JP7776014B2 - Pulse pressure measuring device and pulse pressure measuring method - Google Patents

Description

The present invention relates to a pulse pressure measuring device and a pulse pressure measuring method.

A pulse wave measuring device is known that optically acquires pulse wave information from a body part, calculates pulse rate and pulse wave time information from the acquired pulse wave information, and estimates and outputs blood pressure information based on the pulse rate and pulse wave time information (see Patent Document 1). Examples of estimated blood pressure information include blood pressure, blood pressure status, arteriosclerosis, vascular age, and whether or not the subject is prone to stroke.

WO 205/098977

One indicator of health status is pulse pressure, which is the difference between systolic blood pressure and diastolic blood pressure. Conventional pulse wave measuring devices are unable to measure pulse pressure. The object of the present invention is to provide a pulse pressure measuring device and pulse pressure measuring method that are capable of measuring pulse pressure.

According to one aspect of the present invention,
a peripheral blood pressure index calculation unit that calculates a peripheral blood pressure index related to the steepness of the rising edge of a pulse wave signal measured by a pulse wave sensor worn by a user;
a pulse wave feature amount calculation unit that calculates an ae time index including information about the elapsed time from the peak of the a-wave to the peak of the e-wave of an acceleration pulse wave obtained by second-order differentiation of the waveform of the pulse wave signal;
There is provided a pulse pressure measuring device including a pulse pressure calculation unit that calculates pulse pressure based on the peripheral blood pressure index and the ae time index.

According to another aspect of the present invention,
A pulse wave signal is acquired by a pulse wave sensor attached to the user.
a pulse pressure measuring device calculating a peripheral blood pressure index related to the steepness of the rising edge of the pulse wave signal;
the pulse pressure measuring device calculates an ae time index including information about the elapsed time from the peak of the a-wave to the peak of the e-wave of an accelerated pulse wave obtained by second-order differentiation of the waveform of the pulse wave signal;
There is provided a pulse pressure measuring method in which the pulse pressure measuring device determines the pulse pressure based on the peripheral blood pressure index and the ae time index.

The peripheral blood pressure index, which is correlated with blood pressure, also correlates with pulse pressure. The time elapsed from the peak of the a wave to the peak of the e wave of the accelerated pulse wave also correlates with pulse pressure. By using the peripheral blood pressure index and the ae time index, pulse pressure can be calculated with high accuracy.

FIG. 1 is a block diagram and a schematic diagram of a pulse pressure measuring device according to a first embodiment. FIG. 2 is a block diagram and a schematic diagram of a pulse pressure measuring device according to a modification of the first embodiment. FIG. 3 is a block diagram and a schematic diagram of a pulse pressure measuring device according to another modification of the first embodiment. FIG. 4 is a perspective view and a block diagram of a pulse pressure measuring device according to a modification of the first embodiment shown in FIG. FIG. 5 is a graph showing an example of a pulse wave, a velocity pulse wave, and an acceleration pulse wave. FIG. 6 is a graph showing an example of a pulse wave and an acceleration pulse wave. Figures 7A and 7B are graphs showing the relationship between the peripheral blood pressure index "1/VE0.5" value calculated from the pulse wave measured when the height from the heart to the measurement site (finger) is changed and when the measurement site is adjusted to chest height and the area around the elbow on the side where the measurement site (finger) is located is cooled, and the systolic blood pressure measured at the wrist. Figures 8A and 8B are graphs showing the relationship between the peripheral blood pressure index "a/S" value calculated from the pulse wave measured when the height from the heart to the measurement site (finger) is changed and when the measurement site is adjusted to chest height and the area around the elbow on the side where the measurement site (finger) is located is cooled, and the systolic blood pressure measured at the wrist. Figures 9A and 9B are graphs showing the relationship between the peripheral blood pressure index "(a-b)/(a-d)" calculated from the pulse wave measured when the height from the heart to the measurement site (finger) is changed and when the measurement site is adjusted to chest height and the area around the elbow on the side where the measurement site (finger) is located is cooled, and the systolic blood pressure measured at the wrist. 10A and 10B are scatter plots of multiple subjects, with the horizontal axis representing wrist systolic blood pressure and the vertical axis representing the peripheral blood pressure index "1/VE0.5." 11A and 11B are scatter plots of multiple subjects, with the horizontal axis representing wrist systolic blood pressure and the vertical axis representing the peripheral blood pressure index "a/S." 12A and 12B are scatter plots of multiple subjects, with the horizontal axis representing wrist systolic blood pressure and the vertical axis representing the peripheral blood pressure index "(ab)/(ad)." 13A and 13B are scatter plots of multiple subjects, with the vertical axis representing the time elapsed from the peak of the a-wave to the peak of the b-wave of the accelerated pulse wave and the horizontal axis representing the systolic blood pressure at the wrist. 14A and 14B are scatter plots of multiple subjects, with the vertical axis representing the time elapsed from the peak of the b wave to the peak of the d wave of the accelerated pulse wave and the horizontal axis representing the systolic blood pressure at the wrist. 15A and 15B are scatter plots of multiple subjects, with the vertical axis representing the time elapsed from the peak of the d wave to the peak of the e wave of the accelerated pulse wave and the horizontal axis representing the systolic blood pressure at the wrist. 16A and 16B are scatter plots of multiple subjects, with the vertical axis representing the time elapsed from the peak of the a-wave to the peak of the e-wave of the accelerated pulse wave and the horizontal axis representing the systolic blood pressure at the wrist. FIG. 17 is a scatter plot of multiple subjects, with the pulse interval on the vertical axis and the systolic blood pressure at the wrist on the horizontal axis. 18A and 18B are scatter plots of multiple subjects, with pulse pressure on the horizontal axis and peripheral blood pressure index "1/VE0.5" on the vertical axis. 19A and 19B are scatter plots of multiple subjects, with pulse pressure on the horizontal axis and peripheral blood pressure index "(ab)/(ad)" on the vertical axis. 20A and 20B are scatter plots of multiple subjects, with pulse pressure on the horizontal axis and the feature "de time" on the vertical axis. 21A and 21B are scatter plots of multiple subjects, with pulse pressure on the horizontal axis and the feature "ae time" on the vertical axis. FIG. 22 is a scatter plot of multiple subjects, with pulse pressure on the horizontal axis and pulse interval on the vertical axis. 23A and 23B are scatter plots of multiple subjects, with pulse pressure on the horizontal axis and pulse pressure index value Pa on the vertical axis. 24A and 24B are scatter plots of multiple subjects, with pulse pressure on the horizontal axis and pulse pressure index value Pad on the vertical axis. FIG. 25 is a scatter plot showing multiple measurement results obtained over a long period of time for one subject, with pulse pressure on the horizontal axis and pulse pressure index value Pad on the vertical axis. FIG. 26 is a perspective view and a block diagram of a pulse pressure measuring device according to the third embodiment. FIG. 27 is a schematic diagram for explaining the procedure for measuring the difference in height between the ring device and the heart. FIG. 28 is a flowchart showing the steps of a pulse pressure measuring method executed by the pulse pressure measuring device according to the third embodiment. FIG. 29 is a flowchart showing the steps of a pulse pressure measuring method executed by a pulse pressure measuring device according to a modified example of the third embodiment. FIG. 30 is a perspective view and a block diagram of a pulse pressure measuring device according to the fourth embodiment. FIG. 31 is a flowchart showing the steps of a pulse pressure measuring method executed by the pulse pressure measuring device according to the fourth embodiment. FIG. 32 is a perspective view and a block diagram of a pulse pressure measuring device according to the fifth embodiment. FIG. 33 is a flowchart showing the steps of a pulse pressure measuring method executed by the pulse pressure measuring device according to the fifth embodiment.

[First Example]
A pulse pressure measuring device according to a first embodiment will be described with reference to Figures 1, 2, 3, and 4. The pulse pressure measuring device according to the first embodiment calculates pulse pressure based on the waveform of a pulse wave acquired from a subject.

Pulse waves are used to measure various biological information. For example, they are used to measure pulse rate and oxygen saturation. They are also used to measure autonomic nervous function based on fluctuations in pulse intervals, and to measure respiratory rate based on baseline fluctuations in pulse waves and fluctuations in pulse intervals. Furthermore, technology has been developed to estimate blood pressure from the waveform shape of the pulse wave. Pulse waves are classified into pressure pulse waves (piezoelectric pulse waves), which are measured using piezoelectric sensors, and volume pulse waves (photoelectric pulse waves), which are measured using photoelectric pulse wave sensors.

Pulse pressure is the difference between systolic and diastolic blood pressure, and its normal value is said to be between 40mmHg and 50mmHg. An example of pulse pressure exceeding the normal range is when stroke volume increases. Possible causes of increased stroke volume include exercise, hyperthyroidism, and anemia.

Pulse pressure also exceeds the normal range when the elasticity of large blood vessels decreases. When blood vessels become stiffer, systolic blood pressure rises. When systolic blood pressure rises, a reaction occurs that dilates peripheral blood vessels, and diastolic blood pressure actually falls. As a result, pulse pressure increases. For example, when arteriosclerosis is widespread in the aorta, pulse pressure increases. It has also been reported that pulse pressure of 65 mmHg or higher increases the risk of myocardial infarction and cerebrovascular disease. Measuring pulse pressure can help identify those with these diseases and those at risk.

