Heartbeat interval calculation device, heartbeat interval calculation system, heartbeat interval calculation method, and heartbeat interval calculation program

Abstract

A heartbeat interval calculation device (10) comprises: a phase generation unit (110) that receives a pulse wave signal outputted from a pulse wave detection device (20) which detects a pulse wave of a living body at a predetermined sampling rate, and that calculates a phase of the pulse wave signal; and a heartbeat interval calculation unit (120) that calculates a heartbeat interval (RRI) on the basis of a first phase (θa), which represents a phase calculated on the basis of a pulse wave signal at a first time (ta), and of a second phase (θb), which represents a phase calculated on the basis of a pulse wave signal at a second time (tb) that is different from the first time (ta).

Description

Heartbeat interval calculation device, heartbeat interval calculation system, heartbeat interval calculation method, and heartbeat interval calculation program

[0001] The present disclosure relates to a heart beat interval calculation device, a heart beat interval calculation system, a heart beat interval calculation method, and a heart beat interval calculation program.

[0002] There is a demand for high-resolution acquisition of heartbeat intervals, which are biological information, in order to evaluate the state of a living body (e.g., the state of the autonomic nervous system). Also, there has been proposed a device that acquires heartbeat intervals from the pulse wave of a living body detected by a pulse wave sensor (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2021-45319

[0004] However, if the sampling rate when detecting the pulse wave is low (for example, lower than 200 Hz), there is a problem in that it is not possible to calculate the heartbeat interval from the pulse wave with high resolution.

[0005] The present disclosure aims to provide a heart beat interval calculation device, a heart beat interval calculation system, a heart beat interval calculation method, and a heart beat interval calculation program that enable calculation of a heart beat interval with high resolution from a detected pulse wave.

[0006] The heart beat interval calculation device of the present disclosure is characterized by having a phase generation unit that receives a pulse wave signal output from a pulse wave detection device that detects pulse waves of a living body at a predetermined sampling rate and calculates the phase of the pulse wave signal, and a heart beat interval calculation unit that calculates the heart beat interval based on a first phase, which is the phase calculated based on the pulse wave signal at a first time, and a second phase, which is the phase calculated based on the pulse wave signal at a second time different from the first time.

[0007] The heartbeat interval calculation method disclosed herein is characterized by comprising the steps of receiving a pulse wave signal output from a pulse wave detection device that detects pulse waves of a living body at a predetermined sampling rate and calculating the phase of the pulse wave signal, and calculating the heartbeat interval based on a first phase, which is the phase calculated based on the pulse wave signal at a first time, and a second phase, which is the phase calculated based on the pulse wave signal at a second time different from the first time.

[0008] According to the present disclosure, the heartbeat interval can be calculated with high resolution from the detected pulse wave.

[0009] 1 is a block diagram schematically showing the configuration of a heartbeat interval calculation system and a heartbeat interval calculation device according to an embodiment. FIG. 2 is a block diagram schematically showing another configuration of the heartbeat interval calculation system according to an embodiment. FIG. 3 is a diagram showing an example of the hardware configuration of the heartbeat interval calculation device according to an embodiment. FIG. 4 is a flowchart showing the operation of the heartbeat interval calculation device according to an embodiment. FIG. 5 is an explanatory diagram showing the operation of a phase generation unit. FIG. 6 is a diagram showing an electroencephalogram signal and a first phase and a second phase (when there is a phase difference of less than 2π between the first phase and the second phase). FIG. 7 is a diagram showing, on a phase plane, the first phase and the second phase (when there is a phase difference of less than 2π between the first phase and the second phase) used in calculation of the heartbeat interval by the heartbeat interval calculation unit. FIG. 8 is a diagram showing, as a time-phase graph (when there is a phase difference of 2π or more between the first phase and the second phase), the first phase and the second phase (when there is a phase difference of 4π or more between the first phase and the second phase). 12 is a diagram showing, on a phase plane, a first phase and a second phase (when there is a phase difference of 4π or more between the first and second phases) used in calculating a heartbeat interval by a heartbeat interval calculation unit. FIG. 13 is a diagram showing, as a time-phase graph (when there is a phase difference of 4π or more between the first and second phases) a first phase and a second phase used in calculating a heartbeat interval by a heartbeat interval calculation unit. FIG. 14 is a diagram showing an example of a method for determining the thresholds in FIGS. 8 and 11. FIG. 15A is a diagram showing an example of a pulse wave signal, and FIG. 15B is a diagram showing an example of outputting a heartbeat interval RRI at regular intervals.

