Oxygen saturation determination device, oxygen saturation determination method, and oxygen saturation determination program

Abstract

An oxygen saturation determination device disclosed herein comprises a processor that: calculates the intensity and the waveform amplitude of a second pulse wave signal; calculates a first threshold value for the waveform amplitude of a first pulse wave signal on the basis of the waveform amplitude of the second pulse wave signal and a preset reference value of oxygen saturation; determines that the oxygen saturation in an artery is equal to or greater than the reference value when the amplitude of the acquired first pulse wave signal is equal to or less than the calculated first threshold value; and determines that the oxygen saturation in the artery is less than the reference value when the amplitude of the acquired first pulse wave signal is greater than the calculated first threshold value.

Description

Oxygen saturation determination device, oxygen saturation determination method, and oxygen saturation determination program

[0001] The present disclosure relates to an oxygen saturation level determination device, an oxygen saturation level determination method, and an oxygen saturation level determination program.

[0002] Conventionally, oxygen saturation in arterial blood (SpO ２ A technique such as that disclosed in Japanese Patent No. 3238813 is known as an example of a method for measuring the blood pressure (PPP) of a subject. Specifically, red light and infrared light having different wavelengths are projected onto arterial blood, and transmitted light or reflected light corresponding to each light is received by a light-receiving element. For ease of explanation, the light received by the light-receiving element, such as transmitted light and reflected light, will hereinafter be collectively referred to as "received light."

[0003] Japanese Patent No. 3238813 discloses a pulse oximeter that calculates oxygen saturation from pulse wave signals of received red and infrared light and pre-entered information. In this patent, the calculated oxygen saturation is compared with a predetermined number of previous oxygen saturation calculations to select a median. The latest median calculated using the selected median and the average of the predetermined number of previous medians is then displayed as the oxygen saturation measurement value.

[0004] Patent Document 1: Patent No. 3238813

[0005] In recent years, there has been a growing demand for devices that can measure oxygen saturation on a daily basis with medical-grade accuracy while the measuring device is continuously attached to the skin of the wrist, upper arm, etc. However, when measuring on the wrist or upper arm, the amplitude of the pulse wave signal acquired by the device is much lower than when measuring on the fingertip, etc.

[0006] When the oxygen saturation level in arterial blood is relatively high, the absorption of red light by hemoglobin is smaller than that of other light having wavelengths for which the absorption coefficient of oxyhemoglobin is higher than that of red light. This reduces the change in the intensity of the received red light due to arterial blood pulsation, resulting in a smaller amplitude of the waveform of the acquired pulse wave signal. This reduces the accuracy of oxygen saturation measurement, which is problematic. However, since Japanese Patent No. 3238813 does not consider the fact that the absorption coefficient of red light by hemoglobin decreases with oxygen saturation, there is a concern that oxygen saturation measurement may be impossible if the red pulse wave signal cannot be detected.

[0007] Another possible method for improving the accuracy of oxygen saturation measurement is to increase the amount of red light received by increasing the amount of red light projected, i.e., to increase the intensity of the pulse wave signal. Another possible method is to increase the frequency with which the pulse wave signal is acquired, even if the amplitude of the waveform of the red pulse wave signal is reduced. However, simply applying a method for increasing the intensity or frequency of the pulse wave signal to a conventional pulse oximeter such as that disclosed in Japanese Patent No. 3238813 would result in an increase in the power consumption of the device.

[0008] The present disclosure has been made in light of the above, and provides an oxygen saturation level determination device, an oxygen saturation level determination method, and an oxygen saturation level determination program that can reduce power consumption.

[0009] An oxygen saturation determination device according to a first aspect of the present disclosure includes a first light-emitting element that emits red light onto an artery, a second light-emitting element that emits reference light having a wavelength with a higher absorption coefficient of oxyhemoglobin than the red light onto the artery, a light-receiving element that receives transmitted light or reflected light corresponding to the emitted red light and the reference light, respectively, as received light, and outputs a first pulse wave signal corresponding to the intensity of the received red light and a second pulse wave signal corresponding to the intensity of the received reference light, and a light-receiving element electrically connected to the light-receiving element and configured to output the first pulse wave signal and the second pulse wave signal. a processor that acquires a pulse wave signal, calculates the intensity and waveform amplitude of the second pulse wave signal, calculates a first threshold value for the waveform amplitude of the first pulse wave signal based on the waveform amplitude of the second pulse wave signal and a preset reference value for oxygen saturation, and determines that the arterial oxygen saturation is equal to or greater than the reference value if the acquired amplitude of the first pulse wave signal is equal to or less than the calculated first threshold, and determines that the arterial oxygen saturation is less than the reference value if the acquired amplitude of the first pulse wave signal is greater than the calculated first threshold.

[0010] According to the above configuration, when determining the oxygen saturation state, the reference light, which makes it easier to obtain the waveform amplitude, is used instead of simply increasing the intensity of the red first pulse wave signal, which becomes difficult to obtain as the absorption coefficient of hemoglobin for red light decreases as the oxygen saturation state increases. Therefore, when determining the oxygen saturation state, the power consumption of the device can be reduced overall compared to methods that simply increase the intensity or frequency of the pulse wave signal.

[0011] In a second aspect, the processor of the first aspect is configured, in the process of acquiring the amplitude of the first pulse wave signal, to estimate the amplitude of the first pulse wave signal based on the intensity of the first pulse wave signal at peak timings of the waveform of the second pulse wave signal and the intensity of the first pulse wave signal at valley timings of the waveform of the second pulse wave signal, and to acquire the estimated amplitude as the amplitude of the first pulse wave signal.

[0012] With this configuration, even for a red first pulse wave signal in which peak and valley timings of the waveform are difficult to distinguish, it is easy to extract the peak and valley values ​​of the intensity, thereby enabling the amplitude of the first pulse wave signal to be obtained with increased accuracy.

[0013] In a third aspect, in the first or second aspect, the reference light is infrared light.

[0014] According to the above configuration, the second pulse wave signal can be generated using infrared light as the reference light.

[0015] In a fourth aspect, in any one of the first to third aspects, the processor is configured to compare the amplitude of the second pulse wave signal of the reference light with a preset second threshold value before the process of calculating the first threshold value, and to calculate the first threshold value when the amplitude of the second pulse wave signal of the reference light is equal to or greater than the second threshold value.

[0016] According to the above configuration, the first threshold value of the first pulse wave signal is calculated when the amplitude of the second pulse wave signal is equal to or greater than the second threshold value, so that the accuracy of calculating the first threshold value is high.

[0017] In a fifth aspect, in the fourth aspect, the processor is configured to increase the amount of light emitted by the second light-emitting element when the amplitude of the second pulse wave signal is less than the second threshold value.

