Speed of sound based capnographic sensor using acoustic virial
equation
1. FIELD OF INVENTION
The present invention generally relates to design and fabrication of non-invasive and portable Biosensor for capnography application, particularly relates to speed of sound based sensor using acoustic virial equation.
2. BACKGROUND OF THE INVENTION
Capnography, the measurement of CO2 in respiratory gases has become an integral
.part of anaesthesia monitoring and helps to prevent life threatening events. Capnography is
the graphic display of instantaneous CO2 concentration versus time (Time Capnogram) or
expired volume (Volume Capnogram) during a respiratory cycle. It is a standard for
monitoring during anaesthesia.
The existing device for capnography works on the principle of IR spectroscopy. A cuvette containing the CO2 sensor is inserted between the breathing circuit and the endotracheal tube. The IR rays traverse the respiratory gases to.an IR detector within the cuvette. It-is relatively heavy and must be" supported to prevent endotracheal tube kinking.-In addition, the sensor's window must be kept clean of mucus and particles to prevent, false readings. Infra-red analyzers, and mass spectrometers are some of the present measurement systems used for monitoring CO2 gas. Although mass spectrometers possess many advantages like quick response, time, the ability to measure multiple gases, the stable and
. accurate readings, due to the necessity for continuous preventive maintenance as well as the cost and size, these systems are ho longer popular. However, in situations where cost and
' space are insignificant, they are still considered the best choice for a system to monitor respiratory gas. The most common approach for CO2 measurement is based on.non-dispersive infrared (NDIR) sensors, which are the simplest of the spectroscopic sensors. However, the physical size' and power consumption restrict the technology considerably. US 5,060,514 relates to the ultrasonic gas measuring device.US 5,627,323 relates to the ultrasonic binary gas measuring device. The above cited prior art documents; do not howeverj give a wholesome method to predict the concentration of a gas in a mixture containing more than one gas.US US 2007/0048181 Al relates to a carbon dioxide nanosensor and respiratory CO2 monitors.US 2008/0214948 AI relates to a method and.apparatus to monitor respiration. The prior art documents do not address the concern of providing a capnography system that is portable and non-invasive. Therefore, there is a need in the art to provide an advanced sensor system for capnography that addresses all the above-mentioned requirements and that can be used in all areas of health care ranging from pre-hospital to extended care.

3. SUMMARY OF THE INVENTION
The need for inserting the sensor device into the endotracheal tube limits the use of capnography device to intensive care units and ventilators. In spite of the extensive use of capnography for intubated patients, capnography is an underutilized tool for the diagnosis of many pulmonary and respiratory diseases. The development of a portable, non-invasive, low power consuming, and small size sensor for capnography can extend the use of capnography from intubated patients to pre-hospital applications. Such a sensor can be designed and developed using the speed of sound technique and acoustic virial equations.
The speed of sound depends on the medium through which it travels. The concentration of CO2 in exhaled air can be monitored by measuring the speed of sound through the gas. The gas concentration measurement using speed of sound is based on the change of sound velocity according to the concentration of medium through which the sound wave is travelling. The speed of sound measurement device consists of an acoustic transmitter and receiver. Acoustic transducers are required to convert electrical energy to sound energy and vice versa. The sound wave transmitted from transmitter is allowed to travel through the path through which exhaled air sample is passed through. Depending on the concentration of CO2 in exhaled, air, the time taken for the sound wave to reach the receiver varies. From the time of transit of sound wave, speed of sound and thereby concentration of CO2 in exhaled air'can be calculated using acoustic virial equation. The real " time variation of partial pressure of CO2 in exhaled air is' graphically displayed as capnograph.
4. BRIEF DESCRIPTION OF THE DRAWINGS
. Figure 1 represents the process flow diagram of the sensor system of the present invention.
Figure 2 represents the sampling of exhaled air. .. Figure 3 represents the sensor system

5. DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a non invasive speed of sound based sensor system, providing the display of capnograph from the exhaled air, comprising of:
a. an enclosed cylindrical chamber (2) as in Figure.3, wherein an inlet (1) and outlet
(4) allows the passage of exhaled air
b. a plurality of ultrasonic sensors, including the first being an ultrasonic transmitter
(5) and second being an ultrasonic receiver (3).
c. a microcontroller which is connected to the ultrasonic sensors wherein the speed of
sound through exhaled air is calculated.
d. plurality of sensors, including the first being a temperature sensor and second being a pressure sensor.
In an embodiment of the present invention, the enclosed cylindrical chamber provides a closed path for the exhaled air sample to pass through. The chamber incorporates a plurality of ultrasonic sensors, including a transmitter operatively mounted on one end wall, and a receiver, operatively mounted on the other end wall. In this embodiment, an inlet and outlet are incorporated to allow the passage of exhaled air through the chamber.
In this embodiment, all of the sensors are connected to a microcontroller. The said sensors send data to the'logic controller which dictates the actions of the other components of the system. In the present embodiment, the timers present in the microcontroller continuously monitors the time taken by the ultrasonic wave to pass through the exhaled air' sample in the chamber. The micro controller constantly logs data from the ultrasonic sensors and perform the required algorithm to predict the instantaneous values of carbon dioxide in. exhaled air using the acquired data. .
In the. present embodiment, the pressure sensor and the temperature sensor present in the
chamber continuously monitors the pressure- and temperature maintained within the
chamber.. The gas concentration measurement using speed of sound is based on the change
of sound velocity according to the concentration of medium, through which the sound wave
is travelling. The speed of sound measurement device consists of a Microelectromechanical
acoustic transmitter and receiver. Acoustic transducers are required to convert electrical
energy to sound energy' and vice versa. The sound wave transmitted from transmitter is
allowed to travel through the path through which exhaled air sample is passed through.
■Depending on the concentration of CO2 in exhaled air, the time'taken for the sound wave to
reach'the receiver varies. A timer can be used to measure the time taken by the sound wave
transmitted by the acoustic transmitter to reach the receiver. Knowing the time of transit (T)
and the distance travelled by the sound wave which is nothing but the distance between the
acoustic transmitter and receiver (D), speed of sound can be calculated using the well-known
equation v = D/T . Thus from the time of transit of sound wave, speed of sound and thereby
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6. CLAIMS
We claim
1. A portable non-invasive sensor system to display capnograph, which is the instantaneous concentration of carbon dioxide in exhaled air.
2. The non-invasive sensor system for capnography as claimed in claim 1, consists of a cylindrical chamber with one inlet and outlet for sampling of exhaled air.
3. The non-invasive sensor system for capnography as claimed in claim 1, wherein an ultrasonic transmitter and detector are mounted on the cylindrical chamber as claimed in claim 2.
4. The non-invasive sensor system for capnography as claimed in claim I. wherein a disposable nasal mask, which consists of a mouth piece and a tube connecting lo the inlet of the cylindrical chamber is used to sample the exhaled air.
5. The non-invasive sensor system for capnography as claimed in claim I, wherein a microcontroller is connected to the ultrasonic transmitter and receiver as claimed in claim 3, which performs the necessary operations to convert the speed of sound ' through the exhaled air to the concentration of carbon dioxide.
6. The microcontroller as claimed in claim 5, consist of a timer to measure the time the ultrasonic wave is taking to reach the receiver from the transmitter through the.gas.
7. The microcontroller as claimed in claim 5, is connected to a display wherein the instantaneous concentration of carbon dioxide in exhaled air is displayed.
8. The noninvasive sensor system- for capnography as claimed in claim .1, wherein
. acoustic virial equations are used to predict the concentration of carbon dioxide from
the measured speed of sound through the exhaled air.-