[0010] Hereinafter, a heart beat interval calculation device, a heart beat interval calculation system, a heart beat interval calculation method, and a heart beat interval calculation program according to embodiments will be described with reference to the drawings. The following embodiments are merely examples, and the embodiments can be modified as appropriate.

[0011] FIG. 1 is a block diagram showing a schematic configuration of a heartbeat interval calculation system 1 and a heartbeat interval calculation device 10 according to an embodiment. The heartbeat interval calculation system 1 includes a heartbeat interval calculation device 10 and a pulse wave detection device 20. The heartbeat interval calculation device 10 is a device for calculating a heartbeat interval (RRI: R-R Interval) from a pulse wave signal indicating a pulse wave of a living body. The heartbeat interval calculation device 10 may have a function for measuring or determining a living body condition (e.g., autonomic nervous state, stress level, organ condition, etc.) using the calculated heartbeat interval. The heartbeat interval calculation device 10 is a device capable of implementing the heartbeat interval calculation method according to the first embodiment. The heartbeat interval calculation device 10 is, for example, a computer. The heartbeat interval calculation device 10 may be a computer system configured by cloud computing using a computer network.

[0012] The pulse wave detection device 20 detects the pulse wave of a living body (e.g., a human) at a predetermined sampling rate and outputs a pulse wave signal (i.e., a signal containing pulse wave information). The pulse wave detection device 20 is a pulse wave sensor that senses changes in the volume of the blood vessels of the living body (i.e., changes in blood flow rate) in time series. There are no limitations on the detection principle or structure of the pulse wave detection device 20, as long as it is a device that can generate a signal containing information indicating the pulse wave. The sampling rate is 5 [s －１ In this embodiment, the sampling rate is preferably 200 [s －１ ], the heartbeat interval RRI can be calculated with high resolution.

[0013] Pulse wave detection device 20 is, for example, a photoplethysmographic sensor. Photoplethysmographic sensors detect pulse waves by utilizing the property of hemoglobin in blood, which strongly absorbs green light. The photoplethysmographic sensor includes a light-emitting diode (LED) as a light source that irradiates light onto the living body from outside the skin, and a photodetector element such as a photodiode that detects reflected light from the irradiated living body and outputs a pulse wave signal based on the reflected light.

[0014] The heartbeat interval calculation device 10 includes a phase generation unit 110 and a heartbeat interval calculation unit 120. The phase generation unit 110 receives a pulse wave signal and calculates the phase of the pulse wave signal. The phase generation unit 110 can calculate the phase from the pulse wave detected by the pulse wave detection device 20.

[0015] The heartbeat interval calculation unit 120 calculates the heartbeat interval RRI based on a first phase θa, which is a phase calculated based on the pulse wave signal at a first time ta, and a second phase θb, which is a phase calculated based on the pulse wave signal at a second time tb, which is different from the first time ta.

[0016] FIG. 2 is a block diagram showing a schematic configuration of another heartbeat interval calculation system 1a according to an embodiment. The heartbeat interval calculation system 1a includes a heartbeat interval calculation device 10 and a pulse wave detection device 20a. The pulse wave detection device 20a is a camera serving as an imaging device that captures an image of a living body from outside the skin of the living body and outputs a video signal. In this case, the video signal is a pulse wave signal that contains pulse wave information. The video signal of a living body captured by the imaging device (for example, a video signal capturing an image of a face) can sense changes in the volume of the blood vessels of the living body (i.e., changes in blood flow) in time series. When the pulse wave detection device 20a is an imaging device, the sampling rate [s －１ The frame rate [fps] is generally 20 fps to 120 fps (for example, 30 fps, 60 fps, or 120 fps).