[0018] According to the above configuration, even if the amplitude of the second pulse wave signal is less than the second threshold value, the amount of light emitted by the second light-emitting element is increased, so that the amplitude of the second pulse wave signal can be obtained with high accuracy, and as a result, the accuracy of calculating the first threshold value can be improved.

[0019] In a sixth aspect, in any one of the first to fifth aspects, the processor is configured to perform a filtering process on at least one of the first pulse wave signal and the second pulse wave signal before the process of calculating the first threshold value.

[0020] According to the above configuration, filtering is performed on at least one of the first pulse wave signal and the second pulse wave signal, thereby improving the accuracy of calculating the first threshold value.

[0021] In a seventh aspect, in any of the first to sixth aspects, when the amplitude of the acquired first pulse wave signal is equal to or less than the first threshold, the processor determines that the arterial oxygen saturation is equal to or greater than the reference value and calculates that the oxygen saturation is normal; and when the amplitude of the acquired first pulse wave signal is greater than the first threshold, the processor determines that the oxygen saturation is less than the reference value and calculates that the oxygen saturation is abnormal.

[0022] According to the above configuration, a variety of oxygen saturation determination results can be realized.

[0023] An oxygen saturation determination method according to an eighth aspect projects red light and reference light having a wavelength at which the absorption coefficient of oxyhemoglobin is higher than that of the red light onto an artery, receives transmitted light or reflected light corresponding to the projected red light and the reference light, respectively, and obtains a first pulse wave signal corresponding to the intensity of the received red light and a second pulse wave signal corresponding to the intensity of the received reference light. The method calculates the intensity and waveform amplitude of the second pulse wave signal, and calculates a first threshold value for the waveform amplitude of the first pulse wave signal based on the waveform amplitude of the second pulse wave signal and a preset reference value for oxygen saturation. If the amplitude of the acquired first pulse wave signal is equal to or less than the calculated first threshold, the oxygen saturation of the artery is determined to be equal to or greater than the reference value. If the amplitude of the acquired first pulse wave signal is greater than the calculated first threshold, the method determines that the oxygen saturation of the artery is less than the reference value.

[0024] According to the eighth aspect, similarly to the first aspect, when determining the oxygen saturation state, the power consumption of the device can be reduced overall.

[0025] an oxygen saturation determination program for determining an oxygen saturation level on a processor; projecting red light and reference light having a wavelength at which the absorption coefficient of oxyhemoglobin is higher than that of red light onto an artery; receiving transmitted light or reflected light corresponding to the projected red light and the reference light, respectively, as received light; acquiring a first pulse wave signal corresponding to the intensity of the received red light and a second pulse wave signal corresponding to the intensity of the received reference light; calculating the intensity and amplitude of the second pulse wave signal; and calculating a first threshold value for the amplitude of the waveform of the first pulse wave signal based on the amplitude of the waveform of the second pulse wave signal and a preset reference value for oxygen saturation; and determining that the oxygen saturation level of the artery is equal to or greater than the reference value if the amplitude of the acquired first pulse wave signal is equal to or less than the calculated first threshold, and determining that the oxygen saturation level of the artery is less than the reference value if the amplitude of the acquired first pulse wave signal is greater than the calculated first threshold.

[0026] According to the ninth aspect, similarly to the first aspect, when determining the state of oxygen saturation, it is possible to reduce the overall power consumption of the device.

[0027] According to the oxygen saturation level determination device, oxygen saturation level determination method, and oxygen saturation level determination program of the present disclosure, power consumption can be reduced.

[0028] FIG. 1 is a perspective view illustrating an oxygen saturation determination device according to an embodiment of the present disclosure. FIG. 2 is a block diagram comprehensively illustrating the hardware configuration of a processor of the oxygen saturation determination device according to the embodiment. FIG. 3 is a block diagram illustrating the hardware configuration of the processor of the oxygen saturation determination device according to the embodiment based on functions. FIG. 4 is a flowchart illustrating an oxygen saturation determination method using the oxygen saturation determination device according to the embodiment. FIG. 5 is a diagram illustrating a method for calculating a first threshold value and an estimated amplitude of the waveform of a first pulse wave signal using peak timings and valley timings of the waveform of a second pulse wave signal. FIG. 6 is a flowchart illustrating an oxygen saturation determination method according to a modified example.

[0029] Embodiments of the present disclosure are described below. However, the present disclosure is not limited to the following embodiments. When embodiments are described with reference to drawings in the present disclosure, the configuration of the embodiment is not limited to the configuration shown in the drawings. Furthermore, the sizes of components in each drawing are conceptual, and the relative size relationships between components are not limited to these. In the following description of the drawings, identical and similar parts are denoted by the same or similar symbols. However, the drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratios of each device and each component, etc., differ from the actual ones. Therefore, specific thicknesses and planar dimensions should be determined with reference to the following explanation. Furthermore, parts with different dimensional relationships and ratios are included between the drawings. Furthermore, unless otherwise specified in the specification, the number of each component element of the present disclosure is not limited to one and may be present in multiple numbers. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges and do not limit the present disclosure.

[0030] <Oxygen Saturation Measuring Device> The structure of an oxygen saturation measuring device 10 according to this embodiment will be described with reference to Figures 1 to 3. As shown in Figures 1 and 2, the oxygen saturation measuring device 10 according to this embodiment is a portable wearable device that can be worn by a person being measured, and includes a band 12, a housing 14, a PPG sensor unit 16, a display unit 18, and an arithmetic and control unit 20.

[0031] The oxygen saturation measuring device 10 according to this embodiment is also provided with a power supply unit 11. The power supply unit 11 may be a primary battery or a secondary battery. In the present disclosure, the shape of the oxygen saturation measuring device is not limited to a wearable device that can be worn by the subject. The oxygen saturation measuring device of the present disclosure may be a portable device that can be worn or not, such as a portable device that can be placed near the wrist, or a stationary device, or may be configured in any manner.

[0032] In this specification, the "direction E in which the forearm extends" overlaps with the direction in which the radius, ulna, and artery of the subject extend. Furthermore, strictly speaking, the "direction E in which the forearm extends" differs for each subject. In other words, the "direction E in which the forearm extends" is not uniquely determined by coordinates in three-dimensional space, but is individually determined based on the direction in which the radius, ulna, and artery of each subject extend.

[0033] 1, the band 12 is wrapped around the subject's wrist in the circumferential direction C of the wrist. The band 12 may be made of any material, such as resin, fabric, or metal. The band 12 is also provided with a clasp to adjust the length when wrapped around the wrist and to secure the band in place.

[0034] (Housing) The housing 14 is attached to the band 12 and comes into contact with the surface of the back of the subject's wrist (i.e., the dorsum of the hand). The housing 14 may be made of any material, such as resin or metal. The housing 14 has a constant width along the direction E in which the forearm extends. The housing 14 is provided with a PPG sensor unit 16, a display unit 18, and an arithmetic and control unit 20.

