JP2015217060A - Heartbeat detection method and heartbeat detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately detect a heartbeat and the time through data of an electrocardiogram waveform partially having a small amplitude.SOLUTION: A heartbeat detector includes: a detector 3 which detects a point where the reduction width between the electric potentials of pieces of sampling data at continuous two points exceeds a reduction width threshold and becomes a maximum value from the sampling data of the electrocardiogram waveform of a living body; and a heartbeat time calculation part 4 which regards sampling the pieces of data at the continuous two points when the reduction width of the electric potential exceeds the reduction width threshold and becomes maximum value as pieces of data at two points representing a change from a R-wave to a S-wave in the electrocardiogram waveform and calculates the time when a line connecting the two points data on the electrocardiogram waveform crosses a predetermined electric potential for determining a heartbeat time as a heartbeat time.

Description

  The present invention relates to a heartbeat detection method and a heartbeat detection apparatus for extracting biological information such as a heartbeat interval (RR interval) from an electrocardiogram waveform.

  The ECG (Electrocardiogram) waveform is an observation of the electrical activity of the heart with electrodes placed on the body surface. There are various types of ECG waveform induction methods, that is, electrode arrangements using the extremities and the chest. Among the chest leads, V3 to V5 leads are electrodes placed on the left chest. In the CC5 lead used when the ECG waveform is monitored for a long time, electrodes are arranged on the left chest and the right chest. These inductions have the advantage that a stable waveform with a large amplitude can be obtained.

FIG. 7 shows an example of an ECG waveform. In FIG. 7, the vertical axis represents potential and the horizontal axis represents time. The ECG waveform is composed of a continuous heartbeat waveform, and one heartbeat waveform is composed of components such as a P wave, a Q wave, an R wave, an S wave, and a T wave that reflect the activities of the atrium and the ventricle.
It is known that biological information such as the RR interval obtained from the ECG waveform is an index reflecting the function of the autonomic nerve. Taking an ECG waveform in daily life and analyzing heart rate variability data from the detected heart rate is useful for evaluating the autonomic nervous function.

  The following documents are known as conventional heartbeat detection methods. Patent Document 1 discloses a configuration for removing perturbation of the baseline of the ECG waveform. Patent Document 2 discloses a configuration for recognizing an R wave with a threshold value based on the amplitude of peaks and valleys of an ECG waveform.

JP 2002-78695 A JP 2003-561 A

  However, the conventional heart rate detection method has the following problems. When recording or analyzing heart rate data in daily life, ECG data including a large number of heartbeats over a long period of time does not provide a clear waveform for all heartbeats. Each component or part may be reduced. The decrease in the amplitude of the waveform is caused by a decrease in the signal level due to the increase in impedance between the living body and the electrode accompanying the activity of the living body (human body), and the change in the waveform is rapidly changed with respect to the sampling frequency along with the activity of the living body. This is due to the data being lost.

  FIG. 8A to FIG. 8C are diagrams for explaining conventional problems, and are diagrams showing examples of ECG waveforms in which the amplitude is partially reduced. 8A to 8C, the vertical axis represents potential and the horizontal axis represents time. In the example of FIG. 8A, the amplitude of the R wave is small in the portion 100, and in the example of FIG. 8B, the amplitude of the S wave is small in the portion 101. In the example of FIG. 8C, the amplitudes of the R wave and the S wave are small at the portion 102. If an attempt is made to detect the heartbeat based on the amplitude level of the R wave or S wave with respect to the ECG waveform as shown in FIGS. 8A to 8C, the heartbeat cannot be correctly detected and the heartbeat cannot be counted. End up.

  The present invention has been made in view of the above points, and a heartbeat detection method and a heartbeat capable of accurately detecting a heartbeat and its time even from data in which the amplitude of an ECG waveform is partially reduced. An object is to provide a detection device.