The pulse pressure measuring device according to the first embodiment can be applied to both piezoelectric and photoplethysmograms. More information can be obtained from photoplethysmograms than from piezoelectric pulse waves. The following explanation will use photoplethysmograms as an example.

Figure 1 is a block diagram and schematic diagram of a pulse pressure measuring device according to the first embodiment. The pulse pressure measuring device according to the first embodiment includes a processing device 30 and a photoplethysmographic sensor 50. The photoplethysmographic sensor 50 includes a light-emitting element 51 and a light-receiving element 53. The processing device 30 includes a light-emitting control unit 31, a pulse wave measuring unit 32, a peripheral blood pressure index calculation unit 33, a pulse wave feature calculation unit 34, a pulse pressure calculation unit 35, a control unit 36, and a display unit 37.

The light-emitting element 51 and the light-receiving element 53 are used by contacting the user's body surface 70. The light-emitting element 51 emits measurement light toward the body surface 70. The emitted light is absorbed, reflected, or scattered (hereinafter sometimes simply referred to as "reflected") by the epidermal region 71, arterioles 72, and capillaries 73 within the body surface 70. A portion of the reflected light is incident on the light-receiving element 53.

Arterioles 72 are small blood vessels, for example, with a diameter of 20 μm or more and 200 μm or less, and exist between arteries and capillaries 73. Multiple capillaries 73 branch off from arterioles 72. Capillaries 73 are small blood vessels, for example, with a diameter of about 10 μm, and connect arteries and veins. Multiple capillaries 73 are distributed in a region shallower than the area where arterioles 72 are distributed.

The light-emitting element 51 outputs measurement light under the control of the processing device 30. A signal indicating the light intensity measured by the light-receiving element 53 is input to the processing device 30. The signal indicating the light intensity detected by the light-receiving element 53 is referred to as the "pulse wave signal." Arterial blood contains hemoglobin, which has the property of absorbing measurement light. Blood flow changes with the beating of the heart, and the amount of light absorbed also changes in accordance with the change in blood flow. Therefore, the intensity of the pulse wave signal changes with the beating of the heart.

The light-emitting element 51 may be, for example, one that emits light in the wavelength range from blue to yellow-green (450 nm to 550 nm), preferably in the wavelength range from 500 nm to 550 nm. The light-emitting element 51 may be, for example, a light-emitting diode (LED) or a vertical-cavity surface-emitting laser (VCSEL). The light-receiving element 53 may be, for example, a photodiode (PD) or a phototransistor.

Light in the blue to yellow-green wavelength range is highly absorbed by biological tissue. Therefore, pulse waves acquired using light in the blue to yellow-green wavelength range reflect information from regions shallower than the skin surface, particularly regions shallower than the arterioles 72 and primarily containing capillaries 73. The arrows in Figure 1 do not indicate the light propagation path, but rather indicate that light output from the light-emitting element 51 passes through the epidermal region 71 and the region primarily containing capillaries 73 and then enters the light-receiving element 53. To ensure that information from regions shallower than the arterioles 72 and primarily containing capillaries 73 is reflected in the acquired pulse waves, it is preferable to shorten the distance L1 between the light-emitting element 51 and the light-receiving element 53. For example, it is preferable to set the distance L1 to between 1 mm and 3 mm.

Light with wavelengths shorter than 450 nm can damage biological tissue. To avoid damaging biological tissue, it is preferable that the wavelength of light used to measure pulse waves be 450 nm or longer.

Figure 2 is a block diagram and schematic diagram of a pulse pressure measuring device according to a modification of the first embodiment. In the modification shown in Figure 2, a light emitting element 52 with a different emission wavelength is used instead of the light emitting element 51 of the pulse pressure measuring device according to the first embodiment (Figure 1). The light emitting element 52 used in the modification shown in Figure 2 outputs light in the wavelength range from red to near-infrared light, for example, a wavelength range of 750 nm to 950 nm. Light in the wavelength range from red to near-infrared light is less absorbed by biological tissue than light in the wavelength range from blue to yellow-green light. Therefore, pulse waves obtained using light in the wavelength range from red to near-infrared light reflect information from areas deeper than the skin surface.

For example, information about the area where capillaries 73 and arterioles 72 are distributed is reflected. The arrows shown in Figure 2 do not indicate the path of light propagation, but rather indicate that the light output from the light-emitting element 52 passes through the area where not only capillaries 73 but also arterioles 72 are distributed and is incident on the light-receiving element 53. In order to ensure that information about the area where arterioles 72 and capillaries 73 are distributed is significantly reflected in the acquired pulse wave, it is preferable to set the distance L2 between the light-emitting element 52 and the light-receiving element 53 to be 5 mm or more and 20 mm or less.

Hemoglobin absorbance decreases in wavelengths longer than 950 nm. Therefore, it is preferable to use light in the wavelength range of 950 nm or less to obtain pulse wave signals.

Figure 3 is a block diagram and schematic diagram of a pulse pressure measuring device according to another modification of the first embodiment. The pulse pressure measuring device according to the second modification includes both a light-emitting element 51 (Figure 1) that outputs light in the wavelength range from blue to yellow-green, and a light-emitting element 52 that outputs light in the wavelength range from red to near-infrared. The light-receiving element 53 detects light in both wavelength ranges, the light output from the light-emitting element 51 and the light output from the light-emitting element 52. The preferred range for the distance L1 between the light-emitting element 51 and the light-receiving element 53 is 1 mm or more and 3 mm or less, and the preferred range for the distance L2 between the light-emitting element 52 and the light-receiving element 53 is 5 mm or more and 20 mm or less.

In the modified example shown in Figure 3, one light receiving element 53 is arranged for two light emitting elements 51 and 52, but it is also possible to arrange one light receiving element for one light emitting element 51 and another light receiving element for the other light emitting element 52.

Next, the function of the processing device 30 (FIGS. 1, 2, and 3) will be described.
The control unit 36 of the processing device 30 controls the start and end of measurement, controls display of the measurement results on the display unit 37, controls storage of the measurement results, etc. The light emission control unit 31 controls pulsed emission of the light emitting element 51 or the light emitting element 52. For example, the light emitting element 51 or the light emitting element 52 emits pulsed light at a predetermined frequency of 100 Hz or more and 1000 Hz or less. In the second modified example shown in Fig. 3, the light emitting element 51 and the light emitting element 52 emit light alternately.

The pulse wave measurement unit 32 generates a pulse wave waveform (hereinafter sometimes simply referred to as a "pulse wave") from the measurement result (pulse wave signal) input from the light-receiving element 53. For example, the pulse wave measurement unit 32 generates a pulse wave by reading the measured light intensity from the light-receiving element 53 at a predetermined sampling rate in synchronization with the pulsed light emitted by the light-emitting element 51 or light-emitting element 52. In the second modified example shown in FIG. 3, the pulse wave measurement unit 32 separately generates a pulse wave based on the light output from the light-emitting element 51 and a pulse wave based on the light output from the light-emitting element 52 by reading the measured light intensity from the light-receiving element 53 at a predetermined sampling rate in synchronization with the pulsed light emitted by the light-emitting element 51 or light-emitting element 52.

The peripheral blood pressure index calculation unit 33 calculates a peripheral blood pressure index related to the steepness of the rising edge of the pulse wave from the pulse wave generated by the pulse wave measurement unit 32. The pulse wave feature calculation unit 34 calculates the ae time index from the pulse wave generated by the pulse wave measurement unit 32. The peripheral blood pressure index and the ae time index will be explained in detail later. The pulse pressure calculation unit 35 calculates the pulse pressure based on the peripheral blood pressure index and the ae time index, thereby obtaining a measured value of the pulse pressure.

Figure 4 is an oblique view and block diagram of a pulse pressure measuring device according to a modified example of the first embodiment shown in Figure 3. The pulse pressure measuring device according to this modified example includes a ring device 61 and a portable mobile terminal 62.

The ring device 61 will now be described.
Two light-emitting elements 51 and 52 and one light-receiving element 53 are attached to the inner surface of an annular attachment member 60. The attachment member 60 is attached to a user's finger when in use. A variety of sizes of attachment members 60 are prepared according to the thickness of the user's finger. When the attachment member 60 is attached to the finger, the light-emitting elements 51 and 52 output light toward the finger. The light-receiving element 53 is attached at a position where light reflected inside the finger is incident.

The attachment member 60 further incorporates a light emission control unit 31, a pulse wave measurement unit 32, and a communication unit 55. The light emission control unit 31, the pulse wave measurement unit 32, and the communication unit 55 may be configured as a single integrated circuit.

The functions of the processing device 30 (Figure 3) are realized by a ring device 61 and a portable mobile terminal 62. Examples of the mobile terminal 62 include a smartphone, tablet terminal, and laptop computer. The mobile terminal 62 includes a communication unit 64, a peripheral blood pressure index calculation unit 33, a pulse wave feature amount calculation unit 34, a pulse pressure calculation unit 35, a control unit 36, and a display unit 37. The functions of the peripheral blood pressure index calculation unit 33, the pulse wave feature amount calculation unit 34, and the pulse pressure calculation unit 35 may also be realized by a server. When this configuration is adopted, data communication is performed between the mobile terminal 62 and the server via a communication line.

Data communication is performed between the communication unit 55 of the ring device 61 and the communication unit 64 of the mobile terminal 62. Communication between the ring device 61 and the mobile terminal 62 uses, for example, short-range wireless communication methods of various standards.

