WO2016178986A1

WO2016178986A1 - System and method for spo2 determination using reflective ppg

Info

Publication number: WO2016178986A1
Application number: PCT/US2016/030088
Authority: WO
Inventors: Laurence Richard OLIVIER; Franco Bauer Du Preez; Tiaan Andre VAN DER MERWE
Original assignee: Lifeq Global Limited
Priority date: 2015-05-01
Filing date: 2016-04-29
Publication date: 2016-11-10

Abstract

A system and method for performing SpO₂measurements using reflective PPG technology. The method of the invention is to be applied to physiological signal analysis. The system and method of the invention allows accurate measurement of SpO₂ by wearable devices.

Description

SYSTEM AND METHOD FOR SPO₂DETERMINATION USING REFLECTIVE PPG

FIELD OF INVENTION

[0001] The present invention relates to the field of non‐invasive digital health monitoring and signal processing. In particular, a system and method for determining oxygen saturation (SpO₂) non‐invasively is presented. In an embodiment, the invention comprises a wearable device to be placed in contact with the user’s skin.

BACKGROUND OF INVENTION

[0002] Traditionally, determining the degree of saturation of the oxygen‐carrying molecules found in red blood cells (SpO₂) was only applicable in a fixed medical/hospital setting. However, there is an increasing demand for wearable devices which can provide monitoring in almost any environment. This includes, but is not limited to, sports, quantified‐self, and medical applications. This data may also be useful in a scientific and clinical research setting.

[0003] While finger and ear clip based pulse oximetry monitors have been proven to provide an accurate measure of SpO₂, they are limited by the location from which the SpO₂measurement is taken. These pulse oximeters interfere with everyday activities, such as typing, and wearing such devices for extended periods of time may cause discomfort. Some monitors make use of a photoplethysmography (PPG) light source and sensor, which utilizes an algorithm to determine the user’s degree of oxygen saturation (SpO₂) from the measured PPG signal. In such instances, the sensor and light source are both in contact with the same area of skin. This method is particularly useful under free‐ living conditions where a subject may be unable to wear a finger or ear clip. A wristwatch form factor is generally a more favorable option. However, the wristwatch form factor creates the challenge of measuring SpO₂under conditions which may greatly reduce the quality and consistency of the PPG signal used for calculating the SpO₂value. In addition, being able to perform SpO₂measurements on a finger that is temporarily placed in contact with a flat sensor in reflectance mode as opposed to using an additional cumbersome finger clip that supports transmission mode, will make it possible to embed the present technology to measure SpO₂ on mobile devices connected to the cloud.

[0004] PPG measurements can be performed in two modes, namely transmission or reflection. In transmission mode PPG, the light emitting diodes (LEDs) and photodetector (PD) within the sensor are placed on opposite sides of the pulsating vascular bed. For example, the sensor can be attached across a fingertip or earlobe allowing the light to be transmitted from the LED through the flesh and detected on the opposite side by the PD. On the other hand, the configuration of reflection mode differs in that the LEDs and PD are situated adjacent to each other, on the same planar surface, which allows the PD to detect the reflected light from tissue, bone and/or blood vessels. The reflection mode arrangement enables readings from various body locations, such as the head and wrist.

[0005] Pulse oximetry is largely based on the principle of the Beer‐Lambert Law which uses variables such as absorbance, intensity of incident and transmitted light, extinction coefficients and path length (distance light is transmitted). In applying the Beer‐Lambert Law, exact knowledge of the path length is required. However, the path length changes due to perturbations in the tissue. There are publications that have innovations based on inferring the optical path length (US 20130331710), have corrected for differential scattering based on path lengths estimated from signal levels (US 20100076319) and inventions that control the optical path length mechanically (US 8060171). While these publications, and others (US 6987994, US 5259381), describe techniques conventional to most reflective PPG devices, they do not describe a calibration method where the ratio of optical path lengths for visible spectrum light, such as red light, and infrared light illumination is the parameter being fitted as in the present invention. Others have assumed that the change in path length, due to factors such as the wavelength shift, is negligible on the accuracy of the SpO₂prediction (WO 2013077808). US Patent Publication No. 20140200423 discloses a wearable, reflective PPG device whereby the optical path length was increased. While the said method improves accuracy of the signal, due to increased absorbance, it does not describe a method by which path length is necessarily appropriately calibrated in the prediction of SpO₂.