The heartbeat detection method of the present invention is a detection step of detecting a point where the decrease width of the potential between two consecutive sampling data exceeds the decrease width threshold and becomes a maximum value from the sampling data of the electrocardiogram waveform of the living body, The sampling data at the two consecutive points when the potential decrease width exceeds the decrease width threshold and reaches a maximum value, and the two points of sampling data representing the change from the R wave to the S wave of the electrocardiogram waveform. And a heartbeat time calculating step of calculating a time at which a straight line connecting the two points of data on the electrocardiogram waveform intersects with a predetermined potential for determining the heartbeat time as a heartbeat time. is there.
Moreover, in one configuration example of the heartbeat detection method of the present invention, the detection step includes a step of updating the decrease width threshold based on an average of maximum values of potential decrease widths between the two consecutive data points. It is characterized by this.

In one configuration example of the heartbeat detection method of the present invention, the heartbeat time calculating step calculates the time when a straight line connecting the two points of data on the electrocardiogram waveform intersects the predetermined potential. It is determined whether or not the immediately preceding heartbeat time is more than a certain time, and when the immediately preceding heartbeat time is not more than a certain time, the calculated time is not adopted as the heartbeat time.
Further, in one configuration example of the heartbeat detection method of the present invention, the heartbeat time calculating step calculates the time when a straight line connecting the two points of data on the electrocardiogram waveform intersects the predetermined potential. Judgment is made whether the heart rate interval when it is regarded as the heart rate time has increased more than a certain rate from the previous heart rate interval, and if the heart rate interval has increased more than a certain rate, the calculated time is adopted as the heart rate time It is characterized by not.

  In addition, the heartbeat detection device of the present invention is a detection means for detecting a point where the decrease width of the potential between two consecutive sampling data exceeds the decrease width threshold and becomes the maximum value from the sampling data of the electrocardiogram waveform of the living body. And the sampling data of the two consecutive points when the potential decrease width exceeds the decrease width threshold and reaches a maximum value, the two points representing the change from the R wave to the S wave of the electrocardiogram waveform. And a heart rate time calculating means for calculating, as a heart rate time, a time at which a straight line connecting the two points of data on the electrocardiogram waveform intersects with a predetermined potential for determining the heart rate time. Is.

  According to the present invention, from the sampling data of the electrocardiogram waveform of the living body, a point where the decrease width of the potential between the two consecutive sampling data exceeds the decrease width threshold and becomes the maximum value is detected, and the continuous time at this time is detected. Two points of sampling data are regarded as two points of data representing a change from an R wave to an S wave of an electrocardiogram waveform, and a straight line connecting the two points of data on the electrocardiogram waveform is a predetermined value for determining a heartbeat time. The time at which the potential intersects is calculated as the heartbeat time. In the present invention, the heartbeat is detected based on the portion where the potential sharply decreases from the R wave to the S wave, and the time at which the potential of the portion should take a predetermined potential is calculated as the heartbeat time. Even from a data string having a low sampling frequency and including fluctuations in waveform amplitude, heartbeats can be detected accurately, and biological information such as RR intervals with high accuracy can be extracted based on the time string.

  In the present invention, the decrease width threshold is updated based on the average of the maximum values of the decrease width of the potential between two consecutive data points. In the electrocardiogram waveform, the amplitude of each component and the degree of change differ depending on individual differences and electrode placement. By setting a decrease width threshold value for identifying two points representative of the change from the R wave to the S wave based on the average of the maximum values of the decrease width between the two points of the ECG waveform data so far, It is possible to reduce the influence of waveform differences caused by individual differences.

  Further, in the present invention, when calculating a time at which a straight line connecting two points of data on the electrocardiogram waveform intersects with a predetermined potential, it is determined whether this time and the immediately preceding heartbeat time are apart from each other by a certain time, The calculated time is not adopted as the heartbeat time if it is not more than a certain time away from the previous heartbeat time. There is a general range of normal values for the heart rate interval. When a very short heart rate interval is detected, noise superimposed on the ECG waveform due to body movement may be mistakenly recognized as a heart rate. High nature. By imposing a condition that the calculated heartbeat time candidate is separated from the immediately preceding heartbeat time by a certain time or more, erroneous detection due to noise or the like can be prevented.