[0035] (Display Unit) As shown in Fig. 1, the display unit 18 is disposed on the surface of the housing 14 opposite the wrist. The display unit 18 is an image display device formed of, for example, a liquid crystal display. The display unit 18 displays the results of calculations performed by the calculation control unit 20 externally so that the result can be visually confirmed by the subject. The display unit 18 may be provided with a storage device for temporarily saving the results of calculations.

[0036] (PPG Sensor Unit) The PPG sensor unit 16 acquires pulse wave signals from an artery in the wrist. The measurement region of the PPG sensor unit 16 is located on the side that comes into contact with the skin. Hereinafter, in the present specification and drawings, the pulse wave signal will also be referred to as a "PPG signal." The PPG sensor unit 16 of this embodiment faces, for example, an artery included in the dorsal carpal artery network on the dorsal side of the wrist of the person being measured. The dorsal carpal artery network includes branches from the radial artery and branches from the ulnar artery. Note that, in the present disclosure, the arteries to be measured are not limited to arteries in the wrist.

[0037] As shown in Fig. 3, the light-emitting unit 16A has a first light-emitting element LED1 and a second light-emitting element LED2. The light-receiving unit 16B has a light-receiving element PD. A light-shielding unit 16C is provided between the light-emitting unit 16A and the light-receiving unit 16B. The light-receiving unit 16B and the light-shielding unit 16C form a sensor unit. The light-shielding unit 16C prevents the light-receiving element from directly receiving light from the light-emitting element.

[0038] In this embodiment, one "PPG sensor unit" is configured by two light-emitting elements, a first light-emitting element LED1 and a second light-emitting element LED2, and one light-receiving element PD corresponding to the two light-emitting elements. Note that in the present disclosure, the number of light-emitting elements and the number of light-receiving elements included in one "PPG sensor unit" can both be set arbitrarily. Furthermore, multiple PPG sensor units may be provided.

[0039] An opening (not shown) may be formed in the housing 14 between the light-projecting unit 16A and the outside. An optical device (not shown) that is translucent to the light projected from the light-projecting element is disposed in the opening. The optical device that is translucent to the projected light is, for example, a diffusion lens that can expand the irradiation area. Although not shown, a diffusion agent may be disposed above the light-projecting element together with or instead of the diffusion lens. Note that, in the present disclosure, an optical device that is translucent to the projected light is not essential.

[0040] An opening (not shown) may be formed in the housing 14 between the light receiving unit 16B and the outside. An optical device (not shown) that is translucent to light reflected from the artery in the wrist is disposed in the opening. The optical device is, for example, a focusing lens that can collect the reflected light. Note that, in the present disclosure, an optical device that is translucent to reflected light is not essential.

[0041] (Light-emitting element) The first light-emitting element LED1 and the second light-emitting element LED2 are both electronic components such as light-emitting diodes (LEDs). Each light-emitting element irradiates light onto the artery in the wrist. The first light-emitting element LED1 and the second light-emitting element LED2 are arranged apart from each other. In the present disclosure, the number of each of the first light-emitting element and the second light-emitting element is one or more and is arbitrary. The wavelengths of the light-emitting elements are not limited to two, red light and infrared light, and one or more other wavelengths, such as a third or fourth wavelength, may be used.

[0042] (First Light-Projecting Element and Second Light-Projecting Element) The first light-projecting element LED1 projects red light onto the artery. The second light-projecting element LED2 projects infrared light onto the artery. From the viewpoint of improving measurement accuracy, it is preferable that the first light-projecting element LED1 and the second light-projecting element LED2 are arranged so that the optical paths of the light projected by each of them are as similar as possible. In this embodiment, the infrared light corresponds to the reference light of the present disclosure.

[0043] (Red Light and Infrared Light) The oxygen saturation measurement process requires the use of a combination of two wavelength bands with different absorption coefficients for oxyhemoglobin and deoxyhemoglobin. A preferred combination of wavelength bands is the red region, which is approximately 590 nm or more and 770 nm or less, and the infrared region, which is approximately 770 nm or more and 1000 nm or less.

[0044] In particular, to measure oxygen saturation with high accuracy, it is desirable to have a large difference between the absorption coefficients of oxyhemoglobin and deoxyhemoglobin. Therefore, it is preferable that the peak wavelength of the red light is in the range of 640 nm to 660 nm, and the peak wavelength of the infrared light is approximately 940 nm. In this embodiment, the red light and the infrared light are projected intermittently, for example, 10 times during one arterial pulsation.

[0045] (Reference Light) The reference light has a wavelength at which the absorption coefficient of oxygenated hemoglobin is higher than that of red light. In this embodiment, the signal-to-noise ratio of the pulse wave signal of the received light corresponding to the reference light obtained by photoplethysmography (PPG) is higher than that of red light.

[0046] The reference light in this embodiment is infrared light. In the present disclosure, the reference light is not limited to infrared light. Light other than infrared light, such as green light, can be used as the reference light. When the reference light is light other than infrared light, a light-projecting element having a wavelength of the reference light is added to the oxygen saturation assessment device, or the light-projecting element having a wavelength of the reference light is replaced with a light-projecting element for infrared light. In the case of replacement, a conversion coefficient for calculating oxygen saturation in a combination of a light-projecting element having a wavelength of reference light other than infrared light and a light-projecting element for red light is set in advance, as in the case of infrared light.

[0047] As the reference light other than infrared light, for example, light in the wavelength range of 430 nm to 590 nm, which has a relatively large absorption coefficient in arterial blood, is preferred. Specifically, the blue region, which is the wavelength range of 430 nm to 490 nm, and the green region, which is the wavelength range of 490 nm to 550 nm, are preferred.

[0048] In this embodiment, green light with a peak wavelength of 530 nm or more and 540 nm or less is particularly preferred because it is easily absorbed by hemoglobin in the blood and easily captures changes in the volume of arterial blood vessels, thereby obtaining a pulse wave signal with a relatively high S / N ratio. Furthermore, LED light sources in the green light wavelength band are readily available on the market and relatively inexpensive, making them advantageous as the second light-emitting element that irradiates the reference light. Note that, in the present disclosure, light sources in the violet and ultraviolet wavelength bands of 430 nm or less are not excluded as light sources of the reference light.

[0049] In the present disclosure, the reference light is not limited to green light. In the present disclosure, light in any band that can be taken from the hemoglobin absorption spectrum can be used as the reference light as long as the absorption coefficient of oxyhemoglobin is higher than that of red light.

[0050] (Light Receiving Element) The light receiving element PD is an electronic component such as a photodiode (PD). The light receiving element PD is disposed at a predetermined position relative to the light emitting element. In the present embodiment, the number of light receiving elements PD is one, but in the present disclosure, the number of light receiving elements may be multiple.

