection,
inversion, and summing of selected pairs of digital signals may be performed for any pairing of
ECG sensing electrodes, and corresponding to the same, or different time periods. The
digitally summed signals may then be processed to monitor the patient' s ECG signals and/or to
detect any arrhythmic cardiac condition.
It should be appreciated that where the signal acquisition circuits described above with
respect to Figs. 9-11 are used with an electrode system associated with a wearable ambulatory
medical device, each of these signal acquisition circuits not only permit the monitoring and
analysis of ECG signals from any pairing of ECG sensing electrodes that are associated with
the wearable ambulatory medical device, but they also permit the signal of any of the plurality
of ECG sensing electrodes lOa-p to be paired with the signal from another source, such as a
wireless ECG sensing electrode. For example, a wireless ECG sensing electrode may be
provided that includes an A/D converter, such as any of A/D converters 1010a-p or 1110a-p
described above, that is coupled to a wireless transmitter or transceiver and can communicate
with the at least one processor 410 via a wireless communication protocol such as Bluetooth,
ZigBee, Wireless USB, Wireless Ethernet, GSM, etc. The signal from the wireless ECG
sensing electrode may then be paired with any of the signals from each of ECG sensing
electrodes 10-p that are associated with the wearable ambulatory medical device, where it is
desirable to do so. In this manner, if additional cardiac information is desired, additional
wireless ECG sensing electrodes may be placed on the patient' s body, and those signals
monitored and analyzed. Indeed, in some embodiments, each of the ECG sensing electrodes
need not be associated with a garment that is worn by the patient, but each of the ECG sensing
electrodes may be self adhesive wireless ECG sensing electrodes that are simply placed, as
desired, on the patient' s body.
Having thus described several aspects of at least one embodiment of this invention, it is
to be appreciated various alterations, modifications, and improvements will readily occur to
those skilled in the art. Such alterations, modifications, and improvements are intended to be
part of this disclosure, and are intended to be within the scope of the invention. Accordingly,
the foregoing description and drawings are by way of example only.
What is claimed is:
CLAIMS
1. An ambulatory medical device, comprising:
a plurality of electrodes configured to be disposed at spaced apart positions about a
body of a patient;
an electrode signal acquisition circuit having a plurality of inputs, each respective input
of the plurality of inputs being electrically coupled to a respective electrode of the plurality of
electrodes, the electrode signal acquisition circuit being configured to sense a respective signal
provided by a plurality of different pairings of the plurality of electrodes; and
a monitoring circuit, electrically coupled to an output of the electrode signal acquisition
circuit, the monitoring circuit being configured to analyze the respective signal provided by
each of the plurality of different pairings and to instruct the electrode signal acquisition circuit
to select at least one of the plurality of different pairings to monitor based upon at least one of
a quality of the respective signal provided by the selected at least one of the plurality of
different pairings, a phase difference between the respective signal provided by the selected at
least one of the plurality of different pairings and the respective signal provided by another
selected at least one of the plurality of different pairings, a position of the respective electrodes
of the selected at least one of the plurality of different pairings relative to the body of the
patient, a plane defined by the respective electrodes of the selected at least one of the plurality
of different pairings, and a cardiac cycle of a heart of the patient.
2. The ambulatory medical device of claim 1, further comprising a garment that is
configured to be worn about the body of the patient, wherein the plurality of electrodes are
integrated into the garment.
3. The ambulatory medical device of claim 2, wherein the plurality of electrodes are
ECG sensing electrodes, and wherein the monitoring circuit is a cardiac monitoring and
arrhythmia detection circuit.
4. The ambulatory medical device of claim 3, wherein the plurality of ECG sensing
electrodes includes at least three ECG sensing electrodes.
5. The ambulatory medical device of claim 4, wherein the plurality of ECG sensing
electrodes are not all located in a common plane.
6. The ambulatory medical device of claim 5, wherein the cardiac monitoring and
arrhythmia detection circuit is configured to analyze the respective signal provided by each of
the plurality of different pairings and to instruct the electrode signal acquisition circuit to select
the at least one of the plurality of different pairings to monitor based upon the quality of the
respective signal provided by the selected at least one of the plurality of different pairings and
the phase difference between the respective signal provided by the selected at least one of the
plurality of different pairings and the respective signal provided by the other selected at least
one of the plurality of different pairings.