  Further, in the present invention, when the time at which a straight line connecting two points of data on the electrocardiogram waveform intersects with a predetermined potential is calculated, the heartbeat interval when this time is regarded as the heartbeat time is constant from the immediately preceding heartbeat interval. It is determined whether or not the rate has increased by more than a percentage, and if the heartbeat interval has increased by a certain percentage or more, the calculated time is not adopted as the heartbeat time. If detection of a certain heartbeat fails, the data obtained as the heartbeat interval of the heartbeat before and after that heartbeat is a value twice as large as the actual one, and is not suitable for use in the evaluation of the autonomic nervous function. By imposing that the heartbeat interval to be detected has not increased by a certain rate or more, erroneous data that fails to detect heartbeats can be excluded from the analysis target of biological information.

It is a figure explaining the principle of this invention. It is a figure explaining the principle of this invention. It is a figure explaining the principle of this invention. It is a block diagram which shows the structure of the heart rate detection apparatus which concerns on embodiment of this invention. It is a flowchart explaining the heart rate detection method which concerns on embodiment of this invention. It is a block diagram which shows another structure of the heart rate detection apparatus which concerns on embodiment of this invention. It is a figure which shows the example of an electrocardiogram waveform. It is a figure explaining the conventional problem.

[Principle of the Invention]
1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B are diagrams for explaining the principle of the present invention. FIGS. 1A and 1B show an example in which the heartbeat time is calculated when the amplitude of the R wave is small (FIG. 8A). FIG. 1A is a diagram showing a part of sampling data of the ECG waveform of FIG. 8A (portion 100 in FIG. 8A), the horizontal axis is time [ms], and the vertical axis is digital. It represents a potential [arbitrary unit] replaced by a value. FIG. 1B is a diagram in which the potential decrease width (that is, the potential difference) at two consecutive points in FIG. 1A is plotted. Here, the potential decrease width ΔV can be calculated as follows.
ΔV = X (i) −X (i + 1) (1)

  In Expression (1), X (i) is the i-th sampling data, and X (i + 1) is the sampling data after one sampling of X (i). At point A in FIG. 1B, the potential decrease width ΔV exceeds the threshold Th for identifying two points that represent a change from the R wave to the S wave, and has a maximum value. By specifying this point A, the heartbeat can be detected. Since the potential decreases with the change from the R wave to the S wave, the potential decrease width ΔV at the point A becomes a positive value. Therefore, a positive value is set as the threshold Th.

  In FIG. 1A, the point A in FIG. 1B corresponds to the sampling data at the point B and the sampling data at the point C, which are representative of the change from the R wave to the S wave. It can be identified as sampling data of points. The time at which the straight line connecting the two consecutive sampling data and the potential L for calculating the heartbeat time can be set as the heartbeat time T of the heartbeat.

Similarly, FIGS. 2A and 2B show examples of calculating the heartbeat time when the amplitude of the S wave is small (FIG. 8B), and FIG. FIG. 2B is a diagram showing sampling data of the portion 101 in FIG. 8B, and FIG. 2B is a diagram plotting the potential decrease width at two consecutive points in FIG. Similarly, FIGS. 3A and 3B show examples of calculating the heartbeat time when the amplitudes of the R wave and the S wave are small (FIG. 8C), and FIG. FIG. 8 is a diagram showing sampling data of a portion 102 in FIG. 8C, and FIG. 3B is a diagram in which the potential decrease width at two consecutive points in FIG. 3A is plotted.
In the examples of FIGS. 2A, 2B, 3A, and 3B, the heartbeat time T is calculated in the same manner as in the examples of FIGS. 1A and 1B. Can be sought.

[Embodiment]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 4 is a block diagram showing a configuration of the heartbeat detecting device according to the embodiment of the present invention, and FIG. 5 is a flowchart for explaining the heartbeat detecting method according to the embodiment of the present invention. The heartbeat detection device includes an electrocardiograph 1, a storage unit 2, a detection unit 3, and a heartbeat time calculation unit 4.

  Hereinafter, the heartbeat detection method of the present embodiment will be described. Here, a procedure from detection of one heartbeat to calculation of the heartbeat time will be described. By repeating such calculation of the heartbeat time over the ECG waveform data period, time-series data of the heartbeat time is sequentially obtained, and an index of heartbeat variability can also be calculated from the time-series data.