[0051] The light-receiving element PD receives reflected light corresponding to red light and outputs a first pulse wave signal corresponding to the intensity of the received reflected light. The light-receiving element PD also receives reflected light corresponding to infrared light and outputs a second pulse wave signal corresponding to the intensity of the received reflected light. The light-receiving element PD also receives reflected light corresponding to the reference light and outputs a third pulse wave signal corresponding to the intensity of the received reflected light. Specifically, two types of reflected light, red light and infrared light, received by the light-receiving element are used to measure oxygen saturation.

[0052] In the present disclosure, the light-receiving elements are not limited to receiving the reflected light of each of the red light, infrared light, and reference light, but may also receive transmitted light corresponding to each of the lights and output pulse wave signals corresponding to the intensity of the received transmitted light. That is, although the oxygen saturation measuring device 10 according to the present embodiment is a reflective type in which the intensity of the pulse wave signal is measured by reflected light, the present disclosure is not limited to a reflective type, and a transmissive oxygen saturation measuring device in which the intensity of the pulse wave signal is measured by transmitted light may also be configured.

[0053] (Principle of Oxygen Saturation Measurement) Here, the principle of oxygen saturation measurement when the absorbance ratio is calculated by absorption spectroscopy using only two types of light, red light and infrared light, will be described. In absorption spectroscopy, multiple amplitudes of the pulse wave signal for each of the red light and the infrared light are calculated from the maximum and minimum values ​​obtained from the pulse wave signal of the received light. From the calculated multiple amplitudes, a fluctuating component (AC) and a fixed component (DC) of the pulse wave signal are calculated, and perfusion indices (PI values) for each of the red light and the infrared light are calculated using the calculated fluctuating and fixed components.

[0054] The oxygen saturation level can be calculated as a measured value by incorporating the ratio of the PI value of red light to the PI value of infrared light (absorbance ratio) into a preset formula for calculating oxygen saturation. That is, the absorbance ratio is calculated as the amplitude ratio of the pulse wave signal of received infrared light to the pulse wave signal of received red light.

[0055] Specifically, the changes in the intensities of the first red pulse wave signal and the second infrared pulse wave signal are monitored over time. Then, the data of the first pulse wave signal and the data of the second pulse wave signal corresponding to each other over time are plotted on an XY coordinate system to generate a scatter diagram of the data. For example, the data values ​​of the second pulse wave signal can be plotted on the X axis as an explanatory variable, and the data values ​​of the first pulse wave signal can be plotted on the Y axis as a response variable.

[0056] Then, a regression line is obtained by performing a regression process on the data contained in the scatter plot. The slope of the obtained regression line corresponds to the ratio of the absorbance of red light (AC / DC) to the absorbance of infrared light (AC / DC), i.e., the absorbance ratio. Hereinafter, the absorbance ratio is also referred to as "Ratio." The absorbance of each light (AC / DC) is the ratio of the variable component AC to the fixed component DC of the pulse wave signal. The calculated absorbance ratio (Ratio) is then used in the following equation (1) to calculate oxygen saturation (SpO ２ ) can be calculated. ２ [%]=a×Ratio+b (1) The conversion coefficients a and b in the formula (1) can be determined by experiment.

[0057] (Calculation control unit) In this embodiment, the calculation control unit 20 is provided in the housing 14. The processor is electrically connected to the first light-emitting element LED1, the second light-emitting element LED2, and the light-receiving element PD. Pulse wave signal data of reflected light corresponding to the light emitted from each light-emitting element is input to the calculation control unit 20 over time from the light-receiving element PD. Based on the pulse wave signal obtained from the reflected light, the calculation control unit 20 measures the oxygen saturation level of the artery irradiated with light by a method using the ratio of PI values, for example.

[0058] The processor calculates the strength and waveform amplitude of the second pulse wave signal. The processor calculates a first threshold value for the waveform amplitude of the first pulse wave signal based on the waveform amplitude of the second pulse wave signal and a preset reference value for oxygen saturation. If the amplitude of the acquired first pulse wave signal is equal to or less than the calculated first threshold value, the processor determines that the arterial oxygen saturation is equal to or greater than the reference value. If the amplitude of the acquired first pulse wave signal is greater than the calculated first threshold value, the processor determines that the arterial oxygen saturation is less than the reference value. The first threshold value and the acquisition of the amplitude of the first pulse wave signal will be described again later in the section <Oxygen Saturation Determination Method>.

[0059] 2, the arithmetic and control unit 20 includes a CPU (Central Processing Unit: processor) 21, a ROM (Read Only Memory) 22, a RAM (Random Access Memory) 23, a storage 24, a user interface 25, and a communication interface 26. Each component is connected to each other via a bus 27 so as to be able to communicate with each other.

[0060] The CPU 21 is a central processing unit that executes various programs and controls each part. That is, the CPU 21 reads programs from the ROM 22 or the storage 24 and executes the programs using the RAM 23 as a work area. The CPU 21 controls the above components and performs various arithmetic processing in accordance with the programs recorded in the ROM 22 or the storage 24. The CPU 21 is a processor of the present disclosure.

[0061] In this embodiment, an oxygen saturation level determination program is stored in the ROM 22 or the storage 24. The oxygen saturation level determination program is a calculation program for measuring oxygen saturation levels.

[0062] The ROM 22 stores various programs and various data. The RAM 23 temporarily stores programs or data as a working area. The storage 24 is configured by an HDD (Hard Disk Drive) or an SSD (Solid State Drive) and stores various programs including the operating system and various data.

[0063] The user interface 25 is an interface through which a subject wearing the oxygen saturation measuring device 10, which is a wearable device, uses the arithmetic and control unit 20. The user interface 25 may include, for example, at least one of a liquid crystal display equipped with a touch panel that allows the subject to perform touch operations, a voice input receiving unit that receives voice input from the subject, and a button that the subject can press. The display unit of this embodiment is an example of the user interface 25.

[0064] The communication interface 26 is an interface for the arithmetic and control unit 20 to communicate with other devices, and uses standards such as Ethernet (registered trademark), FDDI, and Wi-Fi (registered trademark).

[0065] When executing the oxygen saturation determination program, the oxygen saturation measuring device 10 uses the above hardware resources to realize various functions. The functional components realized by the oxygen saturation measuring device 10 include a pulse wave signal acquiring unit, an intensity and amplitude calculating unit, a first threshold calculating unit, a first amplitude acquiring unit, a first comparing unit, a second comparing unit, a current comparing unit, a current increasing unit, and a filtering unit. Each functional component is realized by the CPU 21 reading and executing the oxygen saturation determination program stored in the ROM 22 or the storage 24.

[0066] 3, the calculation and control unit 20 specifically includes a light-emitting and receiving control unit, a control unit, and a storage unit. The light-emitting and receiving control unit is electrically connected to the light-emitting unit 16A and the light-receiving unit 16B. The light-emitting and receiving control unit calculates the signal strength and amplitude of each of the first and second pulse wave signals input over time from the light-receiving unit 16B, and outputs the calculated signal strength and amplitude as waveforms over time.