7. The ambulatory medical device of claim 6, further comprising:
a plurality of therapy electrodes integrated into the garment and configured to deliver a
defibrillating shock to the body of the patient in response to detection of a treatable cardiac
arrhythmia by the cardiac monitoring and arrhythmia detection circuit.
8. The ambulatory medical device of claim 5, wherein the cardiac monitoring and
arrhythmia detection circuit is configured to analyze the respective signal provided by each of
the plurality of different pairings and to instruct the electrode signal acquisition circuit to select
the at least one of the plurality of different pairings to monitor based upon the quality of the
respective signal provided by the selected at least one of the plurality of different pairings and
the plane defined by the respective electrodes of the selected at least one of the plurality of
different pairings.
9. The ambulatory medical device of claim 5, wherein the cardiac monitoring and
arrhythmia detection circuit is configured to analyze the respective signal provided by each of
the plurality of different pairings and to instruct the electrode signal acquisition circuit to select
the at least one of the plurality of different pairings to monitor based upon the position of the
respective electrodes of the selected at least one of the plurality of different pairings relative to
the body of the patient and the cardiac cycle of the heart of the patient.
10. The ambulatory medical device of claim 5, wherein the cardiac monitoring and
arrhythmia detection circuit is configured to select at least three of the plurality of different
pairings to monitor based upon the at least one of the quality of the respective signal provided
by each of the selected at least three of the plurality of different pairings, the phase difference
between the respective signal provided by each of the selected at least three of the plurality of
different pairings, the position of the respective electrodes of the selected at three of the
plurality of different pairings relative to the body of the patient, the plane defined by the
respective electrodes of the selected at least three of the plurality of different pairings, and the
cardiac cycle of the heart of the patient.
11. The ambulatory medical device of claim 1, further comprising a garment that is
configured to be worn about the body of the patient, wherein the plurality of electrodes are
integrated into the garment, and wherein the plurality of electrodes includes at least four ECG
sensing electrode that are not all located in a common plane.
12. The ambulatory medical device of any one of claims 1 and 11, wherein the
monitoring circuit is configured to analyze the respective signal provided by each of the
plurality of different pairings and to instruct the electrode signal acquisition circuit to select the
at least one of the plurality different pairings to monitor based upon the quality of the
respective signal provided by the selected at least one of the plurality of different pairings and
the phase difference between the respective signal provided by the selected at least one of the
plurality of different pairings and the respective signal provided by the other selected at least
one of the plurality of different pairings.
13. The ambulatory medical device of any one of claims 1 and 11, wherein the
monitoring circuit is configured to analyze the respective signal provided by each of the
plurality of different pairings and to instruct the electrode signal acquisition circuit to select the
at least one of the plurality of different pairings to monitor based upon the quality of the
respective signal provided by the selected at least one of the plurality of different pairings and
the plane defined by the respective electrodes of the selected at least one of the plurality of
different pairings.
14. The ambulatory medical device of any one of claims 1 and 11, wherein the
monitoring circuit is configured to analyze the respective signal provided by each of the
plurality of different pairings and to instruct the electrode signal acquisition circuit to select the
at least one of the plurality of different pairings to monitor based upon the position of the
respective electrodes of the selected at least one of the plurality of different pairings relative to
the body of the patient and the cardiac cycle of the heart of the patient.
15. The ambulatory medical device of any one of claims 1 and 11, wherein the
monitoring circuit is configured to select at least three of the plurality of different pairings to
monitor based upon the at least one of the quality of the respective signal provided by each of
the selected at least three of the plurality of different pairings, the phase difference between the
respective signal provided by each of the selected at least three of the plurality of different
pairings, the position of the respective electrodes of the selected at three of the plurality of
different pairings relative to the body of the patient, the plane defined by the respective
electrodes of the selected at least three of the plurality of different pairings, and the cardiac
cycle of the heart of the patient.