The advantageous effects of using the ring device 61 to acquire a pulse wave will now be described.
Fingers have a relatively thin epidermis, making them suitable for acquiring pulse waves using the photoplethysmographic sensor 50. Furthermore, the capillary pathways on the finger are less complex than those on the face, making it easier for the values of pulse wave feature quantities to be stable. This increases the reliability of the pulse pressure obtained from the pulse wave. Furthermore, when using the pulse pressure measuring device continuously or intermittently, there is the excellent effect of minimizing discomfort and discomfort even when wearing the ring device 61 on the finger for long periods of time.

[Peripheral blood pressure index]
Next, with reference to FIGS. 5 to 9B, a peripheral blood pressure index, which is one piece of basic information for determining pulse pressure using the pulse pressure measuring device according to the first embodiment, will be described.

In this specification, "peripheral blood pressure" is defined as the blood pressure in peripheral arterioles and capillaries. Peripheral blood pressure is sometimes used to mean blood pressure at the wrist or ankle measured with a cuff-type sphygmomanometer, but wrist or ankle blood pressure is measured in a large artery (such as the radial artery) and is different from peripheral blood pressure as defined in this specification. Blood pressure in blood vessels decreases as you move from large arteries to arterioles and capillaries. The degree to which blood pressure decreases varies depending on the measurement site, the individual's vascular condition (presence or absence of arteriosclerosis, etc.), mental state (autonomic nervous system state, etc.), environment (temperature, presence or absence of noise, etc.), clothing, etc.

Among the waveform features of the pulse wave, an index that is effective for determining peripheral blood pressure is adopted as the peripheral blood pressure index. The peripheral blood pressure index is considered to have the following characteristics.
First, when blood vessels are healthy, peripheral blood pressure indexes have a positive correlation with blood pressure at the upper arm or wrist under the condition that vascular resistance does not change. Second, when the vicinity of the measurement site is cooled to constrict blood vessels, peripheral blood pressure indexes decrease. When blood vessels constrict, peripheral vascular resistance increases, which can cause blood pressure at the upper arm or wrist to rise.

Next, with reference to Figures 5 and 6, we will explain various characteristics of the pulse wave waveform.

Figure 5 is a graph showing examples of a pulse wave, velocity pulse wave, and acceleration pulse wave. The peripheral blood pressure index calculation unit 33 (Figures 1, 2, and 3) performs first-order and second-order differentiation of the pulse wave. The waveforms obtained by first-order and second-order differentiation of the pulse wave are referred to as the velocity pulse wave and acceleration pulse wave, respectively. For example, the velocity pulse wave is obtained by numerically differentiating the intensity of the pulse wave, which is discretely distributed at time intervals corresponding to the sampling rate, at time intervals equivalent to the sampling rate. Furthermore, the magnitude of the velocity pulse wave is numerically differentiated to obtain the acceleration pulse wave.

The horizontal axis of Figure 5 represents time in units of seconds, the left vertical axis represents the magnitude of the velocity pulse wave and acceleration pulse wave normalized so that the maximum value is 1, and the right vertical axis represents the magnitude of the pulse wave in arbitrary units. The solid line, long dashed line, and short dashed line in the graph shown in Figure 2 represent the pulse wave, velocity pulse wave, and acceleration pulse wave, respectively. Generally, five peaks appear in an acceleration pulse wave within one beat. The first, second, third, fourth, and fifth peaks within one beat are called the a wave, b wave, c wave, d wave, and e wave, respectively.

The full width at half maximum of the first upward peak of the velocity pulse wave is labeled "VE0.5." The difference between the peak values of the a wave and the b wave is labeled "a-b," and the difference between the peak values of the a wave and the d wave is labeled "a-d." A depression called the incisa IC appears slightly behind the maximum peak of the pulse wave.

Figure 6 is a graph showing an example of a pulse wave and an accelerated pulse wave. The horizontal axis represents time, the left vertical axis represents the amplitude of the pulse wave in arbitrary units, and the right vertical axis represents the amplitude of the accelerated pulse wave in arbitrary units. Five divisions on the horizontal axis correspond to 0.2 s. The peak value of the a-wave of the accelerated pulse wave is labeled "a", and the amplitude of the pulse wave is labeled "S". The amplitude S of the pulse wave corresponds to the difference between the minimum and maximum values after waveform correction has been performed so that the minimum values of the pulse wave for two consecutive beats are the same magnitude.

The following three feature amounts can be cited as pulse wave feature amounts that reflect the above two features of the peripheral blood pressure index.
The full width at half maximum is the reciprocal of "VE0.5" (hereinafter referred to as "1/(VE0.5)")
The ratio of the amplitude S of the pulse wave to the peak value a of the a wave of the accelerated pulse wave (hereinafter referred to as "a/S").
The ratio of the difference "a-b" between the peak value of the a wave and the peak value of the b wave of the accelerated pulse wave to the difference "a-d" between the peak value of the a wave and the peak value of the d wave (hereinafter referred to as "(a-b)/(a-d)")
In this specification, these characteristic quantities of the pulse wave waveform are referred to as “peripheral blood pressure indices.” These peripheral blood pressure indices are related to the steepness of the rising edge of the pulse wave.

7A and 7B are graphs showing the relationship between the peripheral blood pressure index "1/VE0.5" calculated from pulse waves measured when the height from the heart to the measurement site (finger) is changed and when the measurement site is adjusted to chest height and the area around the elbow on the side where the finger is located is cooled, and the systolic blood pressure measured at the wrist. Figures 7A and 7B show the measurement results when the pulse wave was measured using the pulse pressure measuring device (green light) shown in Figure 1 and the pulse pressure measuring device (near-infrared light) shown in Figure 2, respectively. The pulse wave measured using green light primarily reflects fluctuations in blood flow in the capillaries 73 (Figure 1), while the pulse wave measured using near-infrared light reflects fluctuations in blood flow in the capillaries 73 and arterioles 72 (Figure 2).

The horizontal axis of the graphs in Figures 7A and 7B represents systolic blood pressure at the wrist in units of mmHg, and the vertical axis represents the peripheral blood pressure index "1/(VE0.5)" in units of s ^-1 . In each graph, the results of measurements performed on three subjects A, B, and C are shown using triangle symbols, square symbols, and circle symbols, respectively. The three hollow symbols shown for each subject represent the values of pulse wave feature quantities calculated from pulse waves acquired when the height of the measurement site (finger) was set to the navel, chest, and forehead, respectively. The values of the peripheral blood pressure index "1/VE0.5" decrease in the order of the measurement site height (navel, chest, and forehead). The solid black symbols shown for each subject represent the values of the peripheral blood pressure index "1/VE0.5" calculated from pulse waves acquired when the height of the measurement site was set to the chest and the area around the elbow was cooled.

Although the degree of correlation varies depending on the subject, it can be seen that when the height of the measurement site is changed, the peripheral blood pressure index "1/VE0.5" generally has a positive correlation with systolic blood pressure at the wrist. Furthermore, although there are some exceptions, it can be seen that when the area near the measurement site is cooled to constrict the blood vessels, the peripheral blood pressure index "1/VE0.5" decreases. This change matches the expected characteristics of peripheral blood pressure indexes. Therefore, it is thought that the peripheral blood pressure index "1/VE0.5" is an effective index for estimating peripheral blood pressure.

The results shown in Figures 7A and 7B indicate that green light is preferable to near-infrared light for measuring the peripheral blood pressure index "1/VE0.5." Furthermore, the inverse of a parameter representing the width of the maximum peak of the velocity pulse wave may be used as an alternative to the peripheral blood pressure index "1/VE0.5." Alternatively, a negative exponent of the parameter representing the width of the maximum peak of the velocity pulse wave may be used. More generally, a function in which the parameter representing the width of the maximum peak of the velocity pulse wave is used as a variable and the value of the function decreases as the width of the peak increases may be used as the peripheral blood pressure index.

Figures 8A and 8B are graphs showing the relationship between the peripheral blood pressure index "a/S" value calculated from the pulse wave measured when the height from the heart to the measurement site (finger) is changed and when the measurement site is adjusted to chest height and the area around the elbow on the side where the measurement site (finger) is located is cooled, and the systolic blood pressure measured at the wrist. Figures 8A and 8B show the measurement results when the pulse wave was measured using the pulse pressure measuring device (green light) shown in Figure 1 and the pulse pressure measuring device (near-infrared light) shown in Figure 2, respectively.

The horizontal axis of the graphs in Figures 8A and 8B represents systolic blood pressure at the wrist in units of mmHg, and the vertical axis represents the peripheral blood pressure index "a/S" in arbitrary units. The symbols in Figures 8A and 8B have the same meaning as those in the graphs shown in Figures 7A and 7B.

The measurement results shown in Figures 8A and 8B show trends similar to those shown in Figures 7A and 7B. Therefore, the peripheral blood pressure index "a/S" is considered to be an effective index for estimating peripheral blood pressure. Furthermore, the results shown in Figures 8A and 8B indicate that green light is preferable to near-infrared light for measuring the peripheral blood pressure index "a/S."

Instead of the peripheral blood pressure index "a/S," the peripheral blood pressure index may be the product of the peak value a of the a-wave of the accelerated pulse wave raised to a positive power and the amplitude S of the pulse wave raised to a negative power. Alternatively, the peripheral blood pressure index may be calculated based on information regarding the peak value of the a-wave of the accelerated pulse wave and the amplitude of the pulse wave signal. For example, the peripheral blood pressure index may be a function in which the peak value a and the amplitude S are variables, and the value of the function increases as the peak value a increases and decreases as the amplitude S increases.