[0017] 3 is a diagram illustrating an example of the hardware configuration of a heartbeat interval calculation device 10 according to an embodiment. The heartbeat interval calculation device 10 includes a processor 11 such as a CPU (Central Processing Unit), a memory 12 serving as a storage device such as a RAM (Random Access Memory), a storage device 13 serving as a non-volatile storage device such as an HDD (Hard Disk Drive) or an SSD (Solid State Drive), and an interface 14. These components may be configured using dedicated processing circuits.

[0018] The processor 11 can execute a heartbeat interval calculation program according to the embodiment. The heartbeat interval calculation program is recorded on a recording medium such as an SD (Secure Digital) memory card or a USB (Universal Serial Bus) memory card, or is provided by downloading via a network. The hardware configuration shown in FIG. 3 is an example, and various modifications are possible to the hardware configuration.

[0019] 4 is a flowchart showing the operation of heart beat interval calculation device 10 according to an embodiment. FIG. 4 shows a heart beat interval calculation method according to the embodiment. The heart beat interval calculation method according to the embodiment includes step S1 of receiving a pulse wave signal output from pulse wave detection device 20, which detects the pulse wave of a living body at a predetermined sampling rate, and calculating the phase of the pulse wave signal. The heart beat interval calculation method according to the embodiment also includes step S2 of calculating the heart beat interval RRI based on a first phase θa, which is a phase calculated based on the pulse wave signal at a first time ta, and a second phase θb, which is a phase calculated based on the pulse wave signal at a second time tb different from the first time ta.

[0020] 5 is an explanatory diagram showing the operation of the phase generator 110. The phase generator 110 includes a reference signal generator 111, comparators 112 and 113, and a phase calculator 114. The reference signal generator 111 applies a window interval to the pulse wave signal, which is the signal to be analyzed and is output from the pulse wave detection device 20, to obtain a first reference signal REF1 and a second reference signal REF2 in advance. The first reference signal REF1 and the second reference signal REF2 are orthogonal to each other. The phase generating unit 110 calculates the first phase θa and the second phase θb ​​using coordinate points on a phase plane determined by a first component IF1 (real part) obtained by a comparison by a comparator 112 between the pulse wave signal, which is the signal to be analyzed and is output from the pulse wave detection device 20, and a first reference signal REF1, and a second component IF2 (imaginary part) obtained by a comparison by a comparator 113 between the pulse wave signal, which is the signal to be analyzed, and a second reference signal REF2.

[0021] The reference signals (e.g., the first reference signal REF1 and the second reference signal REF2 described above) may be periodic signals based on the main frequency components obtained by extracting a portion of the signal to be analyzed using a time window and performing frequency analysis. In this case, two types of reference signals are prepared, and the two prepared reference signals have a known phase difference. The two types of reference signals may be, for example, a sine wave as the first reference signal REF1 and a cosine wave as the second reference signal REF2 having the same frequency components as the first reference signal REF1. Alternatively, a portion of the signal to be analyzed may be extracted using a time window to be used as one of the reference signals (REF1), and a signal with a known phase difference may be generated based on this to be used as the other reference signal (REF2).

[0022] Each of the comparators 112 and 113 may calculate a correlation coefficient between the two input signals, may perform convolution, or may perform a Fourier transform on the entire section of the signal to be analyzed to obtain a reference signal.