[0006] PPG waveforms consist of direct current (DC) and alternating current (AC) components. The DC component of the PPG waveform represents the detected transmitted or reflected optical signal from the tissue and remains relatively constant with regard to respiration. The AC component corresponds to changes in the blood volume, namely between the systolic and diastolic phases of the cardiac cycle, and is dependent on the heart rate signal, which is superimposed onto the DC component. Additionally, modulation amplitude or level is indicative of the effect of the status of the tissue, by the change in blood flow within the artery, and is calculated as the AC/DC component ratio (R). Reflective PPG often results in a reduced AC component and an increased DC component, since the light tends to pass through a more superficial part of the skin and has a reduced interaction with pulsating blood. In an attempt to address this issue, International Patent Publication No. WO 2009064979 described a method whereby the DC component of the analog was filtered out in order to extract an AC component, which could then be further processed in order to identify the rising portion of the AC component. Others have documented that the addition of a third near‐IR wavelength enables the calculation of an additional ratio formed by the combination of the two IR wavelengths (US Patent Publication No. 20020042558). In addition, the US Patent No. 6839580 made use of dynamic device calibration using a database of SpO₂ calibration curves to calibrate the device on a per individual basis. European Patent No. EP 0957747 used a novel sensor design that has a ring‐like geometry and two point‐like light sources to mechanically mitigate variations in the path lengths for red and infrared light sources.

[0007] However, the proposed systems and methods demonstrated in related art remain lacking in the aspects of continuity in measurement, sufficient calibration as well as optimal signal extraction. Therefore, we propose a system and method which addresses these shortcomings and results in an accurate SpO₂ prediction.

SUMMARY OF INVENTION

[0008] The present invention relates to a novel system and method for continuous SpO₂ measurement using reflective PPG. In an aspect, the system facilitates SpO₂ prediction by basing a calibration step on the ratio of red and infrared path lengths. This facilitates SpO₂prediction, on any user for a given optical configuration, by ensuring that path length is accordingly incorporated into the prediction equation. In addition, use of an automatic gain control (AGC) implemented on the device enables optimal signal extraction for the determination of SpO₂. The signal extraction ensures the maximal amplification of the AC component, which is usually a limitation in reflective PPG. In another aspect, the current invention provides a novel system and method that removes discontinuities from the red and infrared signals in a way that preserves the Φ value (equation 20, discussed below), ensuring a more accurate SpO₂prediction.

[0009] In an aspect, the system and method perform SpO₂measurements using PPG technology in reflective mode. The unique aspects pertain to selecting a specific factor for calibrating the PPG device in a manner that is specific to the layout of the optical configuration. This method adds further accuracy and robustness to the SpO₂prediction. Finally, a filter technique is presented that removes discontinuities from the signal while preserving the information content relating to SpO₂calculations, further increasing the robustness of the output.

[00010] Determining SpO₂ using optical PPG technology in reflective mode poses additional challenges as opposed to transmission modes. One challenge is differences in the scattering coefficient of the illumination sources. For example, a source in the red portion of the visible spectrum (e.g., around 660 nm) and a source in the infrared spectrum (e.g., around 950 nm) results in different degrees of light penetration, potentially illuminating tissue areas with different concentrations of blood vessels, as infrared light typically penetrates deeper layers.

[00011] The present invention relates to using the path length ratio (c) (see equation 3 below), which changes depending on the distance between the detector and light source, to calibrate a specific optical layout consisting of photodiode(s) and LEDs for measuring a range of oxygen saturation values (SpO₂) using reflective mode PPG data.

[00012] The present invention further relates to the use of a specific adaptable amplification system to generate the signals used for calculating the SpO₂value.

[00013] In an embodiment, the DC component of the received signal can be filtered out. This results in a larger AC component, therefore producing a better signal‐to‐noise ratio (SNR) due to the increased power in the signal.