Figures 9A and 9B are graphs showing the relationship between the peripheral blood pressure index "(a-b)/(a-d)" calculated from the pulse wave measured when the height from the heart to the measurement site (finger) is changed and when the measurement site is adjusted to chest height and the area around the elbow on the side where the measurement site (finger) is located is cooled, and the systolic blood pressure measured at the wrist. Figures 9A and 9B show the measurement results when the pulse wave was measured using the pulse pressure measuring device (green light) shown in Figure 1 and the pulse pressure measuring device (near-infrared light) shown in Figure 2, respectively.

The horizontal axis of the graphs in Figures 9A and 9B represents systolic blood pressure at the wrist in units of mmHg, and the vertical axis represents the peripheral blood pressure index "(a-b)/(a-d)." The symbols in Figures 9A and 9B have the same meaning as those in the graphs shown in Figures 7A and 7B.

The measurement results shown in Figures 9A and 9B show trends similar to those shown in Figures 7A and 7B. Therefore, the peripheral blood pressure index "(a-b)/(a-d)" is considered to be an effective index for estimating peripheral blood pressure.

Instead of the peripheral blood pressure index "(a-b)/(a-d)," the peripheral blood pressure index may be calculated based on information regarding the difference between the peak values of the a-wave and the b-wave of the accelerated pulse wave and the difference between the peak values of the a-wave and the d-wave. For example, a function may be used as the peripheral blood pressure index, with the difference (a-b) between the peak values of the a-wave and the b-wave and the difference (a-d) between the peak values of the a-wave and the d-wave as variables, such that the value of the function increases as the value of the difference (a-b) increases and decreases as the value of the difference (a-d) increases.

[Relationship between peripheral blood pressure index and systolic blood pressure]
An evaluation experiment was conducted to determine the relationship between wrist systolic blood pressure and peripheral blood pressure indexes by increasing the number of subjects. The results of this evaluation experiment will be explained with reference to Figures 10A to 12B. In the evaluation experiment, healthy subjects and diabetic patients were included in the subjects to collect data with widely varying extremes. A cuff-type blood pressure monitor was worn on the left or right wrist, and a ring device 61 (Figure 4) equipped with a photoplethysmographic sensor (Figures 1, 2, and 3) was worn on the index finger of the hand on the same side as the cuff-type blood pressure monitor.

With the subject sitting still, the hand equipped with a photoplethysmogram sensor was held at chest height to measure both the photoplethysmogram and blood pressure. Because measuring both simultaneously would obstruct blood flow to the finger due to the cuff, blood pressure was measured using a cuff-type sphygmomanometer after the photoplethysmogram measurement was completed.

Figures 10A and 10B are scatter plots of multiple subjects, with wrist systolic blood pressure (hereinafter sometimes simply referred to as blood pressure) on the horizontal axis and the peripheral blood pressure index "1/VE0.5" on the vertical axis. Figures 11A and 11B are scatter plots of multiple subjects, with wrist systolic blood pressure on the horizontal axis and the peripheral blood pressure index "a/S" on the vertical axis. Figures 12A and 12B are scatter plots of multiple subjects, with wrist systolic blood pressure on the horizontal axis and the peripheral blood pressure index "(a-b)/(a-d)" on the vertical axis. Figures 10A, 11A, and 12A show the results of pulse wave measurements using green light, while Figures 10B, 11B, and 12B show the results of pulse wave measurements using near-infrared light. In each graph, filled circles and hollow circles indicate measurement results for healthy subjects and diabetic patients, respectively.

Figures 10A and 10B show that the higher the blood pressure, the smaller the peripheral blood pressure index "1/VE0.5". Furthermore, when measuring pulse waves using green light, the peripheral blood pressure index "1/VE0.5" of diabetic patients is concentrated in a relatively low range, clearly separated from the range in which the peripheral blood pressure index "1/VE0.5" of healthy individuals is distributed. It is hypothesized that the following mechanism explains these measurement results.

Continuing to have high blood sugar levels can lead to vascular disease, in which blood vessels become brittle and worn out, and arteriosclerosis progresses in large blood vessels. Smaller blood vessels are also damaged, reducing vascular function (endothelial function) and reducing blood flow.

Local blood pressure (peripheral blood pressure) decreases as one progresses from large arteries to arterioles and capillaries. It is believed that the degree of decrease in peripheral blood pressure increases when vascular endothelial function declines. It is said that approximately 40% to 60% of diabetic patients also suffer from hypertension. The evaluation results shown in Figure 10A show that the systolic blood pressure of diabetic patients is higher than that of healthy subjects, but this tendency is not significant. In contrast, there is a clear tendency for the peripheral blood pressure index "1/VE0.5" of diabetic patients to be lower than that of healthy subjects. This can be explained by the fact that peripheral vascular disorders occur in diabetic patients, which inhibit blood flow to the peripheral blood vessels, resulting in a decrease in the peripheral blood pressure index "1/VE0.5."

As shown in Figures 11A and 11B, in healthy individuals, the peripheral blood pressure index "a/S" tends to decrease as blood pressure increases, similar to the peripheral blood pressure index "1/VE0.5." In diabetic patients, however, the peripheral blood pressure index "a/S" varies widely, and the correlation between the peripheral blood pressure index "a/S" and blood pressure is unclear. This is presumably because the peak value a of the a-wave of the accelerated pulse wave, and the amplitude S of the pulse wave (Figure 6), are easily affected by various factors.

As shown in Figures 12A and 12B, the peripheral blood pressure index "(a-b)/(a-d)" tends to decrease as blood pressure increases, similar to the peripheral blood pressure index "1/VE0.5." Furthermore, the magnitude of the peripheral blood pressure index "(a-b)/(a-d)" calculated from the pulse wave acquired using green light is clearly separated between healthy subjects and diabetic patients. The three measurement results where the peripheral blood pressure index "(a-b)/(a-d)" is zero on the horizontal axis represent subjects in whom the b wave of the accelerated pulse wave could not be detected.

[Pulse wave features thought to be causally related to blood pressure]
Next, pulse wave feature quantities that are thought to be causally related to blood pressure will be described with reference to Figures 13A to 17. Figures 13A to 17 are scatter plots of multiple subjects, with wrist systolic blood pressure on the horizontal axis and various pulse wave feature quantities on the vertical axis.

The vertical axes of Figures 13A and 13B represent the elapsed time from the peak of the a wave to the peak of the b wave of the acceleration pulse wave (hereinafter referred to as "ab time") in units of [s]. The vertical axes of Figures 14A and 14B represent the elapsed time from the peak of the b wave to the peak of the d wave of the acceleration pulse wave (hereinafter referred to as "bd time") in units of [s]. The vertical axes of Figures 15A and 15B represent the elapsed time from the peak of the d wave to the peak of the e wave of the acceleration pulse wave (hereinafter referred to as "de time") in units of [s]. The vertical axes of Figures 16A and 16B represent the elapsed time from the peak of the a wave to the peak of the e wave of the acceleration pulse wave (hereinafter referred to as "ae time") in units of [s]. The vertical axis of Figure 17 represents the pulse interval in units of [s].

Figures 13A, 14A, 15A, and 16A show the features of the pulse wave measured using green light, while Figures 13B, 14B, 15B, and 16B show the features of the pulse wave measured using near-infrared light. The pulse intervals shown in Figure 17 are the same whether green light or near-infrared light is used.

As shown in Figures 13A and 13B, the feature "ab time" and systolic blood pressure have a weak negative correlation. As shown in Figures 14A and 14B, the feature "bd time" and systolic blood pressure have a positive correlation. As shown in Figures 15A and 15B, the feature "de time" and systolic blood pressure have a negative correlation. As shown in Figures 16A, 16B, and 17, the feature "ae time" and pulse interval showed no correlation with systolic blood pressure.

Furthermore, differences were observed between healthy individuals and diabetic patients in the feature "de time." Specifically, when systolic blood pressure was roughly the same, the feature "de time" of diabetic patients tended to be larger than the feature "de time" of healthy individuals. No clear trends were observed between healthy individuals and diabetic patients in other feature values.

The mechanism behind the negative correlation between the feature "de time" and systolic blood pressure is presumed to be as follows.

As shown in Figure 5, the time when the peak of the d wave of the accelerated pulse wave appears is close to the time when the pulse wave reaches its maximum value. The area near the b wave of the accelerated pulse wave is considered to be the ejection wave, and the area near the d wave is considered to be the reflected wave, and the notch IC of the pulse wave appears after that (around 0.4 seconds as shown in Figure 5). Since there is no clear correlation between blood pressure and the "ae time," the tendency for the "de time" to shorten as blood pressure increases means that the position of the peak of the d wave moves later (closer to the position of the peak of the e wave) as blood pressure increases.

An increase in blood flow means an increase in the ejection wave and reflected wave. Therefore, an increase in blood flow causes the convex part of the pulse wave (ranging from the b wave to the d wave) to spread further back. As a result, it is thought that the position of the peak of the d wave also moves further back. In other words, it is thought that the increase in blood pressure caused an increase in blood flow, and that the increase in blood flow shortened the "de time."