[0023] FIG. 7 is a diagram showing the sampling points (including the sampling points at times ta and tb of the pulse wave signal shown in FIG. 6 ) used in the calculation of the heartbeat interval RRI by the heartbeat interval calculation unit 120, expanded on a phase plane (complex plane) using the method shown in FIG. 5 . Specifically, IF1 (real part) and IF2 (imaginary part) obtained in FIG. 5 are plotted on the phase plane. In FIG. 7 , the sampling points trace a trajectory that rotates around the origin 30 from time ta to time tb. The points corresponding to times ta and tb are PA(θa, ta) and PB(θb, tb), and the amount of rotation is 2π-D. Here, if the number of samples taken between the first time ta and the second time tb is represented by Fa-Fb, the sampling rate (frame rate in the case of an imaging device) of the pulse wave detection device 20 (or 20a) is represented by FR, and the heartbeat interval is represented by RRI, the heartbeat interval calculation unit 120 can calculate the RRI using the following equation (1).

[0024]

[0025] FIG. 8 shows a time series waveform in which the angle between each point plotted on the complex plane shown in FIG. 7 and the horizontal axis (real axis) is defined in the range of −π to +π. FIG. 8 shows an example in which the RRI is calculated each time the threshold value 34 is exceeded. In this way, the RRI can be calculated approximately once per cycle. However, the cycle for calculating the RRI is not limited to the example shown in FIG. 8. If the difference between point PA (θa, ta) and point PB (θb, tb) in FIG. 8 is −D, then θb−θa=2π−D. In the case of FIG. 8 as well, the RRI can be calculated using equation (1).

[0026] Furthermore, if the distance from the origin 30 on the phase plane in FIG. 7 to a coordinate point (sampling point) is outside a predetermined range (for example, a range between a predetermined value r1 and a predetermined value r2) (for example, point 33 in FIG. 7), the heartbeat interval calculation unit 120 does not use the coordinate point (33) in calculating the RRI. In this way, the heartbeat interval can be calculated with high accuracy. Note that the heartbeat interval calculation unit 120 can determine whether the coordinate point serving as a sampling point is within a predetermined range using, for example, a Kalman filter.

[0027] 10 is a diagram showing the sampling points (including the sampling points at times ta and tc of the pulse wave signal shown in FIG. 9 ) used in the calculation of the heartbeat interval RRI by the heartbeat interval calculation unit 120, expanded on a phase plane (complex plane) using the method shown in FIG. 5 . Specifically, IF1 (real part) and IF2 (imaginary part) obtained in FIG. 5 are plotted on the phase plane. In FIG. 10 , the sampling points trace a locus that rotates around the origin 30 from time ta to time tc. The points corresponding to times ta and tc are PA(θa, ta) and PC(θc, tc), and the amount of rotation is 4π+D. Here, if the number of samples between the first time ta and the third time tc is represented by Fa-Fb, the sampling rate (frame rate in the case of an imaging device) of the pulse wave detection device 20 (or 20a) is represented by FR, and the heartbeat interval is represented by RRI, the heartbeat interval calculation unit 120 can calculate the RRI using the above formula (1).

[0028] FIG. 11 shows a time-series waveform in which the angle between each point plotted on the complex plane shown in FIG. 10 and the horizontal axis (real axis) is defined as a range of −π to +π. FIG. 11 shows an example in which the RRI is calculated every two times the threshold 34 is exceeded. In this way, the RRI can be calculated approximately once every two periods. However, the period for calculating the RRI is not limited to the example shown in FIG. 11. If the difference between point PA (θa, ta) and point PC (θc, tc) on the vertical axis of FIG. 11 is D, then θc − θa = 4π + D. In the case of FIG. 11 as well, the RRI can be calculated using equation (1). The heartbeat interval calculation unit 120 may calculate the heartbeat interval RRI for each predetermined number of sampling points.

[0029] Fig. 12 is a diagram showing an example of a method for determining the threshold value 34 in Fig. 8 and Fig. 11. As shown in Fig. 12, the threshold value 34 in Fig. 8 and Fig. 11 can be determined based on a point 35 of the pulse wave signal that corresponds to point R of the heartbeat signal. However, the method for determining the threshold value 34 is not limited to the example shown in Fig. 12.