16. The ambulatory medical device of claim 15, wherein the monitoring circuit is
configured to select at least two of the selected at least three of the plurality of different
pairings to monitor during a first time interval and to select a different at least two of the
selected at least three of the plurality of different pairings to monitor during a second time
interval.
17. The ambulatory medical device of claim 1, wherein the plurality of electrodes are
ECG sensing electrodes, and wherein the monitoring circuit is a cardiac monitoring and
arrhythmia detection circuit.
18. The ambulatory medical device of claim 17, wherein the plurality of ECG sensing
electrodes includes at least three ECG sensing electrodes.
19. The ambulatory medical device of claim 1, wherein the electrode signal acquisition
circuit includes:
a selection circuit having a plurality of inputs and a plurality of outputs, each respective
input of the plurality of inputs of the selection circuit being electrically coupled to a respective
one of the plurality of electrodes; and
a plurality of differential amplifiers, each respective differential amplifier having a pair
of inputs and an output, each respective input of the pair of inputs being electrically coupled to
a respective one of the plurality of outputs of the selection circuit, each respective output of the
plurality of differential amplifiers providing an output signal corresponding to a difference
between the pair of inputs of the respective differential amplifier.
20. The ambulatory medical device of claim 1, wherein the electrode signal acquisition
circuit includes:
a plurality of differential amplifiers each having a pair of inputs and an output, the
plurality of differential amplifiers including a respective differential amplifier for each unique
pairing of the plurality of electrodes; and
a selection circuit configured to select at least one output of the plurality of differential
amplifiers to provide to the monitoring circuit.
21. The ambulatory medical device of claim 1, wherein the electrode signal acquisition
circuit includes:
an analog multiplexor having a plurality of inputs and an output, each of the plurality of
inputs being electrically coupled to a respective one of the plurality of electrodes; and
an analog-to-digital converter having an input electrically coupled to the output of the
analog multiplexer.
22. The ambulatory medical device of claim 21, wherein the analog-to digital
converter has a sampling rate that is at least N times a desired sampling rate of a signal
provided by each of the plurality of electrodes, where N is the number of the plurality of
electrodes that are to be monitored.
23. The ambulatory medical device of claim 21, wherein the monitoring circuit
includes at least one processor configured to receive a first digital signal corresponding to a
first electrode of the plurality of electrodes and a second digital signal corresponding to a
second electrode of the plurality of electrodes, to invert one of the first and second digital
signals and sum the inverted one of the first and second digital signals with the other of the
first and second digital signals to analyze the respective signal provided by a pairing of the first
electrode and the second electrode.
24. The ambulatory medical device of claim 1, wherein the electrode signal acquisition
circuit includes a plurality of analog-to-digital converters, each respective analog-to-digital
converter having a respective input that is electrically coupled to a respective one of the
plurality of electrodes.
25. The ambulatory medical device of claim 24, wherein each of the plurality of
analog-to-digital converters is connected to another of the plurality of analog-to-digital
converters by a serial bus.
26. A method of monitoring ECG signals, the method comprising acts of:
selecting, from among a plurality of ECG sensors, a plurality of different pairings of
ECG sensors;
analyzing a respective ECG signal provided by each of the plurality of different
pairings;
identifying at least one of the plurality of different pairings to monitor based upon at
least one of a quality of the respective ECG signal provided by the identified at least one of the
plurality of different pairings, a phase difference between the respective ECG signal provided
by the identified at least one of the plurality of different pairings and the respective ECG signal
provided by another identified at least one of the plurality of different pairings, a position of
respective ECG sensors of the identified at least one of the plurality of different pairings
relative to a body of a patient, a plane defined by the respective ECG sensors of the identified
at least one of the plurality of different pairings, and a cardiac cycle of a heart of the patient;
and
monitoring the identified at least one of the plurality of different pairings.
27. The method of claim 26, wherein the act of identifying at least one of the plurality
of different pairings to monitor is based upon the quality of the respective ECG signal provided
by the identified at least one of the plurality of different pairings and the phase difference
between the respective ECG signal provided by the identified at least one of the plurality of
different pairings and the respective ECG signal provided by the other identified at least one of
the plurality of different pairings.