[00014] The present invention also relates to post processing of the signal obtained from said amplification system. In order to calculate SpO₂(or S as used in the formulas below; equation 29) the value of Φ is required, which relies on a reading proportional to the light intensity detected by the PPG sensor. In the case where the photodiode is used as the light sensor, the photodiode current is generally proportional to the amount of monochromatic light that the photodiode receives and can be calculated based on an understanding of the signal amplification and quantification (analog‐to‐digital converter or ADC) systems. Imperfections in these calculations, variation in the specification of the components involved and changes in the intensity of the illumination source all affect the calculated PD current and changes in settings typically cause discontinuities that reduce the quality of SpO₂predictions. Looking at the definition of Φ and assuming that PD current is proportional to the light intensity received at the photodiode, it becomes evident that any linear factor that equally affects the AC and DC components of one wavelength, cancels out and does not affect the value of Φ. Hence, it is possible to remove discontinuities from the PD current signal by multiplying the signal with the necessary factor to retain continuity. The improvement in signal continuity subsequently aids the process of separating AC and DC components for the current in the 660 nm and 950 nm channels and thereby calculating a more robust Φ, and therefore SpO₂ value.

[00015] In an aspect, this technology may be deployed in a range of mobile devices, such as smart‐watches, or incorporated in the exterior of mobile phones to enable the calculation of SpO₂values with minimal invasiveness. Such devices can perform further processing to reveal additional physiological information based on the SpO₂value and other information, or this could be sent to a mobile phone for such operations, or could similarly be sent to a cloud computing platform for such operations. The information produced as a result of these processing steps could then be relayed back to a user via a range of terminals including, but not limited to, a wearable display, a smartwatch display or other computing devices connected to the internet.

BRIEF DESCRIPTION OF DRAWINGS

[00016] The preferred embodiments of the invention will be described by way of example only, with reference to the accompanying drawings:

[00017] FIG. 1 is a schematic diagram of reflective and transmission mode PPG.

[00018] FIG. 2 shows the differences in the trajectory for light of different wavelengths, exemplified with a red (660 nm) and near infrared (950 nm) light source.

[00019] FIG. 3 shows signal adjustment using scaling continuity filter, according to an embodiment of the present invention.

[00020] FIG. 4 shows data flow, information flow, and connectivity of the sensor to mobile computing platforms and the internet, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF INVENTION AND DRAWINGS

[00021] The following detailed description and drawings describe different aspects of the current invention. The description and drawings serve to enable one skilled in the art to fully understand the current invention and are not intended to limit the scope of the invention in any manner. Before the present methods and systems are disclosed and described, it is to be understood that the methods and systems are not limited to special methods, special components, or to particular implementations. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. As used in the specification and the appended claims, the word “comprise” and variations of the word, such as “comprising” and “comprises,” means “including but not limited to, ”and is not intended to exclude, for example, other components or steps. “Exemplary” means “an example of” and is not intended to convey an indication of a preferred or ideal embodiment. “Such as” is not used in a restrictive sense, but for explanatory purposes. The singular forms “a,” “an,” and “the” also include plural elements unless the context clearly dictates otherwise. “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur and that the description includes instances where said event or circumstance occurs and instances where it does not. The system of the current invention typically comprises electrodes which are placed in a specific configuration on an area of the body which includes, but is not limited to, areas such as the upper arm, forearm and wrist.

[00022] FIG. 1 depicts the difference between transmission and reflective mode PPG. In transmission mode PPG, a light source 160 illuminates tissue 170 and a sensor or detector 150 is situated opposite the light source 160 across the tissue 170. The light path 180 through the tissue 170 is direct. In reflective mode PPG, a light source 100 illuminates tissue 120 and a sensor or detector 110 is situated on the same side of the tissue 120 as the light source 100. The light that is detected at the sensor 110 is the light that follows a reflected light path 130. Although the same tissue can be illuminated in transmission mode as in reflective mode PPG, differences are pronounced in reflective mode. Furthermore, the reflective mode receives more light that is less modulated by interaction with the pulsating tissue, thereby causing a smaller modulation (AC/DC ratio). Lastly, in reflective mode, motion artifacts are pronounced due to the specular nature of the light reflecting off the skin surface.