[0067] The light emission and reception control unit can be configured using an integrated analog front end, which is an analog circuit. The control unit is connected to the light emission and reception control unit. The control unit can be configured using a microprocessor (Micro Processing Unit, MPU), etc. The memory unit is connected to the control unit. The memory unit can be configured using RAM, ROM, etc.

[0068] <Oxygen Saturation Determination Method> Next, an example of an oxygen saturation determination method and an oxygen saturation measurement method using the oxygen saturation measurement device 10 according to this embodiment will be described with reference to FIGS. 4 and 5. FIG.

[0069] (Red Light and Infrared Light Projection Process and Pulse Wave Signal Acquisition Process) First, in step S10 in FIG. 4 , processor 21 projects red light and infrared light onto an artery using light projector 16A. Next, in step S20, processor 21 receives, as received light, reflected light corresponding to the projected red light and infrared light, respectively, using light receiver 16B. Next, in step S30, the pulse wave signal acquisition unit of processor 21 acquires a first pulse wave signal corresponding to the intensity of the received red light and a second pulse wave signal corresponding to the intensity of the received reference light.

[0070] (Filtering Process) Next, in this embodiment, the filtering unit of the processor 21 performs filtering on both the first pulse wave signal and the second pulse wave signal in step S40 before calculating the first threshold value. The filtering process involves signal preprocessing, such as a band pass filter (BPF) or moving averaging. In the present disclosure, it is sufficient that the filtering process is performed on at least one of the first pulse wave signal and the second pulse wave signal. In the present disclosure, filtering is not essential.

[0071] Next, in step S50, the intensity and amplitude calculation unit of processor 21 calculates the intensity and waveform amplitude of each of the first pulse wave signal and the second pulse wave signal.

[0072] (Processing for Comparing Amplitude of Second Pulse Wave Signal with Second Threshold Value) Next, prior to the processing for calculating the first threshold value, the second comparing unit of processor 21 compares the amplitude of the second pulse wave signal of the reference light with a preset second threshold value in step S60. If the result of the comparison in step S60 indicates that the amplitude of the second pulse wave signal of the reference light is equal to or greater than the second threshold value, the processing proceeds to step S70, and the first threshold value calculating unit of processor 21 calculates the first threshold value.

[0073] In the present disclosure, it is not essential to compare the amplitude of the second pulse wave signal of the infrared light as the reference light with the second threshold value in step S60. If the comparison in step S60 shows that the amplitude of the second pulse wave signal of the reference light is less than the second threshold value, the process proceeds to step S160. The process from step S160 onward will be described later.

[0074] (Processing for Calculating First Threshold Value for First Pulse Wave Signal) Next, in step S70, the first threshold value calculation unit of processor 21 calculates a first threshold value for the amplitude of the waveform of the first pulse wave signal based on the amplitude of the waveform of the second pulse wave signal and a preset reference value for oxygen saturation. The method for calculating the first threshold value will be described together with the processing for obtaining the amplitude of the first pulse wave signal in step S80.

[0075] (Processing for Obtaining Amplitude of First Pulse Wave Signal) Next, in step S80, the first amplitude obtainment unit of processor 21 estimates the amplitude of the first pulse wave signal and obtains the estimated amplitude as the amplitude of the first pulse wave signal. In this specification, the estimated amplitude is referred to as the “estimated amplitude.”

[0076] (Method of Calculating First Threshold and Estimated Amplitude) Here, a method of calculating the first threshold and estimated amplitude of the first pulse wave signal will be described. ２ The relationship between the SpO 2 and the absorbance ratio (Ratio) is expressed by the following formula (1), as described above. ２ [%] = a × Ratio + b (1) where: SpO ２ : Oxygen saturation a: SpO ２ Conversion coefficient slope b: SpO ２ Conversion factor intercept Ratio: Absorbance ratio

[0077] Next, solving equation (1) for Ratio, we obtain the following equation (2): Ratio = (SpO ２ / a) - b (2) Ratio is defined as in the following equation (3): Ratio = PI_red / PI_ir (3) where, PI_red: Perfusion index of red light PI_ir: Perfusion index of infrared light

[0078] Therefore, the following equation (4) can be derived from equations (2) and (3): PI_red = {(SpO ２ / a) - b}× PI_ir ... Formula (4)

[0079] PI_red is defined as in the following equation (5): PI_red = AC_red / DC_red (5) where AC_red: estimated amplitude of the red pulse wave signal DC_red: received light intensity of the red light

[0080] Therefore, by substituting equation (5) into equation (4) and solving AC_red, the following equation (6) is obtained: AC_red = {(SpO ２ / a) - b} × PI_ir × DC_red ... (6) where, PI_ir: Perfusion index of infrared light (= AC_ir / DC_ir) AC_ir: Amplitude of infrared pulse wave signal DC_ir: Received light intensity of infrared light

[0081] In equation (6), the preset SpO ２ By substituting the reference value of the value, "conversion coefficients a and b," the perfusion index of infrared light "PI_ir," and the received light intensity of red light "DC_red," the SpO ２ The first threshold value "AC_red_th" of the amplitude of the red pulse wave signal corresponding to the reference value of the value can be calculated. In addition, by substituting the preset "conversion coefficients a, b," the infrared perfusion index "PI_ir" obtained by measuring the pulse wave signal, and the red received light intensity "DC_red" into equation (6), the estimated amplitude "AC_red" of the first pulse wave signal obtained when the current oxygen saturation level is obtained can be calculated.

[0082] It is known that the conversion coefficient a is always negative. Therefore, from equation (6), SpO ２The direction of change of the first threshold "AC_red_th" relative to the direction of change of the value is negative. Therefore, if the estimated amplitude of the first red pulse wave signal is equal to or less than the first threshold "AC_red_th," it can be determined that the current oxygen saturation level is equal to or greater than the reference value. On the other hand, if the estimated amplitude of the first red pulse wave signal is greater than the first threshold "AC_red_th," it can be determined that the current oxygen saturation level is less than the reference value.

[0083] In the present embodiment, the calculation method of the first threshold and the estimated amplitude has been described above by way of example in which infrared light is used as the reference light. In the present disclosure, when light other than infrared light is used as the reference light, the calculation may be performed in the same manner as in the case of infrared light, with the reference light substituted for the position of infrared light in the description of the calculation method of the first threshold and the estimated amplitude.

[0084] In this embodiment, the estimated amplitude of the first pulse wave signal PS1 is obtained based on the peak and valley timings of the waveform of the second pulse wave signal PS2, as shown in Figure 5. The peak timing is measured during a period in which the signal strength changes over time and is the timing at which the rate of change in the signal strength changes from positive to negative. The valley timing is measured during a period in which the signal strength changes over time and is the timing at which the rate of change in the signal strength changes from negative to positive.