[0030] Fig. 13(A) is a diagram showing an example of a pulse wave signal, and Fig. 13(B) is a diagram showing an example of outputting a heartbeat interval RRI at regular intervals. While the graphs of Fig. 8 and Fig. 11 illustrate an example in which "phase gradient = angular velocity," Figs. 13(A) and (B) illustrate an example in which "phase gradient ≠ angular velocity." When "phase gradient ≠ angular velocity," the effects of noise can be reduced.

[0031] FIG. 13B shows the heartbeat interval RRI calculated by the heartbeat interval calculation unit 120. ｋ (k is the sampling number) α (Time s α ) and the second phase θ ｋ (Time s ｋ ) as a time-phase graph (when there is a phase difference of about 2π between the first phase and the second phase). α (Time s α ) point P1(θ α , t α ) and the second phase θ ｋ (Time s ｋ ) point P2(θ ｋ , t ｋ ) is shown. In FIG. 13B, the phase θ ｋ When sampling, the threshold phase (θ ｋ ) to search for the phase and its sampling number α of approximately one cycle in the past, and for each search, RRI (here, RRI ｋ In this way, the RRI can be calculated for each sampling period. In this case, the RRI ｋ The calculation formula is the following formula (2): where sr is a predetermined constant.

[0032]

[0033] FIG. 13B shows the heart rate interval RRI calculated by the heart rate interval calculation unit 120. ｋ＋１ (k+1 is the sampling number) β (Time s β ) and the second phase θ ｋ＋１ (Time s ｋ＋１) as a time-phase graph (when there is a phase difference of about 2π between the first phase and the second phase). β (Time s β ) point P3(θ β , t β ) and the second phase θ ｋ＋１ (Time s ｋ＋１ ) point P4(θ ｋ＋１ , t ｋ＋１ ) is shown. In FIG. 13B, the phase θ ｋ＋１ When sampling, the threshold phase (θ ｋ＋１ ) to search for the phase and its sampling number β of approximately one cycle in the past, and for each search, RRI (here, RRI ｋ＋１ In this way, the RRI can be calculated for each sampling period. In this case, the RRI ｋ＋１ The calculation formula is the following formula (3).

[0034]

[0035] As described above, by using the device, system, method, and program according to this embodiment, the phase can be calculated for every predetermined number of samples (e.g., every sample), and therefore the cardiac interval RRI can be calculated with high resolution.

[0036] For example, when a pulse wave signal is acquired based on a video signal from an imaging device with a sampling rate (frame rate) of 30 Hz, the insufficient number of samples makes it impossible to calculate the heart rate interval RRI with high resolution. However, by using the device, system, method, and program according to the present embodiment, it is possible to calculate the heart rate interval RRI with high resolution from a pulse wave signal with a low sampling rate.

[0037] Furthermore, by using the device, system, method, and program according to this embodiment, the current heart rate interval RRI can be detected with high resolution from the pulse wave without using an electrocardiogram, and therefore the heart rate interval RRI can be obtained with high resolution using a mobile device equipped with a camera (for example, a smartphone, a tablet device, a personal computer, or a wearable device such as smart glasses or a smart watch).

[0038] In addition, by combining software that measures or determines the state (e.g., autonomic nervous system state, stress level, organ state, etc.) of a living organism (e.g., human, animal, etc.) based on the heartbeat interval and its fluctuations, it is possible to continuously observe the state of the living organism over a long period of time.

[0039] REFERENCE SIGNS LIST 1, 1a Heartbeat interval calculation system, 10 Heartbeat interval calculation device, 20, 20a Pulse wave detection device, 110 Phase generation unit, 120 Heartbeat interval calculation unit

Claims

1. A heart beat interval calculation device comprising: a phase generation unit that receives a pulse wave signal output from a pulse wave detection device that detects pulse waves of a living body at a predetermined sampling rate, and calculates the phase of the pulse wave signal; and a heart beat interval calculation unit that calculates the heart beat interval based on a first phase, which is the phase calculated based on the pulse wave signal at a first time, and a second phase, which is the phase calculated based on the pulse wave signal at a second time different from the first time.