[00023] FIG. 2 shows an example of how light of different wavelengths can illuminate different sections of tissue due to the preferential scattering of lower wavelengths, exemplified by light from a light source 210, such as an LED, with a 660 nm wavelength compared to light from a light source 210, such as an LED, with a 950 nm wavelength. Note that light source 210 of FIG. 2 generates light of both wavelengths. This difference forms the basis for considering different path lengths, such as a longer path length 230 and a shorter path length 240, for the different wavelengths, even though the light source 210 to sensor 220 distance could be identical in both cases.

[00024] Although 660 nm and 950 nm are used as examples throughout, any of a number of combinations of wavelengths could be used. As examples, ultraviolet and infrared, visible and ultraviolet, visible and terahertz, or infrared and mm‐wave light could be used. Furthermore, two distinct wavelengths within the same portion of the spectrum can be used, such as two separate wavelengths of infrared light. The combination of red light and infrared light is used in the preferred embodiment due to the availability of these light sources in convenient components for integrated devices and the ability of these wavelengths to penetrate into the body. These two wavelengths of light can be generated by one or two or more light sources 210, including, but not limited to LEDs, diode lasers, or incandescent lamps. The light source 210 may be tunable in wavelength. Likewise, the light may be sensed by one or two or more light sensors 220. The light sensors 220, in an embodiment, may be, among others, photodiodes, photodetectors, photoconductors, photomultiplier tubes, or solar cells. A single photodiode may receive the signal at two wavelengths simultaneously or in a time modulated manner to act as sensors for more than one wavelength.

[00025] The following section shows the derivation of oxygen saturation using the principles of the Beer‐Lambert Law describing the absorption of light in various media. The calculations are carried out on the device via a processor. This processor, in an embodiment, may be embedded in the wrist band of the device in the wristwatch form factor.

[00026] Light transmission (T) is defined as the amount of light that leaves a medium that is illuminated (I) relative to the amount of light used in the illumination process (I₀):

Absorbance (A) is then defined as the negative log of transmission:

We also know according to the Beer‐Lambert Law that

where A is absorbance, ∈ is the molar extinction coefficient, c is concentration and l is the path length.

[00027] If we assume that light travels through layers of different compounds (such as bone, skin pigment, arterial and venous blood vessels, etc.), then we can think of the resulting transmittance as the product of the transmittance components associated with the different layers.

The different layers can be seen as filters with different characteristics. In an aspect, we view the transmitted light as the light traveling between the light source 210, such as an emitters LED, and the sensor 220, such as a photodiode or other type of photodetector.

[00028] We can add two multiplicative factors to equation 4,

Where η_LED−Tissue is the coupling efficiency of emitted light and η_Tissue−PD is the efficiency of the photo‐diode to collect light.

[00029] Now using the Beer‐Lambert Law, equation 3

This gives the total absorbance (A_Total) and can be equivalently expressed as sum of the individual absorbances,

[00030] We assume that all other layers/filters remain fairly constant during a relatively short time period, except for the arterial absorption layer, for which its changes in concentration are induced by a beating heart. Since pulse oximetry focuses on the pulsating arterial blood, the derivative of equation 8 with respect to time is calculated. Most terms will cancel out because they are constant with respect to time:

[00031] An additional assumption is made that there are only two absorbers in the arterial blood: oxyhemoglobin (HbO₂) and deoxyhemoglobin (Hb). This implies that

If we assume the incoming light intensity is constant (I_E), then the only changing intensity is that which is measured at the end of the path length, at the photodiode (due to pulsating blood), and therefore equation 13 becomes (using the chain rule of differentiation):

[00032] If we take the ratio of delta absorbances for the two different wavelengths 660 nm (red visible light) and 950 nm (infrared),

Note that this is approximately equal to:

with the denominator I_E associated with the DC component and the numerator derivative term dI_E/dtbeing associated with the AC component. Note that if we take the gradient calculated from the trough to the peak of the PPG waveform (AC), then we essentially calculate the average gradient over that period. Similarly, the DC component is the average intensity of transmitted light. Note that while the calculations are performed here using 660 nm and 950 nm light, any two wavelengths can be substituted into the equations.