Figures 15A and 15B show that as blood pressure increases, the "de time" shortens, meaning that the position of the peak of the d wave tends to move closer to the position of the peak of the e wave. Furthermore, in diabetic patients, the position of the peak of the d wave tends to be farther from the position of the peak of the e wave compared to healthy individuals. As speculated above, it is thought that the "de time" shortens as blood flow increases, so the fact that the "de time" in diabetic patients tends to be longer than the "de time" in healthy individuals does not contradict the speculation that peripheral blood pressure decreases in diabetic patients, making it more difficult for blood to flow.

In the scatter plots in Figures 13A to 14B, the measurement results of some diabetic patients are located on the horizontal axis where the "ab time" or "bd time" is 0 seconds. This indicates that the b wave of the accelerated pulse wave could not be detected. It can be seen that the number of subjects for whom the b wave could not be detected was greater when measured with green light than when measured with near-infrared light. In subjects with poor peripheral circulation, such as diabetic patients, the b wave of the accelerated pulse wave is small, making it often difficult to detect.

The evaluation results shown in Figures 10A to 17 reveal that pulse wave features that showed significant differences between diabetic patients and healthy subjects include the peripheral blood pressure index "1/VE0.5" when green light was used (Figure 10A), the peripheral blood pressure index "(a-b)/(a-d)" when green light was used, and the feature "de time" when green light or near-infrared light was used.

[Features that are thought to have a causal relationship with pulse pressure]
Next, with reference to FIGS. 18A to 22, a description will be given of the feature quantities of the pulse wave that are thought to have a causal relationship with the pulse pressure.

Figures 18A to 22 are scatter plots of multiple subjects, with pulse pressure on the horizontal axis and various features of the pulse wave on the vertical axis.

The vertical axes of Figures 18A and 18B represent the peripheral blood pressure index "1/VE0.5" in units of [s ^-1 ]. The vertical axes of Figures 19A and 19B represent the peripheral blood pressure index "(a-b)/(a-d)". The vertical axes of Figures 20A and 20B represent the feature "de time" in units of [s]. The vertical axes of Figures 21A and 21B represent the feature "ae time" in units of [s]. The vertical axis of Figure 22 represents the pulse interval in units of [s].

Figures 18A, 19A, 20A, and 21A show the features of the pulse wave measured using green light, while Figures 18B, 19B, 20B, and 21B show the features of the pulse wave measured using near-infrared light. The pulse interval shown in Figure 22 is the same whether green light or near-infrared light is used.

Feature quantities that showed a certain degree of correlation with pulse pressure include the peripheral blood pressure index "1/VE0.5" (Figures 18A and 18B), the peripheral blood pressure index "(a-b)/(a-d)" (Figures 19A and 19B), and the feature quantity "ae time" (Figures 21A and 21B). For the peripheral blood pressure index "1/VE0.5," a stronger correlation was observed when green light was used than when near-infrared light was used. The feature quantity "de time" (Figures 20A and 20B) and the pulse interval (Figure 22) did not show a significant correlation with pulse pressure.

The peripheral blood pressure index "1/VE0.5" and the peripheral blood pressure index "(a-b)/(a-d)" have been confirmed to have a negative correlation with pulse pressure. This is similar to the relationship between the peripheral blood pressure index "1/VE0.5" and the peripheral blood pressure index "(a-b)/(a-d)" and systolic blood pressure. Subjects with low peripheral blood pressure index "1/VE0.5" and the peripheral blood pressure index "(a-b)/(a-d)" are expected to have high vascular resistance. It is generally known that high vascular resistance leads to high pulse pressure. In other words, the negative correlation between the peripheral blood pressure index "1/VE0.5" and the peripheral blood pressure index "(a-b)/(a-d)" and pulse pressure is consistent with this general finding.

The feature "ae time" has been confirmed to have a negative correlation with pulse pressure. No significant difference is observed in the feature "ae time" when measured using green light or near-infrared light. The dicrotic notch IC (Figure 5) that appears on the pulse wave is said to be the end of systole, and the position of the peak of the e wave on the accelerated pulse wave corresponds to the position of the dicrotic IC. Since no difference is observed in the "ae time" when green light or near-infrared light is used, it is inferred that the "ae time" is less affected by vascular conditions, etc.

A long "AE time" means that the left ventricle is contracting for a long time. Therefore, "AE time" is thought to have a positive correlation with stroke volume. The fact that pulse pressure has a positive correlation with "AE time" can be explained by the fact that pulse pressure increases as stroke volume increases.

[Pulse pressure measurement method]
Next, a pulse wave measurement method according to a first embodiment will be described. In the first embodiment, pulse pressure is calculated based on a peripheral blood pressure index "1/VE0.5" that has a correlation with pulse pressure and a feature value "ae time." For example, pulse pressure is calculated using a function that uses the exponentiated value of the peripheral blood pressure index "1/VE0.5" and the exponentiated value of the feature value "ae time" as variables.

Next, this function will be explained. In the first embodiment, the pulse pressure index value Pa is calculated using the following formula.
Pa=(1/VE0.5) ^-α ×(ae time) ^β ...(1)
Here, α and β are positive fitting parameters. The actual pulse pressure value can be calculated by multiplying the pulse pressure index value Pa by a coefficient. The fitting parameters α and β and the coefficient can be determined by actually conducting an evaluation experiment.

Next, the results of an actual evaluation experiment will be described with reference to Figures 23A and 23B. The pulse waves of multiple subjects were measured, and the pulse pressure index value Pa was calculated from the pulse waves. Furthermore, pulse pressure was calculated from the systolic and diastolic blood pressure measured at the wrist. The subjects included multiple healthy individuals and multiple diabetic patients. Pulse wave measurements were performed by wearing the ring device 61 (Figure 4) on the subject's finger, with the height of the measurement site approximately aligned with the height of the heart.

Figure 23A is a scatter plot of the measurement results when the fitting parameters α and β in equation (1) are set to α = 1 and β = 1.5, respectively. The horizontal axis represents pulse pressure in units of mmHg, and the vertical axis represents the pulse pressure index value Pa. The peripheral blood pressure index "1/VE0.5" was obtained from the pulse wave measured using green light, and the feature "ae time" was obtained from the pulse wave measured using near-infrared light. The black and hollow circles in Figure 23A represent the measurement results for healthy subjects and diabetic patients, respectively. Overall, it can be seen that there is a positive correlation between the pulse pressure index value Pa and pulse pressure.

FIG. 23B is a scatter plot showing the measurement results without distinguishing between healthy subjects and diabetic patients. In FIG. 23B, the regression line is indicated by a dashed line. In the example shown in FIG. 23B, the coefficient of determination ^R2 was approximately 0.47. The correlation coefficient was approximately 0.69, indicating a sufficient correlation between the pulse pressure index value Pa and pulse pressure. Thus, the pulse pressure measurement method of the first embodiment can be used to determine pulse pressure using a photoplethysmographic sensor.

From the measured pulse wave, the peripheral blood pressure index "1/VE0.5" and the feature "ae time" can be determined, and the pulse pressure can be calculated using equation (1). It is also possible to gather more subjects and conduct evaluation experiments to improve the accuracy of the fitting parameters α and β.

In Figures 23A and 23B, the peripheral blood pressure index "1/VE0.5" is calculated using green light, and the feature "ae time" is calculated using near-infrared light. As another example, the peripheral blood pressure index "1/VE0.5" may be calculated using near-infrared light, or the feature "ae time" may be calculated using green light. Furthermore, light in the wavelength range from blue to yellow-green may be used instead of green light.

In place of the peripheral blood pressure index "1/VE0.5", other peripheral blood pressure indices related to the steepness of the rising edge of the pulse wave may be used, such as the peripheral blood pressure index "a/S" or "(a-b)/(a-d)" which have a positive correlation with peripheral blood pressure. Furthermore, instead of the value of the feature "ae time" itself, an index which has a positive correlation with "ae time" (referred to as the "ae time index" in this specification) may be used. Furthermore, instead of equation (1), a function may be used in which the value of the function decreases as the peripheral blood pressure index increases and the value of the function increases as the ae time index increases.

The data for the scatter plots shown in Figures 23A and 23B was collected by wearing the ring device 61 (Figure 4) on the subject's finger, but it may also be worn on the subject's fingertip.

[Modification of the first embodiment]
Next, a modification of the first embodiment will be described.
In the first embodiment, a ring-shaped ring device 61 ( FIG. 5 ) is worn on a finger, but instead of the ring device 61, a device shaped to be worn on a part of the body other than a finger may be used. For example, a wearable device shaped to be worn on the wrist, neck, face, ear, etc. may be used. For example, the wearable device may be a wristband or watch type worn on the wrist, an earphone type worn on the ear, a patch type attached to the skin, or a neckband type worn around the neck.

Furthermore, the pulse pressure measuring device does not necessarily have to be wearable, and may be a device that measures pulse waves by pressing a finger against the photoelectric pulse wave sensor 50 as needed. For example, the pulse pressure measuring device may be a portable device such as a smartphone, or a fixed device.

In the first embodiment, light reflected by biological tissue such as a finger is detected, but light that has passed through biological tissue may also be detected. When detecting light that has passed through a finger, the light-emitting element and the light-receiving element are positioned opposite each other with the finger in between. Depending on the relative positions of the light-emitting element and the light-receiving element, it may be possible for the light-receiving element to detect both light reflected by biological tissue and light that has passed through biological tissue. In other words, the light-receiving element detects light that is output from the light-emitting element and has passed through biological tissue.