[0085] 5, the signal strength at the peak timing of the waveform of first pulse wave signal PS1 is the signal strength at the same peak timing as the peak timing of the waveform of second pulse wave signal PS2. Also, the signal strength at the valley timing of the waveform of first pulse wave signal PS1 is the signal strength at the same valley timing as the valley timing of the waveform of second pulse wave signal PS2.

[0086] The estimated amplitude of the waveform of first pulse wave signal PS1 is obtained as the difference between the intensity of first pulse wave signal PS1 at the same peak timing as the peak timing of the waveform of second pulse wave signal PS2 and the intensity of first pulse wave signal PS1 at the same valley timing as the peak timing of the waveform of second pulse wave signal PS2. In practice, the average or median of multiple amplitudes obtained within a predetermined period can be used as the estimated amplitude of the waveform of first pulse wave signal PS1.

[0087] In the present disclosure, the process of obtaining the estimated amplitude of the first pulse wave signal PS1 is not limited to a method using the peak and valley timings of the waveform of the second pulse wave signal PS2. The amplitude may also be obtained based on the waveform of the obtained first pulse wave signal PS1 itself.

[0088] (Processing for Comparing the Amplitude of the First Pulse Wave Signal with the First Threshold Value) Next, the first comparator of processor 21 compares whether the amplitude of the acquired first pulse wave signal PS1 is equal to or less than the calculated first threshold value. If the comparison in step S90 shows that the amplitude of the acquired first pulse wave signal PS1 is equal to or less than the calculated first threshold value, processor 21 determines that the arterial oxygen saturation is equal to or greater than the reference value.

[0089] In the present disclosure, when the amplitude of the acquired first pulse wave signal PS1 is equal to or less than the first threshold, the processor 21 may determine that the oxygen saturation is equal to or greater than the reference value and calculate that the oxygen saturation is normal. In other words, it is not necessary to output the determination result that the oxygen saturation is equal to or greater than the reference value to the outside. The determination result that the oxygen saturation is normal may also be output to the outside. In the present disclosure, various oxygen saturation determination results can be realized.

[0090] Then, in step S100, the processor 21 notifies the subject that the arterial oxygen saturation is equal to or higher than the reference value by, for example, displaying on the display unit 18. Numerical values, messages, icons, etc. may be used as information to be displayed on the screen of the display unit 18. Furthermore, the information may be displayed in combination with other means such as blinking of the display information or a change in the background color of the screen. Notification methods other than the display screen include, for example, sound, vibration, and transmitting information to another device using a communication function.

[0091] (Oxygen Saturation Measurement Process) Next, in step S110, the processor 21 calculates the arterial oxygen saturation based on the calculated absorbance ratio. The calculated oxygen saturation is displayed as a measurement value on the display unit 18. If the oxygen saturation is equal to or greater than the reference value, it is determined that there is no need to improve the measurement accuracy. Therefore, an operating mode is maintained in which power consumption is reduced and repeated determinations of whether the oxygen saturation is equal to or greater than the reference value are made. Specifically, for example, processes such as reducing the frequency of light projection and reception or reducing the light projection power may be performed. In the present disclosure, the oxygen saturation calculation process in step S110 is not essential.

[0092] On the other hand, if the comparison result in step S90 indicates that the amplitude of the acquired first pulse wave signal PS1 exceeds the calculated first threshold, the processor 21 determines that the arterial oxygen saturation is below the reference value. The process proceeds to step S120.

[0093] In the present disclosure, when the amplitude of the acquired first pulse wave signal PS1 exceeds the first threshold, the processor 21 may determine that the oxygen saturation is below the reference value and calculate that the oxygen saturation is abnormal. In other words, it is not necessary to output the determination result that the oxygen saturation is below the reference value to the outside. The determination result that the oxygen saturation is abnormal may also be output to the outside. In the present disclosure, a variety of oxygen saturation determination results can be realized.

[0094] Then, in step S120, the processor 21 notifies the subject that the arterial oxygen saturation is below the reference value, for example, by displaying on the display unit 18. The information displayed on the screen of the display unit 18 may include the same information as in step S100, as well as a notification encouraging the subject to take action to improve their health. Specific examples of such action include oxygen inhalation and visiting a medical institution. The notification encouraging the subject to take action helps the subject to improve their health.

[0095] If the oxygen saturation level is below the reference value, the measurement accuracy is increased. Therefore, in step S130, the device switches to a high-precision operating mode, which prioritizes output of more accurate measurement values. Specifically, for example, the device may increase the frequency of light projection and reception, increase the light projection power, or perform other operations.

[0096] Then, in step S140, similar to step S30, the pulse wave signal acquirer of processor 21 acquires a first pulse wave signal corresponding to the intensity of the received red light and a second pulse wave signal corresponding to the intensity of the received infrared light used as the reference light. Then, in step S150, similar to step S50, the intensity and amplitude calculator of processor 21 calculates the intensity and waveform amplitude of the second pulse wave signal. If the amplitude of the second pulse wave signal is too small to read, i.e., if it is difficult to calculate the amplitude, the amplitude may be considered to be zero.

[0097] Next, the processing from step S160 onward will be described for the case where the comparison result in step S60 indicates that the amplitude of the second pulse wave signal PS2 is less than the second threshold value. If the amplitude of the second pulse wave signal PS2 is less than the second threshold value in steps S160 and S170, processor 21 increases the amount of infrared light projected as the reference light.

[0098] (Driving current comparison process) Specifically, first, in step S160, the current comparison unit of the processor 21 compares the current driving current driving the second infrared light-emitting element LED2 with the upper limit value of the driving current of the second light-emitting element LED2.

[0099] (Light projection amount increase process) If the comparison result in step S160 shows that the current drive current is less than the upper limit value, in step S170, the current increase unit of the processor 21 increases the drive current of the second light-projecting element LED2, thereby increasing the amount of infrared light projected as reference light.

[0100] On the other hand, if the comparison in step S160 indicates that the current drive current is equal to the upper limit, the processor 21 notifies the user in step S180 that the determination is impossible. The notification is displayed on the display unit 18, for example, and the determination process ends.

[0101] The above series of processes constitutes the oxygen saturation determination method according to this embodiment. In the present disclosure, an oxygen saturation measurement method may be configured that includes the oxygen saturation calculation process in step S110.

[0102] (Effects) In this embodiment, the second pulse wave signal PS2 is output according to the light intensity of the received infrared light used as the reference light. A first threshold value for the amplitude of the waveform of the first pulse wave signal PS1 is calculated based on the amplitude of the waveform of the second pulse wave signal PS2 and a preset reference value for oxygen saturation. If the amplitude of the first pulse wave signal PS1 is equal to or less than the first threshold value, the oxygen saturation is determined to be equal to or greater than the reference value. If the amplitude of the first pulse wave signal PS1 exceeds the first threshold value, the oxygen saturation is determined to be less than the reference value.