[00033] We define oxygen saturation, S, as the percentage of total concentration (given previous assumption this consists of only hemoglobin and oxyhemoglobin):

This implies that:

We do this simply to get the equation in terms of one variable. Substituting this definition in equation 15 we get

[00034] Using equation 19, we have

Note that in biological tissue, l is the effective mean path length to account for the effects of scattering:

[00035] Note if we assume that the path lengths for hemoglobin and oxyhemoglobin, given the same wave length, are the same (i.e. l_Hb,950= l_HbO2,950and l_Hb,660= l_HbO2,660) and that the ratio of path lengths for visible spectrum light, such as red light, and infrared light are constant

then equation 29 reduces to:

[00036] Note that the dc_Hb/dt cancels in the ratio, so this calculation is essentially independent of the instantaneous change in hemoglobin concentration (due the way we defined it in equation 22). By calibrating one value for the path‐length ratio (c) in equation 29, for a specific reflectance PPG sensor layout, we can obtain a theoretical SpO₂ prediction. The parameter Φ would then be calculated from the photodiode current, after applying the additional filter described below.

[00037] FIG. 3 depicts the basic operation of the proportional discontinuity filter, which uses a multiplicative factor to rebase the signal after adjustments, such as LED intensity increases or changes in the settings of the amplification system have been applied. The key is that the signal is multiplied from the discontinuity onwards with a constant value that brings about continuity of the signal for improved amplitude (AC) and baseline (DC) calculations, while maintaining the AC/DC ratio for any channel that it is applied to. As additional discontinuities are encountered, the constant multiplier is adjusted similarly to retain continuity (i.e. k₁*k₂l_PD in FIG. 3).

[00038] FIG. 4 demonstrates a basic embodiment of the inventions described above concerning SpO2 prediction whereby the sensor is embodied together with a micro controller or other type of processor into a wearable device 1 containing the necessary sensor means to measure SpO2, including light sources, such as LEDs, laser diodes, and the like, and sensors, such as photodiodes and/or photodetectors. The processor is operatively connected to the sensors to receive the measurements of the light intensity or other received signal appropriate for the sensor type. This may include an analog to digital conversion by the processor or by the sensor. The wearable device may be a wrist mount, such as a wristwatch or other type of wrist strap device. The wearable device optionally contains a display 2 and is capable of encoded transmitting digital encoded or analog modulated data to wireless device over a wireless radio connection, such as Bluetooth or Wi‐Fi. The wireless device may be a mobile device 3 or a personal computer 5 with a wireless connection. The wearable device may directly connect to an internet based platform 4. The data can be stored and further processed on a server / internet based platform 6 for future retrieval and to be viewed on a computing platform exemplified by the personal computer 5, the mobile device 3 and or wearable device 1.

[00039] Having thus described exemplary embodiments of a method to produce metallic composite material, it should be noted by those skilled in the art that the within disclosures are exemplary only and that various other alternatives, adaptations, and modifications may be made within the scope of this disclosure. Accordingly, the invention is not limited to the specific embodiments as illustrated herein, but is only limited by the following claims.

Claims

WHAT IS CLAIMED:

1. An apparatus for measuring the oxygen saturation of blood, the apparatus comprising

a. a first light source operating in the visible spectrum;

b. a first light sensor configured to detect the intensity of received light from the first light source and configured to operate in reflective mode photoplethysmography with the first light source;

c. a second light source operating in the infrared spectrum;

d. a second light sensor configured to detect the intensity of received light from the second light source and configured to operate in reflective mode photoplethysmography with the second light source; e. a processor configured to calculate the oxygen saturation, wherein the calculation uses the ratio of the optical path length of the first light source to the first light sensor versus the optical path length of the second light source to the second light sensor.

2. The apparatus of claim 1, wherein the first light source generates light substantially having a wavelength of 660 nm.

3. The apparatus of claim 1, wherein the first light source comprises at least one light emitting diode.

4. The apparatus of claim 1, wherein the second light source generates light substantially having a wavelength of 950 nm.