  In the present embodiment, a data string obtained by sampling the ECG waveform is assumed to be X (i). i (i = 1, 2,...) is a number assigned to one sampling data. Needless to say, the larger the number i, the later the sampling time. In addition, a data string of the maximum value of the decrease width of the potential between two consecutive sampling data representing the change from the R wave to the S wave is P (k) (k = 1, 2,...).

  In addition, as described in FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 3A, and FIG. It is assumed that a decrease width threshold value for identifying sampling data of two consecutive points representing changes is Th, and a potential for calculating a heartbeat time is L. In the present embodiment, the decrease width threshold Th is updated every time a heartbeat is detected. Specifically, the reduction width threshold Th is calculated by multiplying the average of the maximum value of the reduction width of the potential between two consecutive sampling data by a coefficient r.

  The initial value of the decrease width threshold Th is, for example, a value of about 0.7 times the maximum value based on the maximum value of the decrease width of the potential between two consecutive sampling data in the first few seconds of the ECG waveform. It is appropriate to set to. The potential L may be an intermediate value between the potential of the R wave and the potential of the S wave having a typical amplitude, or a baseline value of the ECG waveform. The coefficient r (0 <r <1) is suitably about 0.5 to 0.7.

The electrocardiograph 1 measures an ECG waveform of a living body (human body) (not shown) and outputs a sampling data string X (i) of the ECG waveform. At this time, the electrocardiograph 1 adds the sampling time information to each sampling data and outputs it. Since a specific method for measuring an ECG waveform is a well-known technique, detailed description thereof is omitted.
The storage unit 2 stores the sampling data string X (i) of the ECG waveform output from the electrocardiograph 1 and information on the sampling time.

  The detection unit 3 detects a point (timing) at which the potential decrease width between two consecutive sampling data exceeds the decrease width threshold Th and becomes a maximum value. FIG. 5 shows a case where the mth heartbeat is about to be detected, starting from the nth data in the ECG waveform data string.

  First, the detection unit 3 sets a number (counter variable) i for sequentially reading the sampling data string X (i) to an initial value (here, n) (step S1 in FIG. 5). Next, the detection unit 3 reads the data X (i) and the next data X (i + 1) from the storage unit 2, and the potential decrease width ΔV (i) = X between the two consecutive data points. (I) -X (i + 1) is compared with the decrease width threshold Th (step S2 in FIG. 5).

  When the potential decrease width ΔV (i) = X (i) −X (i + 1) is equal to or smaller than the decrease width threshold Th, the detection unit 3 changes the R wave to the S wave in the vicinity of the data X (i). It is determined that there is no data of two points representing the change, i = i + 1 is set (step S3 in FIG. 5), and the process returns to step S2. Thus, the processes of steps S2 and S3 are repeated until the potential decrease width ΔV (i) = X (i) −X (i + 1) becomes larger than the decrease width threshold Th.

  When the potential decrease width ΔV (i) = X (i) −X (i + 1) is larger than the decrease width threshold Th in step S2, the detecting unit 3 converts the R wave to the S wave in the vicinity of the data X (i). It is determined that there are two points of data representative of the change, and the procedure moves to a procedure for obtaining the maximum value of the potential decrease width between the two consecutive points of data.

  First, the detection unit 3 sets the initial value of the maximum value P (m) of the potential decrease width between two consecutive data points of the m-th heart to be detected as X (i) −X (i + 1). Is stored (step S4 in FIG. 5), and a counter variable j for detecting the maximum value P (m) is set to 1 (step S5 in FIG. 5).

  The detection unit 3 reads the data X (i + j) and the next data X (i + 1 + j) from the storage unit 2, and the potential decrease width ΔV (i + j) = X (i + j) between the two consecutive data points. -X (i + 1 + j) and the maximum value P (m) are compared (step S6 in FIG. 5), and the potential decrease width ΔV (i + j) = X (i + j) −X (i + 1 + j) is equal to the maximum value P (m). If it is smaller, the value of the maximum value P (m) is not changed and the process proceeds to step S8.