[0103] That is, when determining the oxygen saturation state, infrared light, which makes it easier to obtain the waveform amplitude, is used instead of simply increasing the intensity of the red first pulse wave signal PS1, which becomes difficult to obtain as the absorption coefficient of hemoglobin for red light decreases as the oxygen saturation state increases. Therefore, compared to methods that simply increase the intensity or frequency of the pulse wave signal, the overall power consumption of the device can be reduced.

[0104] Furthermore, the lower the subject's arterial blood perfusion state, the smaller the waveform amplitude of the red pulse wave signal. In this embodiment, oxygen saturation is determined using the second infrared pulse wave signal PS2, whose waveform amplitude is easy to obtain, which is advantageous in that measurement accuracy can be improved even in subjects with low arterial blood perfusion states.

[0105] In recent years, there has been an increasing demand for wearable devices that can measure oxygen saturation on a daily basis with medical-grade accuracy while the measuring device is continuously attached to the skin of the wrist, upper arm, etc. However, when measuring on the wrist or upper arm, the amplitude of the pulse wave signal acquired by the device is much lower than that of a fingertip, etc., and as a result, the accuracy of the oxygen saturation measurement is likely to decrease.

[0106] In order to improve the accuracy of oxygen saturation measurement, it is possible to increase the accuracy of pulse wave detection from PPG signals by increasing the amount of light or the frequency of measurement, but these methods have the problem of increasing power consumption. In this embodiment, power consumption can be reduced in the oxygen saturation measurement device, which is a wearable device, and the power saved by the reduction can be used to improve measurement accuracy.

[0107] In this embodiment, the process of acquiring the amplitude of the first pulse wave signal estimates the amplitude of the first pulse wave signal based on the intensity of the first pulse wave signal at the peak timing of the waveform of the second pulse wave signal and the intensity of the first pulse wave signal at the valley timing of the waveform of the second pulse wave signal. The estimated amplitude is acquired as the amplitude of the first pulse wave signal. As a result, even for a red first pulse wave signal in which the peak timing and valley timing of the waveform are difficult to distinguish, it is easy to extract the peak and valley values ​​of the signal intensity. This allows the amplitude of the first pulse wave signal to be acquired with high accuracy.

[0108] In this embodiment, the second pulse wave signal can be generated using infrared light as the reference light. Furthermore, since the second pulse wave signal can be generated using infrared light, which is commonly used in combination with red light in oxygen saturation measurement, there is no need to provide a separate light source other than red and infrared light as the reference light. This allows for a compact device configuration.

[0109] In this embodiment, before the process of calculating the first threshold value, the amplitude of the second pulse wave signal of the infrared light used as the reference light is compared with a preset second threshold value. Then, the first threshold value is calculated if the amplitude of the second pulse wave signal is equal to or greater than the second threshold value. Therefore, the calculation accuracy of the first threshold value is high.

[0110] Furthermore, in this embodiment, if the amplitude of the second pulse wave signal of infrared light used as reference light is less than the second threshold, the amount of light emitted by the second light-emitting element LED2 is increased. Therefore, even if the amplitude of the second pulse wave signal is less than the second threshold, the amplitude of the second pulse wave signal PS2 can be obtained with high accuracy, thereby improving the accuracy of calculating the first threshold.

[0111] Furthermore, in this embodiment, filtering is performed on both the first and second pulse wave signals before the first threshold value is calculated, thereby improving the accuracy of the calculation of the first threshold value.

[0112] 6, in a modified oxygen saturation determination method, after steps S10 to S30, the intensity and amplitude calculation unit of processor 21 calculates the intensity and waveform amplitude of the first pulse wave signal in step S43. Next, in step S50, the intensity and amplitude calculation unit of processor 21 calculates the intensity and waveform amplitude of the second pulse wave signal.

[0113] Next, step S55 is executed to determine whether or not a first pulse wave signal of red light is present. In step S55, processor 21 determines whether or not the first pulse wave signal is present, whether the amplitude of the first pulse wave signal is equal to or greater than a preset value. An example of an amplitude calculation method is a peak-valley method, in which the amplitude is calculated from the intensity at the peak and valley times of the waveform of the first pulse wave signal. The preset value may be a value calculated from noise in the device design and the signal-to-noise ratio (SNR) required for measuring oxygen saturation.

[0114] That is, in step S55 of the modified example, while measuring oxygen saturation with increased accuracy, it is confirmed that the difficulty in acquiring the red light first pulse wave signal due to a change in the subject's oxygen saturation state or a weakening of the arterial pulsation is caused by determining whether the amplitude of the first pulse wave signal is equal to or greater than a set value. For example, when the subject is placed in a cold environment, the blood vessels in the skin at the measurement site may contract, reducing the difference between the amount of blood inflow and outflow due to the heartbeat. As a result, the arterial pulsation may weaken.

[0115] If it is determined in step S55 that the amplitude of the first pulse wave signal is equal to or greater than the preset value, the process proceeds to step S110. Then, in step S110, the oxygen saturation level is calculated using the intensity and waveform amplitude of the first pulse wave signal calculated in step S43 and the intensity and waveform amplitude of the second pulse wave signal calculated in step S50. On the other hand, if it is determined in step S55 that the amplitude of the first pulse wave signal is less than the preset value, the process proceeds to steps S70, S90, S100, and S120, where the oxygen saturation level is determined. That is, it is determined that it is difficult to calculate the oxygen saturation level using the intensity and waveform amplitude data of the first pulse wave signal calculated in step S43 and the intensity and waveform amplitude data of the second pulse wave signal calculated in step S50 as they are.

[0116] The processes of steps S10 to S30, S50, S70, and S90 to S120 in FIG. 6 are the same as the processes of the corresponding steps with the same names in FIG. 4 , and therefore will not be described again. In the modified example, the filtering process of step S40 in FIG. 4 may be performed, for example, between steps S30 and S43, as in the present embodiment. Also in the modified example, the comparison process of the amplitude of the second pulse wave signal with a second threshold value of step S60 in FIG. 4 may be performed between steps S55 and S70, as in the present embodiment. Furthermore, depending on the comparison result of step S60, the drive current comparison process of step S160, the light projection amount increase process of step S170, and other processes may be performed.

[0117] (Effects of the Modification) As with the present embodiment, the modification also reduces power consumption. In particular, in the oxygen saturation determination method according to the modification, as shown in step S110, the process basically proceeds with the objective of calculating the oxygen saturation. However, as explained in the process flow from step S55 to step S70, even if the first pulse wave signal PS1 is too weak to calculate the oxygen saturation, it is determined whether the oxygen saturation is equal to or greater than the reference value. The determination result may indicate whether the oxygen saturation state is normal or abnormal.

