his invention relates to a non-invasive system for measurement of oxygen saturation in arterial blood.
When the oxygen saturation in arterial blood is measured using the principle of pulse oximetry, it is expressed as Sp02.
In all the known methods that have been proposed so far, a part of the body is exposed to red light (R) of the visible spectrum and also by infra-red light (IR), some of which light (R + IR) enters into the body part. From within the body part such entry light (R + IR) exits the body part, the exiting light being detected to obtain photo plethysmo graph (PPG) signals.
The known methods extract the DC and AC (pulsatile) portions of R and IR PPGs and compute Sp02 from these values employing approximate empirical relationships based on ratios. The pulse oximeters used for the purpose require extensive calibration, the time and labour involved in which not only render the procedure tedious but also error-prone.
It is, therefore, an object of this invention to do away with the abovementioned drawbacks by proposing a method and device for the non-invasive measurement of oxygen saturation in arterial blood requiring little calibration for its use and operation.
The method for the non-invasive measurement of oxygen saturation in arterial blood, according to this invention, comprises the steps of exposing a part of the patient's body to R and IR

light; detection of these lights on exiting the body part by photo fliode circuits; normalizing the R and IR PPGs; removing the noise in the R and IR PPGs, by filters, to obtain signals vR and VIR; obtaining the natural logarithm of v* and v1R to get \\\(vR) and ln(y#), and the peak amplitudes of ln(vj?) and \n(vJRX that is, VpR and VPfA:9 and obtaining the oxygen saturation Sp02as

Normalizing R and IR PPG signals is carried out in the following manner: After the sig leaving the body eithr by transmission or reflection, the signal picked up by the detecto lowpass filtered for removal of noise and other interference and then natural logarithm effected on the signal in order to completely normalize it, that is, to make the signal fre< from the undesirable influence of the patient and circuit parameters.
Tiie device for the non-invasive measurement of oxygen saturation in arterial blood, according to this invention, comprises means for exposing a part of the patient's body to R and IR light; a photo diode circuit for detection of these lights on exiting the body part; means for the normalization of the R and IR PPGs; filters ibr removing the noise in the R and IR PPGs to obtain signals vR and vIR: means for calculating the natural logarithm of Vje and vIR to get ln(vR) and \n(vIR\ for calculating the peak to peak amplitudes of ln(vR ) and ln(vIR)9 that is, VpR and VpIR> and calculating the oxygen saturation Sp02as

This invention will now be described with reference to the following Example and drawings , which illustrate, but which do not limit, the scope of this invention.

In the known art oxygen saturation (Sp02) measurement pulse oximeters from different manufacturers are available in the medical field. All these pulse oximeters work on the principle of photoplethysinography (PPG). Atypical PPG signal has an AC component at heart rate due to arterial blood pulsations and a DC component due to non-pulsatile skin, tissue and bone. The ratio of the pulsatile signals measured at R and IR wavelengths is related to oxygen saturation. In modern pulse oximeters, the method of Sp02 estimation relies on an empirical equation obtained by fitting a curve to the data obtained from a group of healthy volunteers and the ratio of red and IR PPG signals. However, the PPGs are influenced by the patient-dependent parameters such as skin pigmentation, volume of the tissue and so on. Pulse oximeters are, therefore, extensively calibrated to remove the effect of patient- and sensor-dependent parameters on the computation of Sp02. Each manufacturer has his own calibrating factors depending upon the characteristics of the group of volunteers participating in the calibration process.
This invention, however, arrives at the value of SpO? without requiring extensive calibration The R and IR PPGs are processed appropriately, as mentioned earlier, with a view to removing the source, detector and patient dependent parameters. Sp02 is then computed utilizing the processed red and IR PPG?: and the well-known extinction coefficients of hemoglobin aud oxygenated hemoglobin at the chosen red and IR wavelengths. Thus the procedures developed for estimation of SpO? remove the effect of input intensity of the light source, detector sensitivity and patient-dependent parameters.