2. The heart beat interval calculation device according to claim 1, characterized in that the phase generation unit obtains in advance a first reference signal and a second reference signal using the pulse wave signal to which a window section has been applied, and calculates the first phase and the second phase using coordinate points on a phase plane determined by a first component indicating the relationship between the pulse wave signal to be analyzed output from the pulse wave detection device and the first reference signal, and a second component indicating the relationship between the pulse wave signal to be analyzed and the second reference signal.

3. The heartbeat interval calculation device according to claim 2, wherein the first reference signal and the second reference signal are orthogonal to each other.

4. The heart beat interval calculation device according to claim 1, characterized in that the phase generation unit acquires in advance a first reference signal obtained by Fourier transforming the pulse wave signal and a second reference signal obtained by delaying the phase of the first reference signal by 90°, and calculates the first phase and the second phase using coordinate points on a phase plane determined by a first component indicating the relationship between the pulse wave signal to be analyzed output from the pulse wave detection device and the first reference signal, and a second component indicating the relationship between the pulse wave signal to be analyzed and the second reference signal.

5. When the first phase is represented by θa, the second phase is represented by θb, the number of samples from the first time to the second time is represented by Fa-Fb, the sampling rate is represented by FR, and the heartbeat interval is represented by RRI, the heartbeat interval calculation unit:

5. The heart beat interval calculation device according to claim 1, wherein the RRI is calculated using the following formula:

6. The heart beat interval calculation device according to claim 2, characterized in that the heart beat interval calculation unit does not use a coordinate point in calculating the heart beat interval if the distance from the origin on the phase plane to the coordinate point is outside a predetermined range.

7. The heartbeat interval calculation device according to claim 6, wherein the heartbeat interval calculation section uses a Kalman filter to determine whether the coordinate point is within the range.

8. A heart beat interval calculation device according to any one of claims 1 to 7, characterized in that the phase generation unit calculates the phase for each predetermined number of samplings by the pulse wave detection device.

9. The heartbeat interval calculation device according to claim 8, wherein the predetermined number of samples is one.

10. A heart beat interval calculation system comprising: a pulse wave detection device that detects pulse waves of a living body at a predetermined sampling rate and outputs a pulse wave signal; a phase generation unit that receives the pulse wave signal and calculates the phase of the pulse wave signal; and a heart beat interval calculation unit that calculates the heart beat interval based on a first phase, which is the phase calculated based on the pulse wave signal at a first time, and a second phase, which is the phase calculated based on the pulse wave signal at a second time different from the first time.

11. The heart beat interval calculation system according to claim 10, characterized in that the pulse wave detection device has a photoelectric pulse wave sensor that detects reflected light when light is irradiated towards the living body and outputs a signal based on the reflected light as the pulse wave signal.

12. The heart beat interval calculation system according to claim 10, wherein the pulse wave detection device has an imaging device that captures an image of the living body and outputs the image signal as the pulse wave signal.

13. A method for calculating a heart beat interval comprising the steps of: receiving a pulse wave signal output from a pulse wave detection device that detects pulse waves of a living body at a predetermined sampling rate, and calculating the phase of the pulse wave signal; and calculating the heart beat interval based on a first phase, which is the phase calculated based on the pulse wave signal at a first time, and a second phase, which is the phase calculated based on the pulse wave signal at a second time different from the first time.

14. A heart beat interval calculation program that causes a computer to execute the steps of: receiving a pulse wave signal output from a pulse wave detection device that detects pulse waves of a living body at a predetermined sampling rate, and calculating the phase of the pulse wave signal; and calculating the heart beat interval based on a first phase, which is the phase calculated based on the pulse wave signal at a first time, and a second phase, which is the phase calculated based on the pulse wave signal at a second time different from the first time.

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