[0118] The result of the determination is then notified to the user, i.e., the person being measured. Therefore, even if the first pulse wave signal is too weak to immediately calculate the oxygen saturation value, the person being measured can grasp their own oxygen saturation state based on the determination result. In other words, compared to when the person being measured simply waits without understanding their oxygen saturation state until the oxygen saturation value is calculated, the modified example provides a longer period of time for the person being measured to grasp their oxygen saturation state. Other advantages of the modified example are similar to those of the present embodiment, and therefore will not be described again.

[0119] <Other Embodiments> The present disclosure has been described with reference to the above disclosed embodiments, but the descriptions and drawings forming a part of this disclosure should not be understood as limiting the present disclosure.

[0120] For example, in the present disclosure, the oxygen saturation level determination process executed by the CPU 21 after reading the software (program) in the above embodiment may be executed by various processors other than the CPU. Examples of such processors include a programmable logic device (PLD) (such as a field-programmable gate array (FPGA)) whose circuit configuration can be changed after manufacture, and a dedicated electrical circuit such as an application-specific integrated circuit (ASIC) that is a processor having a circuit configuration designed specifically for executing a specific process.

[0121] The oxygen saturation determination process may be performed by one of these various processors, or by a combination of two or more processors of the same or different types (e.g., multiple FPGAs, a combination of a CPU and an FPGA, etc.).More specifically, the hardware structure of these various processors is an electric circuit that combines circuit elements such as semiconductor elements.

[0122] In addition, in each of the above embodiments, the oxygen saturation assessment program is described as being pre-stored (installed) in the ROM 22 or the storage 24, but this is not limiting. The program may be provided in a form recorded or stored on a recording medium such as a CD-ROM (Compact Disk Read Only Memory), a DVD-ROM (Digital Versatile Disk Read Only Memory), or a USB (Universal Serial Bus) memory. The program may also be downloaded from an external device via a network.

[0123] The present disclosure includes various embodiments not described above, and the technical scope of the present disclosure is defined only by the invention-specific matters of the claims that are appropriate from the above explanation.

[0124] The disclosure of Japanese Patent Application No. 2024-041699, filed on March 15, 2024, is incorporated herein by reference in its entirety.

[0125] Furthermore, all publications, patent applications, and technical standards mentioned in this specification are incorporated by reference herein to the same extent as if each individual publication, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A first light-emitting element that emits red light onto an artery, a second light-emitting element that emits reference light having a wavelength at which the absorption coefficient of oxygenated hemoglobin is higher than that of the red light onto the artery, and a light-receiving element that receives transmitted light or reflected light corresponding to the emitted red light and reference light, respectively, as received light, and outputs a first pulse wave signal corresponding to the intensity of the received red light and a second pulse wave signal corresponding to the intensity of the received reference light. and a processor electrically connected to the light-receiving element, which acquires the first pulse wave signal and the second pulse wave signal, calculates the intensity and waveform amplitude of the second pulse wave signal, calculates a first threshold value for the waveform amplitude of the first pulse wave signal based on the waveform amplitude of the second pulse wave signal and a preset reference value for oxygen saturation, and determines that the arterial oxygen saturation is equal to or greater than the reference value if the acquired amplitude of the first pulse wave signal is equal to or less than the calculated first threshold, and determines that the arterial oxygen saturation is less than the reference value if the acquired amplitude of the first pulse wave signal is greater than the calculated first threshold.

2. The oxygen saturation determination device according to claim 1, wherein, in the process of acquiring the amplitude of the first pulse wave signal, the processor estimates the amplitude of the first pulse wave signal based on the intensity of the first pulse wave signal at peak times of the waveform of the second pulse wave signal and the intensity of the first pulse wave signal at valley times of the waveform of the second pulse wave signal, and acquires the estimated amplitude as the amplitude of the first pulse wave signal.

3. The oxygen saturation determination device according to claim 1 or 2, wherein the reference light is infrared light.

4. The oxygen saturation determination device according to any one of claims 1 to 3, wherein the processor compares the amplitude of the second pulse wave signal of the reference light with a preset second threshold before calculating the first threshold, and calculates the first threshold if the amplitude of the second pulse wave signal of the reference light is equal to or greater than the second threshold.

5. The oxygen saturation determination device according to claim 4, wherein the processor increases the amount of light emitted by the second light-emitting element when the amplitude of the second pulse wave signal is less than the second threshold value.

6. The oxygen saturation determination device according to any one of claims 1 to 5, wherein the processor performs filtering on at least one of the first pulse wave signal and the second pulse wave signal before calculating the first threshold value.

7. The oxygen saturation determination device according to any one of claims 1 to 6, wherein the processor determines that the oxygen saturation is equal to or greater than the reference value when the amplitude of the acquired first pulse wave signal is equal to or less than the first threshold value, and calculates that the oxygen saturation is normal; and determines that the oxygen saturation is less than the reference value when the amplitude of the acquired first pulse wave signal is greater than the first threshold value, and calculates that the oxygen saturation is abnormal.

8. A method for determining oxygen saturation, comprising: projecting red light and reference light having a wavelength at which the absorption coefficient of oxygenated hemoglobin is higher than that of the red light onto an artery; receiving transmitted light or reflected light corresponding to the projected red light and the reference light, respectively; obtaining a first pulse wave signal corresponding to the intensity of the received red light and a second pulse wave signal corresponding to the intensity of the received reference light; calculating the intensity and amplitude of the second pulse wave signal; calculating a first threshold for the amplitude of the waveform of the first pulse wave signal based on the amplitude of the waveform of the second pulse wave signal and a predetermined reference value for oxygen saturation; determining that the oxygen saturation of the artery is equal to or greater than the reference value if the amplitude of the acquired first pulse wave signal is equal to or less than the calculated first threshold; and determining that the oxygen saturation of the artery is less than the reference value if the amplitude of the acquired first pulse wave signal is greater than the calculated first threshold.

9. An oxygen saturation determination program that causes a processor to execute the following steps: projecting red light and reference light having a wavelength at which the absorption coefficient of oxygenated hemoglobin is higher than that of red light onto an artery; receiving transmitted light or reflected light corresponding to the projected red light and the reference light, respectively, as received light; acquiring a first pulse wave signal corresponding to the intensity of the received red light and a second pulse wave signal corresponding to the intensity of the received reference light; calculating the intensity and amplitude of the second pulse wave signal; calculating a first threshold for the amplitude of the waveform of the first pulse wave signal based on the amplitude of the waveform of the second pulse wave signal and a preset reference value for oxygen saturation; and determining that the oxygen saturation of the artery is equal to or greater than the reference value if the amplitude of the acquired first pulse wave signal is equal to or less than the calculated first threshold, and determining that the oxygen saturation of the artery is less than the reference value if the amplitude of the acquired first pulse wave signal is greater than the calculated first threshold.

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