[Second Example]
Next, a pulse pressure measuring device and a pulse wave measuring method according to a second embodiment will be described with reference to Figures 24A, 24B, and 25. Below, a description of the components common to the pulse pressure measuring device and the pulse wave measuring method according to the first embodiment and its modified example will be omitted.

In the first embodiment, as shown in equation (1), pulse pressure is calculated based on the peripheral blood pressure index "1/VE0.5", which has a correlation with pulse pressure, and the feature "ae time". In contrast, in the second embodiment, pulse pressure is calculated based on the feature "de time" in addition to the peripheral blood pressure index "1/VE0.5" and the feature "ae time". For example, pulse pressure is calculated using a function with the exponent value of the peripheral blood pressure index "1/VE0.5", the exponent value of the feature "ae time", and the exponent value of the feature "de time" as variables.

Next, this function will be explained. In the second embodiment, the pulse pressure index value Pad is calculated using the following formula.
Pad=(1/VE0.5) ^-α × (ae time) ^β × (de time) ^-γ ...(2)
Here, α, β, and γ are positive fitting parameters. The actual pulse pressure value can be calculated by multiplying the pulse pressure index value Pad by the coefficient. The fitting parameters α, β, γ, and the coefficients can be determined by actually conducting an evaluation experiment.

Figure 24A is a scatter plot of the measurement results when the fitting parameters α, β, and γ in equation (2) are set to α = 0.8, β = 1.5, and γ = 0.5, respectively. The horizontal axis represents pulse pressure in units of mmHg, and the vertical axis represents the pulse pressure index value Pad. The peripheral blood pressure index "1/VE0.5" was obtained from pulse waves measured using green light, and the feature quantities "ae time" and "de time" were obtained from pulse waves measured using near-infrared light. The black and hollow circles in Figure 24A represent the measurement results for healthy subjects and diabetic patients, respectively. Overall, it can be seen that there is a positive correlation between the pulse pressure index value Pad and pulse pressure.

FIG. 24B is a scatter plot showing the measurement results without distinguishing between healthy subjects and diabetic patients. In FIG. 24B, the regression line is indicated by a dashed line. In the example shown in FIG. 24B, the coefficient of determination ^R2 was approximately 0.52. The correlation coefficient was approximately 0.72, indicating a strong correlation between the pulse pressure index value Pad and pulse pressure. Thus, the pulse pressure measurement method of the second embodiment can be used to determine pulse pressure using a photoplethysmographic sensor.

From the measured pulse wave, the peripheral blood pressure index "1/VE0.5", the feature "ae time", and the feature "de time" can be determined, and the pulse pressure can be calculated using equation (2). It is also possible to gather more subjects and conduct evaluation experiments to improve the accuracy of the fitting parameters α, β, and γ.

In Figures 24A and 24B, the peripheral blood pressure index "1/VE0.5" is calculated using green light, and the feature quantities "ae time" and "de time" are calculated using near-infrared light. As another example, the peripheral blood pressure index "1/VE0.5" may be calculated using near-infrared light, or the feature quantities "ae time" or "de time" may be calculated using green light. Furthermore, light in the wavelength range from blue to yellow-green may be used instead of green light.

Instead of the peripheral blood pressure index "1/VE0.5," other peripheral blood pressure indices related to the steepness of the rising edge of the pulse wave, such as "a/S" or "(a-b)/(a-d)," which have a positive correlation with peripheral blood pressure, may be used. Furthermore, instead of the value of the feature "ae time" itself, an ae time index that has a positive correlation with "ae time" may be used. Instead of the value of "de time" itself, an index that has a positive correlation with "de time" (referred to herein as the "de time index") may be used. Furthermore, instead of equation (2), a function may be used in which the value of the function decreases as the peripheral blood pressure index increases, the value of the function increases as the ae time index increases, and the value of the function decreases as the de time index increases.

Next, referring to Figure 25, we will explain other advantageous effects of calculating pulse pressure using equation (2).

Figure 25 is a scatter plot of multiple measurement results obtained over a long period of time for a single subject. Note that this subject is a healthy individual and is different from the subject used to collect the data shown in Figures 24A and 24B. The horizontal axis of Figure 25 represents pulse pressure in units of mmHg, and the vertical axis represents the pulse pressure index value Pad. The fitting parameters α, β, and γ in equation (2) are the same as those in Figures 24A and 24B. One set of pulse wave measurements was performed by aligning the measurement site height approximately with the navel, chest, and forehead, respectively. 24 sets of measurements were taken over a 20-day period. Measurement times were morning, midday, and evening. Over the 20 days, 9 sets of measurements were taken in the morning, 12 sets of measurements were taken in the midday, and 3 sets of measurements were taken in the evening.

The measurement procedure is as follows.
First, the pulse wave is measured at navel level using a photoplethysmographic sensor, then the wrist blood pressure is measured at navel level. Next, the pulse wave is measured at chest level using a photoplethysmographic sensor, then the wrist blood pressure is measured at chest level. Next, the pulse wave is measured at forehead level using a photoplethysmographic sensor, and finally the wrist blood pressure is measured at forehead level. The pulse wave measurement using the photoplethysmographic sensor and the wrist blood pressure measurement are not performed simultaneously.

The circle, square, and triangle symbols in the scatter plot shown in Figure 25 represent measurement results obtained when the measurement site heights were adjusted to forehead height, chest height, and navel height, respectively. The dashed line shown in Figure 25 is the same as the regression line shown in Figure 24B. As can be seen from Figure 25, pulse pressure tends to be relatively high at navel height and relatively low at forehead height.

The pulse pressure index value Pad varies between the heights of the navel, chest, and forehead, but is distributed within roughly the same range regardless of the height of the measurement site. This distribution nearly overlaps with the distribution of pulse pressure index values Pad for healthy individuals shown in Figure 24A. Therefore, it can be inferred that the pulse pressure index value Pad calculated using equation (2) will not fluctuate significantly even if the height of the measurement site deviates from the height of the heart.

What is useful to the user is the value of pulse pressure at heart level. Considering user convenience, it is desirable to be able to estimate the value of pulse pressure at heart level even if the measurement site, for example, the height of the finger, deviates from heart level. The calculation formula for the pulse pressure index value Pad, equation (2) used in the second embodiment, is also useful from the perspective of user convenience.

[Third Example]
Next, a pulse pressure measuring device and a pulse pressure measuring method according to a third embodiment will be described with reference to Figures 26, 27, and 28. Hereinafter, a description of the configuration common to the pulse pressure measuring device and the pulse pressure measuring method according to the first or second embodiment will be omitted.

When the measurement site is 10 cm higher than the heart, blood pressure drops by approximately 7 mmHg to 8 mmHg. In other words, when the difference in height between the measurement site and the heart changes within a range of ±10 cm, the measured blood pressure varies by approximately ±8 mmHg. Similarly, pulse pressure measurements are also affected by the height from the heart to the measurement site. In order to utilize pulse pressure measurements as medically useful, it is preferable to measure pulse pressure with the height of the measurement site approximately the same as the height of the subject's heart. The pulse pressure measuring device of the third embodiment has the function of measuring the difference in height between the measurement site and the heart.

Figure 26 is an oblique view and block diagram of a pulse pressure measuring device according to the third embodiment. The pulse pressure measuring device according to the third embodiment includes a ring device 61 and a mobile terminal 62, similar to the pulse pressure measuring device according to the modified example of the first embodiment shown in Figure 4. The mobile terminal 62 of the pulse pressure measuring device according to the third embodiment includes a camera 63, an acceleration sensor 65, and a height calculation unit 38 in addition to the configuration of the mobile terminal 62 of the pulse pressure measuring device according to the modified example of the first embodiment (Figure 4). The height calculation unit 38 calculates the difference in height between the ring device 61 and the user's heart.

Next, the procedure performed by the height calculation unit 38 will be described with reference to Figure 27. Figure 27 is a schematic diagram illustrating the procedure for measuring the difference in height between the ring device 61 and the heart. A user who wishes to measure pulse pressure wears the ring device 61 on their finger, holds the mobile terminal 62 in the hand wearing the ring device 61, and captures an image of their face with the camera 63. The height calculation unit 38 (Figure 26) of the mobile terminal 62 displays the captured image on the display unit 37 in real time. Furthermore, the height calculation unit 38 displays an oval or rectangular shape on the image of the user's face on the display unit 37. While looking at the display unit 37, the user adjusts the relative position of the mobile terminal 62 and their face so that the image of their face fits within the oval or rectangular shape. Furthermore, the user maintains a posture such that their torso is aligned vertically.

Physical information such as the user's height and weight is stored in advance in the height calculation unit 38. Furthermore, statistical relationship information between the physical information and face size is stored in the height calculation unit 38. The height calculation unit 38 calculates the size of the user's face from the stored physical information using the statistical relationship information between the physical information and face size. From the calculated face size and the size of the image of the user's face, the distance L1 from the mobile terminal 62 to the user's face (e.g., eyes) is calculated.

Furthermore, the height calculation unit 38 calculates the tilt of the mobile terminal 62 relative to the vertical direction (direction of gravity) from the measurement results of the acceleration sensor 65. Based on the tilt of the mobile terminal 62 and the distance L1 from the mobile terminal 62 to the user's eyes, it calculates the height H1 from the mobile terminal 62 to the user's eyes.