Theoretical and experimental verifications confirm that the computation of Sp02, employing this invention is indeed unaffected either by the variation in the input intensity of the sources or the sensitivities of the detectors employed. Accordingly, extensive calibration which is normally done for the conventional pulse oximeters is not essential in cases where this invention is used.
A part of the body, say, the finger is exposed to two light sources having different wavelengths, one in the red (R) region of the visible spectrum and the other in the infrared (IR) region. These lights enter the body part and immediately on exiting the body part (either after getting reflected within the body part or passing (getting transmitted) through the body part without
reflection) the "reflected" lights (PPGR^) or transmitted lights
(PPGn) are detected by photo diode circuits.
The noise and interference (created by 50Hz / 60 Hz supply frequency) is removed by using low pass filters.
Designating the resulting signals as vR and vIRt the natural logarithm of vR and vIR is calculated to get hi(yR) and in(v/#).
Next, the peak to peak amplitudes of \n(vR ) and \n(vJR) are determined as VpR and VpIR. and the oxygen saturation Sp02 as


Hemoglobin in arterial blood transports nearly 98 % of oxygen and the remaining 2 % is permanently dissolved in plasma and hence is of no use. One hemoglobin molecule can carry up to four oxygen molecules, Thus one can measure the level of oxygen (Sa02) in arterial blood indirectly by determining the amount of oxygenated hemoglobin as a percentage of total hemoglobin in arterial blood. If we denote
the amount of hemoglobin without oxygen as(Hb) and that of
the oxygenated hemoglobin as(Hb02) in arterial blood, then SaG2 as a percentage can be expressed as

Here the symbol ( ) denotes the amount of a particular
quantity. The above equation assumes that the amount of carboxy-hemoglobin and methemoglobin are negligible. When Sa02 is measured using the principle of pulse oximetry, the abbreviation used Sp02. In pulse oximetry, a part of the body is illuminated with two monochromatic light sources, OIIQ in the red region and the other in the infrared (IR) region. Either the reflected or transmitted light is then detected. The detected signal is called PhotoPlethysmoGraph (PPG). Existing methods

extract the dc and pulsatile components of R and IR PPGs and estimate the ratio of quotients of AC and DC amplitudes of the R and IR PPG signal. This ratio is then used to compute the saturation level of oxygen in arterial blood employing a linear approximation to an experimentally derived calibration curve obtained from healthy young volunteers after induction of hypoxemia.
The present invention is intended for the measurement of SpO^ from suitably processed R and IR PPGs. The processing results in the final expression for calculation of Sp02 to become not only independent of human dependent parameters such as skin pigmentation, volume of the intervening tissue but also independent of input source intensity and detector sensitivity. Thus the proposed invention estimation of SpOo does not require extensive calibration.
As in the existing methods, in the case of this invention also, a part of the body is exposed to two monochromatic light sources, one in the red region (visible light spectrum) and the other in the infrared region and either the reflected lights (PPGRA.) or transmitted lights (PPGT0 are detected using photo diode circuits. The sources and the detectors will have to be

housed on the same plane to obtain a PPG^R. but PPG™ can be obtained only with the source and the detector arranged on two different but parallel planes. Hence PPGn is preferably obtained from the extremities of the body, such as, the earlobe and fingertip.
Though the invention proposed is applicable equally for both reflectance type and transmittance type PPGs, the description given herein is for a transmission type PPG only for brevity. However, one can obtain relevant expressions and explanations simply by replacing the word "absorption" by "reflection" and using appropriate symbols.
To obtain the required PPGs two light sources (one R and one IR light emitting diodes) having output light intensities of IR and IIR are placed on one side of an extremity, say, the finger. The transmitted light through the linger is then detected by placing appropriate photo detectors (photo diode) on the side opposite to that of the sources (Fig. 1(a)). For reflection type the light travels through the skin and gets reflected, hence the source and the detector will have to be housed on the same plane (Fig. 1(b)) and Fig. 1 (c)). The incident R and IR waves travel through the finger and the transmitted light after absorption within the finger is detected by the photo detectors.