5. The apparatus of claim 1, wherein the second light source comprises at least one light emitting diode.

6. The apparatus of claim 1, wherein the first light source is a light emitting diode having a wavelength of 660 nm and the second light source is a light emitting diode having a wavelength of 950 nm.

7. The apparatus of claim 1, wherein the first light source, the second light source, the first light sensor, and the second light sensor are substantially located on the same surface.

8. The apparatus of claim 1, wherein the ratio of the optical path length of the first light source to the first light sensor versus the optical path length of the second light source to the second light sensor is calculated based on the equation:

wherein λ₁ is the wavelength of the first light source, and λ₂is the wavelength of the second light source.

9. The apparatus of claim 9, wherein the parameter Φ is based on the ratio of delta absorbances for the two different wavelengths.

10. The apparatus of claim 10, wherein the parameter Φ is approximated by

11. The apparatus of claim 10, further comprising a scaling discontinuity filter configured to adjust the output of the photodiode current so as to remove the discontinuity caused by a light source intensity change.

12. The apparatus of claim 10, further comprising signal conditioning.

13. The apparatus of claim 12, wherein the signal conditioning comprises an automatic gain control.

14. The apparatus of claim 1, further comprising a wireless radio, wherein the wireless radio is configured to transmit a digital encoding of the calculated oxygen saturation.

15. A method for determining the oxygen saturation of blood, the method comprising:

a. placing a first light emitting diode proximate to skin of a first person, wherein the first light source has a first wavelength;

b. placing a second light emitting diode proximate the skin of the first person, wherein the second light source has a second wavelength; c. placing a first photodetector proximate to the skin of the first person, wherein the first photodetector is configured to operate in reflective photoplethysmography mode and to detect the intensity of the reflected light from the first light source;

d. placing a second photodetector proximate to the skin of the first person, wherein the second photodetector is configured to operate in reflective photoplethysmography mode and to detect the intensity of the reflected light from the second light source; e. determining a first optical path length based on the intensity of the reflected light from the first light source;

f. determining a second optical path length based on the intensity of the reflected light from the second light source; and

g. calculating the oxygen saturation, wherein the calculation uses the ratio of the first optical path length versus the second optical path length.

16. The method of claim 15, wherein the ratio of the first optical path length versus the second optical path length is calculated based on the equation:

17. The method of claim 16, wherein the parameter Φ is based on the ratio of delta absorbances for the two different wavelengths.

18. The method of claim 17, wherein the parameter Φ is approximated by

19. A system for determining the oxygen saturation of blood, the system comprising: a wearable device comprising:

i. a wrist mount,

ii. a first light source,

iii. a first light sensor configured for reflectivity mode operation in conjunction with the first light source and generating a first intensity output,

iv. a second light source,

v. a second light sensor configured for reflectivity mode operation in conjunction with the second light source and generating a second intensity output,

vi. a processor configured to receive the first intensity output and determine a first optical path length, receive the second intensity output and determine a second optical path length, and determine an oxygen saturation based on the ratio of the ratio of the first optical path length and the second optical path length.

20. The system of claim 19, wherein the wearable device further comprises a wireless radio.

21. The system of claim 20, wherein the wireless radio of the wearable device is a Wi‐Fi radio, and the processor is further configured to transmit the oxygen saturation over the Wi‐Fi radio.

22. The system of claim 20, further comprising a wireless device, wherein the wireless device comprises a wireless radio that is operatively connected to the wireless radio of the wearable device, and wherein the mobile device comprises an application for displaying the oxygen saturation.

23. The system of claim 19, wherein the first light source operates in the infrared spectrum, and the second light source operates in the visible spectrum.

24. The apparatus of claim 23, wherein the first light source is a light emitting diode that generates light substantially having a wavelength of 660 nm and the second light source is a light emitting diode that generates light substantially having a wavelength of 950 nm.

25. The system of claim 19, wherein the ratio of the first optical path length versus the second optical path length is calculated based on the equation:

wherein

is the wavelength of the first light source, and λ₂is the wavelength of the second light source.

26. The method of claim 25, wherein the parameter Φ is approximated by

wherein

27. The system of claim 19, wherein the first light source and the second light source are the same.

28. The system of claim 19, wherein the first light sensor and the second light sensor are the same.