The height calculation unit 38 statistically determines the height H2 from the heart to the eyes based on the user's physical information. However, errors will occur in the height H2 if the trunk is bent significantly, such as when leaning forward. Here, it is assumed that the trunk is not tilted. For example, the difference between the medial canthus height and the nipple height contained in the "AIST Human Body Dimension Database 1991-1992" can be used as the height H2 from the heart to the eyes.

The height calculation unit 38 (Figure 26) calculates the difference in height between the mobile device 62 and the heart using the height H1 from the mobile device 62 to the user's eyes and the height H2 from the heart to the eyes. The height of the ring device 61 (measurement site) can be assumed to be approximately equal to the height of the mobile device 62.

Figure 28 is a flowchart showing the steps of the pulse pressure measurement method performed by the pulse pressure measuring device according to the third embodiment.

The height calculation unit 38 (Figure 26) calculates the difference in height between the ring device 61 and the heart (step SA1). This calculation can be performed using the method described with reference to Figure 27. The height calculation unit 38 determines whether the calculated difference between the ring device 61 and the heart is within an acceptable range (step SA2). If the difference is within the acceptable range, the pulse pressure calculation unit 35 (Figure 26) calculates the pulse pressure based on the measured pulse wave (step SA3). The pulse pressure measurement method according to the first or second embodiment can be used to calculate the pulse pressure.

Once the pulse pressure value (calculated value) is obtained by calculation, the control unit 36 stores or outputs the calculated pulse pressure value (step SA4). The calculated pulse pressure value is stored, for example, in association with the measurement date and time. The calculated pulse pressure value is output, for example, by displaying it on the display unit 37 (Figure 26) or sending it to a server. The functions of the peripheral blood pressure index calculation unit 33, pulse wave feature calculation unit 34, and pulse pressure calculation unit 35 (Figure 4) may be realized by the server. If this configuration is adopted, the server calculates the pulse pressure, stores the calculated pulse pressure value, and sends the calculated pulse pressure value to the mobile terminal 62 (Figure 4).

If it is determined in step SA2 that the difference is outside the allowable range, the height calculation unit 38 notifies the user that the difference in height between the ring device 61 and the heart is outside the allowable range (step SA5). Upon receiving this notification, the user can adjust the height of the ring device 61 to match the height of the heart and perform the measurement again.

Next, the excellent effects of the third embodiment will be described.
In the third embodiment, the height of the measurement site can be adjusted to the height of the heart to measure the pulse pressure. This improves the accuracy of the pulse pressure measurement. It is also possible to measure the pulse pressure even if it is determined in step SA2 that the difference is outside the allowable range, and output the pulse pressure measurement with a note indicating that the measurement is not reliable.

Next, a modified example of the third embodiment will be described with reference to FIG. 29. FIG. 29 is a flowchart showing the steps of a pulse pressure measurement method executed by a pulse pressure measurement device according to a modified example of the third embodiment. First, the height calculation unit 38 calculates the difference in height between the ring device 61 and the heart (step SA1), as in the third embodiment (FIG. 28). In the first embodiment, if the difference is outside the allowable range, pulse pressure measurement is not performed. However, in this modified example, the pulse pressure calculation unit 35 calculates pulse pressure based on the measured pulse wave, regardless of whether the difference is within the allowable range (step SA3).

Next, the pulse pressure calculation unit 35 corrects the calculated pulse pressure value based on the difference in height between the ring device 61 and the heart (step SA5). The relationship between the difference in height between the ring device 61 and the heart and the pulse pressure can be determined in advance through evaluation experiments. After correcting the calculated pulse pressure value, the corrected pulse pressure value is stored or output (step SA6).

In the modified example shown in Figure 29, even if there is a large difference in height between the measurement site and the heart, a highly accurate pulse pressure value can be obtained by correcting the calculated pulse pressure value.

[Fourth Example]
Next, a pulse pressure measuring device and a pulse pressure measuring method according to a fourth embodiment will be described with reference to Figures 30 and 31. Hereinafter, a description of the configuration common to the pulse pressure measuring device and the pulse pressure measuring method according to the first or second embodiment will be omitted.

When exercising, blood pressure and pulse pressure change depending on the amount of exercise. Furthermore, when the measurement site is moving, inertial forces act on the blood in the blood vessels, causing the pulse wave waveform to fluctuate. Blood pressure and pulse pressure measured in a resting state are medically useful. Furthermore, the contact state between the photoplethysmographic sensor and the skin is likely to change during exercise. If the contact state changes, noise will be superimposed on the measured pulse wave. In the fourth embodiment, it is determined whether the user is at rest, and pulse pressure in a resting state that is medically useful is measured.

Figure 30 is an oblique view and block diagram of a pulse pressure measuring device according to the fourth embodiment. The pulse pressure measuring device according to the fourth embodiment includes a ring device 61 and a mobile terminal 62, similar to the pulse pressure measuring device according to the modified first embodiment (Figure 4). The ring device 61 of the pulse pressure measuring device according to the fourth embodiment is equipped with an acceleration sensor 54 in addition to the configuration of the ring device 61 of the pulse pressure measuring device according to the modified first embodiment (Figure 4). The mobile terminal 62 includes a resting state determination unit 39 in addition to the configuration of the mobile terminal 62 of the pulse pressure measuring device according to the modified first embodiment.

The resting state determination unit 39 determines whether the user is in a resting state using the measurement results of the acceleration sensor 54. For example, if the acceleration measurement value from the acceleration sensor 54 remains below the determination threshold for a predetermined period of time, such as five minutes, the resting state determination unit 39 determines that the user is in a resting state.

Figure 31 is a flowchart showing the steps of the pulse pressure measurement method executed by the pulse pressure measuring device according to the fourth embodiment. The resting state determination unit 39 determines whether the user wearing the ring device 61 is in a resting state (step SB1). If it is determined that the user is not in a resting state, the resting state determination is repeated at regular intervals until the user is in a resting state (step SB2). If it is determined that the user is in a resting state, the pulse pressure calculation unit 35 calculates the pulse pressure based on the pulse wave acquired at that time (step SB3). The calculated pulse pressure value is then stored or output (step SB4).

Next, the excellent effects of the fourth embodiment will be described.
In the fourth embodiment, the pulse pressure is calculated based on the pulse wave when the user is in a resting state, and therefore the accuracy of the calculated pulse pressure value can be improved.

Next, a modified example of the fourth embodiment will be described. In the fourth embodiment, whether or not the subject is in a resting state is determined based on the acceleration measurement value obtained by the acceleration sensor 54 mounted on the ring device 61. A gyro sensor may be mounted instead of the acceleration sensor 54, and whether or not the subject is in a resting state may be determined based on the angular acceleration measurement value obtained by the gyro sensor. Furthermore, the acceleration sensor 54 and the gyro sensor may be used together to determine whether or not the subject is in a resting state.

[Fifth Example]
Next, a pulse pressure measuring device and a pulse pressure measuring method according to a fifth embodiment will be described with reference to Figures 32 and 33. Hereinafter, a description of the configuration common to the pulse pressure measuring device and the pulse pressure measuring method according to the first or second embodiment will be omitted.

Figure 32 is an oblique view and block diagram of a pulse pressure measuring device according to the fifth embodiment. The pulse pressure measuring device according to the fifth embodiment includes a ring device 61 and a mobile terminal 62, similar to the pulse pressure measuring device according to the modified first embodiment (Figure 4). The ring device 61 of the pulse pressure measuring device according to the fifth embodiment is equipped with an acceleration sensor 54, similar to the ring device 61 of the pulse pressure measuring device according to the fourth embodiment (Figure 30). The mobile terminal 62 includes a sleep state determination unit 40 in addition to the configuration of the mobile terminal 62 of the pulse pressure measuring device according to the modified first embodiment.

The sleep state determination unit 40 determines whether the user is in a resting state using the measurement results of the acceleration sensor 54. Below, we will explain how to determine whether the user is in a sleeping state or an awake state.

The sleep state determination unit 40 determines that body movement is occurring, for example, when the acceleration measurement value from the acceleration sensor 54 exceeds a predetermined threshold. It determines that the subject is asleep when the number of body movements occurring within a predetermined time period is equal to or less than the predetermined threshold. The predetermined time period may be selected, for example, from a range of 15 minutes to 90 minutes. While acceleration may suddenly increase during sleep due to factors such as turning over in bed, this occurs less frequently than when awake. Therefore, whether or not the subject is asleep can be accurately determined based on the number of body movements occurring within a predetermined time period. Fingers move more frequently during wakefulness than the waist, chest, wrist, etc. By utilizing this characteristic, wearing the ring device 61 on the finger allows for more accurate determination of whether or not the subject is asleep compared to wearing a photoplethysmographic sensor on another part of the body, such as the wrist.

Figure 33 is a flowchart showing the steps of a pulse pressure measurement method executed by a pulse pressure measuring device according to the fifth embodiment. The sleep state determination unit 40 (Figure 32) determines whether the user wearing the ring device 61 is asleep or awake (step SC1). If it is determined that the user is not asleep, the determination of whether the user is asleep or awake is repeated at regular intervals (step SC2).