Since the total volume covered by the blood vessels is quite small, majority of the photons emitted from the sources go through the path made of epidermis-tissue-soft bone-tissue-epidermis and reach the detectors. Only a very small fraction, 1 in 50,000 to 1 in 100,000 of the photons emitted by the source goes through the path that includes blood vessels (epidermis-tissue-blood-tissue-bone-tissue-epidermis or epidermis-tissue-bone-tissue-blood-tissue-epidennis or epidennis-tissue-blood-tissue-bone-tissue-blood-tissue-epidennis).
The output of the photo diode will have three components, one
at the DC due to the light path epidermis-tissue-bone-tissue-
epidermis, the second at very low frequency due to the light
path epidermis-tissue-venous blood-bone-tissue-epidermis and
the third at the frequency of heart rate due to the light path
epidermis-tissue-arterial blood-bone-tissue-epidermis.
The attenuation of the beam of light assumed to have a uniform cross section of area A having a total length I as indicated in Fig. 2. Consider a disc of thickness dx in this path. Let the input intensity of light on the disc be ix and the attenuation across the disc dix. 99 % of this volume under consideration is made of molecules of dermis, tissue and bones. Hence the attenuation due to these cells is considered first. Each cell of a particular type.attenuates certain amount of light passing through that cell

*'CpCJlUlIlg UI1 lib UjLUlV/fcll pi uppity tlliui mov ^vain^i o mv iiiwunug
Bght
Assuming the dermis, tissue and bone cells have a cross sectional area of ADEf AT] and ABO and optical attenuation at a wavelength X be ann, « r(, and cr, respectively, each cell will
^ DEA TIA BOA
then attenuate light depending on their optical characteristics as given in Fig. 3(a). This attenuation can be considered as though a fraction of the cross sectional area of a particular cell is completely opaque and the rest of the cross sectional area of that cell completely transparent as indicated in Fig. 2(b). Then the total attenuation across the disc of area A and thickness dx due to a particular type of cell say tissue, will be o.rr7,ArrfN_7A
dx , where N is the number of tissue cells per unit volume
(cells/in ). The equation for attenuation dix across the disc of length dx will be


Here KDX is the sensitivity of the photo detector at a given wavelength, X. A small fraction, say a I0%, of the light also
travels through blood vessels carrying blood that contains 55% plasma, 43% red blood cells (mostly made of Hemoglobin Hb) 1.5% of white blood cells (leukocytes) and 0.5% platelets. Since the amount of white blood ceils and platelets are negligible compared to the red blood cells and the plasma has nearly zero optical attenuation in the wavelength region of interest, we need to consider only the attenuation diw to red blood cells. The red blood cells are made of hemoglobin molecules some carrying oxygen molecules (oxy-hemoglobin) and the rest without oxygen molecules attached (hemoglobin) to them. Let the cross sectional areas of hemoglobin and oxyhemoglobin areAffb and AHho and the optical attenuations a,.
and a x respectively. Then the portion of light that goes
through arteries is attenuated depending on the amount of blood and hence on the quantum of hemoglobin and oxy-hemoglobin contained in the arterial blood at a given time. Then the attenuation due to arterial blood is

Here x(t) is the total equivalent path length of light at a given instant of time t x is the maximum path length and h the

We Claim:
L A method for the non-invasive measurement of oxygen
saturation in arterial blood comprising the steps of exposing a
part of the patient's body to R and IR light; detection of these
lights on exiting the body part by photo diode circuits;
normalizing the R and IR PPGs; removing the noise in the R
and IR PPGs, by filters, to obtain signals vR and vm; obtaining
the natural logarithm of vR and vIR to get ln(yR) and ln(vJR), and
the peak amplitudes of ln(v^ ) and h^v^X that is, VpR and Vp!R>
and obtaining the oxygen saturation Sp02 as

2. A method for the non-invasive measurement of oxygen
saturation in arterial blood substantially as herein described
with reference to the Example.
3. A device for the non-invasive measurement of oxygen
saturation in arterial blood, comprising two light emitting
sources for exposing a part of the patient's body to red light (R)
in the visible spectrum and infra red light (IR); a photo diode
circuit for detection of these lights on exiting the body part;
means for the normalization of the exiting red (R) and infrared
(IR) PPGs (PPG-a or PPGju); filters for removing the noise in
the R and IR PPGs to obtain signals vR and VJR; means for
calculating the natural logarithm of vR and V/R to get ln(va) and
ln(v/jeX for calculating the peak to peak amplitudes o£ln(vR )
and in(vjft), that is, VpR and ¥pIR> and calculating the oxygen


4. A device for the non-invasive measurement of oxygen saturation in arterial blood, substantially as herein described with reference to the Example.