28. The method of claim 27, wherein the act of selecting the plurality of different
pairings of ECG sensors from among the plurality of ECG sensors includes an act of selecting,
from among the plurality of ECG sensors, each unique pairing of ECG sensors; and wherein
the act of analyzing the respective ECG signal provided by each of the plurality of different
pairings includes analyzing the respective ECG signal provided by each unique pairing of ECG
sensors.
29. The method of claim 28, wherein the act of monitoring includes:
monitoring the identified at least one of the plurality of different pairings to detect a
cardiac arrhythmia.
30. The method of claim 29, further comprising acts of:
detecting the cardiac arrhythmia responsive to the act of monitoring;
determining that the detected cardiac arrhythmia is a type of cardiac arrhythmia that
can be treated by applying defibrillation to the body of the patient; and
applying at least one defibrillation pulse to the body of the patient.
31. The method of claim 29, further comprising acts of:
detecting the cardiac arrhythmia responsive to the act of monitoring;
selecting at least one additional pairing of ECG sensors in response to detecting the
cardiac arrhythmia and analyzing the respective ECG signal provided by the at least one
additional pairing;
determining that the detected cardiac arrhythmia is also present on the respective ECG
signal of the at least one additional pairing;
determining that the detected cardiac arrhythmia is a type of cardiac arrhythmia that
can be treated by applying defibrillation to the body of the patient; and
applying at least one defibrillation pulse to the body of the patient.
32. The method of claim 29, further comprising acts of:
detecting the cardiac arrhythmia responsive to the act of monitoring;
selecting at least one additional pairing of ECG sensors in response to detecting the
cardiac arrhythmia and analyzing the respective ECG signal provided by the at least one
additional pairing;
determining that the detected cardiac arrhythmia is also present on the respective ECG
signal of the at least one additional pairing; and
increasing a confidence level that the cardiac arrhythmia has been detected.
33. The method of claim 29, further comprising acts of:
detecting the cardiac arrhythmia responsive to the act of monitoring;
selecting at least one additional pairing of ECG sensors in response to detecting the
cardiac arrhythmia and analyzing the respective ECG signal provided by the at least one
additional pairing;
determining that the detected cardiac arrhythmia is not present on the respective ECG
signal of the at least one additional pairing; and
decreasing a confidence level that the cardiac arrhythmia has been detected.
34. The method of any one of claims 26 and 27, wherein the acts of selecting,
analyzing, and identifying are repeated at periodic intervals.
35. The method of any one of claims 26 and 27, wherein the plurality of ECG sensors
are integrated in a garment that is worn about the body of the patient, and wherein the acts of
selecting, analyzing, and identifying are performed each time the garment is placed about the
body of the patient.
36. The method of any one of claims 26 and 27, wherein the plurality of ECG sensors
are integrated in a garment that is worn about the body of the patient, the method further
comprising an act of:
detecting strenuous physical activity of the patient; and
repeating the acts of selecting, analyzing, and identifying in response to the act of
detecting the strenuous activity of the patient.
37. The method of claim 27, further comprising acts of:
determining that the quality of the respective ECG signal provided by a first pairing of
ECG sensors of the identified at least one of the plurality of different pairings is below a
determined threshold;
selecting another paring of ECG sensors to replace the first pairing of ECG sensors; and
monitoring the other pairing of ECG sensors.
38. The method of any one of claims 26 and 27, further comprising acts of:
determining, from the quality of the respective ECG signal provided by a first pairing
of ECG sensors of the identified at least one of the plurality of different pairings, that one or
more of the ECG sensors of the first pairing may have at least partially lost contact with the
body of the patient;
selecting another paring of ECG sensors to replace the first pairing of ECG sensors; and
monitoring the other pairing of ECG sensors.
39. The method of claim 26, wherein the act of identifying at least one of the plurality
of different pairings to monitor is based upon the quality of the respective ECG signal provided
by the identified at least one of the plurality of different pairings and the plane defined by the
respective ECG sensors of the identified at least one of the plurality of different pairings.
40. The method of claim 26, wherein the act of identifying at least one of the plurality
of different pairings to monitor is based upon the position of respective ECG sensors of the
identified at least one of the plurality of different pairings relative to the body of the patient
and the cardiac cycle of the heart of the patient.