If the sleep state is determined, the pulse wave measurement unit 32 (Figure 4) measures the pulse wave, and the pulse pressure calculation unit 35 (Figure 4) calculates the pulse pressure (step SC3). After the sleep state is determined, a further determination may be made as to whether the subject is in a resting state, as in step SB1 shown in Figure 31, and the pulse wave may be measured and the pulse pressure calculated only if the subject is determined to be in a resting state. Once the calculated pulse pressure value has been obtained, the control unit 36 stores or outputs the calculated pulse pressure value (step SC4).

Next, the excellent effects of the fifth embodiment will be described.
In the fifth embodiment, the pulse pressure during sleep can be automatically determined. If it is determined that the subject is not asleep, the pulse wave is not measured, thereby reducing battery consumption.

Eating, drinking alcohol, caffeine intake, smoking, etc. are known to affect blood pressure and pulse pressure. In addition, exercise, walking, physical activities (such as cleaning), bathing, conversation, mental stress, noisy or vibrating environments, and cold environments also affect blood pressure and pulse pressure. These events occur frequently during wakefulness, making it difficult to determine whether the timing of pulse wave measurements coincides with these events. During sleep, the influence of these events is reduced, allowing for stable measurements by measuring pulse pressure while asleep.

Next, a modification of the fifth embodiment will be described.
In the fifth embodiment, whether or not a person is asleep is determined based on the frequency of body movements, but other methods may also be used to determine whether or not a person is asleep. It is known that the temperature of the fingers rises during sleep. A temperature sensor may be mounted on the ring device 61 (FIG. 32), and the circadian rhythm may be estimated from the temperature sensor's measurements. The frequency of body movements may be combined with the temperature measurements to determine whether or not a person is asleep.

In addition, pulse rate tends to decrease during sleep compared to when awake. Furthermore, respiratory fluctuations are more likely to affect pulse rate. Utilizing this characteristic, the frequency of body movement can be combined with the tendency for pulse rate fluctuations to determine whether or not a person is asleep.

In the fifth embodiment, a determination of whether or not a person is asleep is made based on the frequency of body movement. As a variation of the fifth embodiment, measurement of the pulse wave may be started when the frequency of body movement falls below a certain frequency, and a determination of whether or not a person is asleep may be made based on the frequency of body movement and the measured pulse wave. By additionally using the pulse wave to determine whether or not a person is asleep, the accuracy of determining whether or not a person is asleep can be improved.

In the fifth embodiment, pulse wave measurement is not performed when an awake state is determined, but if the battery capacity is large enough, pulse wave measurement and pulse pressure calculation may be performed in both the asleep and awake states. In this case, it is advisable to store the calculated pulse pressure value in association with identification information that distinguishes between the awake and asleep states. This makes it possible to know the difference in tendency between leg pressure when awake and pulse pressure when asleep.

The above-described embodiments are merely illustrative, and it goes without saying that partial substitution or combination of the configurations shown in different embodiments is possible. Similar effects resulting from similar configurations in multiple embodiments will not be mentioned sequentially for each embodiment. Furthermore, the present invention is not limited to the above-described embodiments. For example, it will be obvious to those skilled in the art that various modifications, improvements, combinations, etc. are possible.

30 Processing device 31 Light emission control unit 32 Pulse wave measurement unit 33 Peripheral blood pressure index calculation unit 34 Pulse wave feature amount calculation unit 35 Pulse pressure calculation unit 36 Control unit 37 Display unit 38 Height calculation unit 39 Resting state determination unit 40 Sleep state determination unit 50 Photoplethysmographic sensors 51, 52 Light emitting element 53 Light receiving element 54 Acceleration sensor 55 Communication unit 56 Gyro sensor 60 Wearable member 61 Ring device 62 Portable terminal 63 Camera 64 Communication unit 65 Acceleration sensor 70 User's body surface 71 Epidermal region 72 Arterioles 73 Capillaries

Claims (21)

a peripheral blood pressure index calculation unit that calculates a peripheral blood pressure index related to the steepness of the rising edge of a pulse wave signal measured by a pulse wave sensor worn by a user;
a pulse wave feature amount calculation unit that calculates an ae time index including information about the elapsed time from the peak of the a-wave to the peak of the e-wave of an acceleration pulse wave obtained by second-order differentiation of the waveform of the pulse wave signal;
a pulse pressure calculation unit that calculates a pulse pressure based on the peripheral blood pressure index and the ae time index. the pulse wave feature amount calculation unit further includes a function of calculating a de time index including information on the elapsed time from the peak of the d wave to the peak of the e wave of the acceleration pulse wave;
The pulse pressure measuring device according to claim 1 , wherein the pulse pressure calculation unit calculates the pulse pressure based on the peripheral blood pressure index, the ae time index, and the de time index. A pulse pressure measuring device according to claim 1 or 2, wherein the pulse wave sensor is configured so that fluctuations in the user's peripheral capillary blood pressure are reflected in the pulse wave signal. The pulse pressure measuring device of claim 3, wherein the pulse wave sensor is a photoplethysmographic sensor that uses light with a wavelength in the blue to yellow-green wavelength range. The pulse wave sensor
a first light-emitting element that outputs light having a wavelength included in a wavelength range from blue to yellow-green;
a first light receiving element that receives light output from the first light emitting element and transmitted through biological tissue;
5. The pulse pressure measuring device according to claim 4, wherein the distance between the first light emitting element and the first light receiving element is 1 mm or more and 3 mm or less. 3. The pulse pressure measuring device according to claim 1 , wherein the pulse wave sensor is configured so that fluctuations in blood pressure in the user's peripheral arterioles are reflected in the pulse wave signal. The pulse pressure measuring device of claim 6, wherein the pulse wave sensor is a photoplethysmographic sensor that uses light with a wavelength ranging from red to near-infrared. The pulse wave sensor
a second light-emitting element that outputs light of a wavelength included in a wavelength range from blue to yellow-green;
a second light receiving element that receives light output from the second light emitting element and transmitted through biological tissue;
8. The pulse pressure measuring device according to claim 7, wherein the distance between the second light emitting element and the second light receiving element is 5 mm or more and 20 mm or less. 3. The pulse pressure measuring device according to claim 1, wherein the pulse wave sensor includes a mounting member that is mounted on a user's finger, radiates light toward the user's finger, and generates the pulse wave signal based on the intensity of the light that passes through the biological tissue of the finger. 3. The pulse pressure measuring device according to claim 1 , wherein the pulse pressure calculation unit calculates the value of a function having a value of the power of the peripheral blood pressure index and a value of the power of the ae time index as variables to determine the pulse pressure. A pulse pressure measuring device as described in claim 10, wherein the peripheral blood pressure index has a positive correlation with peripheral blood pressure, the ae time index has a positive correlation with the elapsed time from the peak of the a wave to the peak of the e wave of the accelerated pulse wave, and the value of the function decreases as the peripheral blood pressure index increases and increases as the ae time index increases. 3. The pulse pressure measuring device according to claim 1 , further comprising a height calculation unit that calculates a difference in height between the pulse wave sensor and the user's heart, and notifies the user when the difference in height exceeds an allowable range. The device further includes a height calculation unit that calculates a difference in height between the pulse wave sensor and the user's heart,
3. The pulse pressure measuring device according to claim 1 , wherein the pulse pressure calculation unit corrects the calculated pulse pressure value in accordance with the difference in height measured by the height calculation unit. 3. The pulse pressure measuring device according to claim 1, wherein the peripheral blood pressure index calculation unit calculates the peripheral blood pressure index based on information regarding the width of the first peak that appears within one beat of a velocity pulse wave waveform obtained by first - order differentiation of the waveform of the pulse wave signal. 3. The pulse pressure measuring device according to claim 1 , wherein the peripheral blood pressure index calculation unit calculates the peripheral blood pressure index based on information relating to the peak value of the a-wave of the accelerated pulse wave and the amplitude of the pulse wave signal. 3. The pulse pressure measuring device according to claim 1, wherein the peripheral blood pressure index calculation unit calculates the peripheral blood pressure index based on information regarding the difference between the peak value of the a-wave and the peak value of the b-wave of the accelerated pulse wave and the difference between the peak value of the a-wave and the peak value of the d -wave. The device further includes a resting state determination unit that determines whether the user is in a resting state,
The pulse pressure measuring device according to claim 1 , wherein the pulse pressure calculation unit calculates the pulse pressure from a pulse wave acquired when the resting state determination unit determines that the user is in a resting state. The device further includes a sleep state determination unit that determines whether the user is asleep,
3. The pulse pressure measuring device according to claim 1, wherein the pulse pressure calculation unit calculates the pulse pressure from the pulse wave acquired when the sleep state determination unit determines that the user is asleep, and does not calculate the pulse pressure when the sleep state determination unit determines that the user is not asleep. A pulse wave signal is acquired by a pulse wave sensor attached to the user.
a pulse pressure measuring device calculating a peripheral blood pressure index related to the steepness of the rising edge of the pulse wave signal;
the pulse pressure measuring device calculates an ae time index including information about the elapsed time from the peak of the a-wave to the peak of the e-wave of an accelerated pulse wave obtained by second-order differentiation of the waveform of the pulse wave signal;
A pulse pressure measuring method in which the pulse pressure measuring device determines the pulse pressure based on the peripheral blood pressure index and the ae time index. The pulse pressure measurement method according to claim 19, wherein the pulse wave signal is acquired so as to reflect fluctuations in the user's peripheral capillary blood pressure. A pulse pressure measurement method according to claim 19 or 20, wherein the pulse wave signal is acquired so as to reflect fluctuations in the user's peripheral arteriolar blood pressure.