  When the potential decrease width ΔV (i + j) = X (i + j) −X (i + 1 + j) is larger than the maximum value P (m), the detection unit 3 sets the value of the maximum value P (m) to X (i + j). Update to -X (i + 1 + j) and store it (step S7 in FIG. 5), and proceed to step S8. In step S8, the detection unit 3 sets a predetermined value jmax that specifies a range from when the potential decrease width ΔV exceeds the decrease width threshold Th until the maximum value P (m) of the decrease width ΔV is obtained, to the counter variable j Judge whether or not is exceeded.

  If the counter variable j does not exceed the predetermined value jmax, the detection unit 3 sets j = j + 1 (step S9 in FIG. 5) and returns to step S6. Thus, the processes of steps S6 to S9 are repeated until the counter variable j exceeds the predetermined value jmax.

  When the counter variable j exceeds the predetermined value jmax, the detection unit 3 ends the search for the maximum value P (m) of the potential decrease width ΔV, and the value 2 when the maximum value P (m) is updated to the latest value. The point data X (i + x) and X (i + 1 + x) are specified as data of two consecutive points representing a change from the R wave to the S wave (that is, P (m) = X (i + x) −X ( i + 1 + x), where x is any value between 0 and jmax), and a predetermined coefficient is added to the average value of the maximum values P (k) (k = 1, 2,..., m) detected after the start of measurement. The value multiplied by r is updated as the latest decrease width threshold Th (step S10 in FIG. 5). As described above, FIG. 5 illustrates that the m-th heartbeat is to be detected. Therefore, (m−1) maximum values have already been detected, and a new maximum value P (m ) Is detected, a total of m maximum values are obtained, and the average value of the m maximum values P (k) is multiplied by the coefficient r.

  Next, the heartbeat time calculation unit 4 calculates a heartbeat time T that is a time when the heart of the living body pulsates. Specifically, the heartbeat time calculation unit 4 has a straight line (or an extension thereof) connecting two points of data X (i + x) and X (i + 1 + x) on the ECG waveform detected by the detection unit 3. A time T at which the predetermined potential L intersects is calculated (step S11 in FIG. 5). As described above, the sampling time information corresponding to the data X (i + x) and X (i + 1 + x) is stored in the storage unit 2. The heartbeat time calculation unit 4 reads the information of the sampling time of the data X (i + x) and the sampling time of X (i + 1 + x) from the storage unit 2, and calculates the time T by a method such as linear interpolation from these two sampling times. That's fine.

Subsequently, when the time T is calculated in step S11, the heartbeat time calculation unit 4 determines whether or not the time T and the immediately preceding heartbeat time T _(-1) are separated by a certain time or more (step in FIG. 5). S12) If the time T is not separated from the immediately preceding heartbeat time T ₍₋₁₎ by a certain time or more, the calculated time T is discarded without being adopted as the heartbeat time, and the process proceeds to step S3.

Furthermore, the heartbeat time calculation unit 4 determines that the heartbeat interval (T−T ₍₋₁₎ ) when the time T calculated in step S11 is regarded as the heartbeat time is the previous heartbeat interval (T ₍₋₁₎ −T _{( -2))} ), it is determined whether it has increased by more than a certain rate (step S13 in FIG. 5), and the rate of increase in heart rate interval (T−T ₍₋₁₎ ) / (T ₍₋₁₎ −T ₍₋₂₎ ) Is greater than or equal to a certain value, it is determined that the heartbeat interval has increased by a certain percentage or more, and the calculated time T is discarded without being adopted as the heartbeat time, and the process proceeds to step S3.

The heartbeat time calculation unit 4 determines that the time T and the immediately preceding heartbeat time T _(-1) are separated by a certain time or more, and the rate of increase in heartbeat interval (T−T ₍₋₁₎ ) / (T ₍₋₁₎ − If T _(-2) ) is less than a certain value, the time T calculated in step S11 is adopted as the heartbeat time (step S14 in FIG. 5).

  After completion of step S14, i = i + 1 is set, and the process returns to step S2. As a result, detection of the next heartbeat is started. Alternatively, the value of i may be skipped by a number smaller than the minimum value of the heartbeat interval to be detected and corresponding to a certain fixed time, and the process may return to step S2. Thus, by repeating the processes of steps S2 to S14, time-series data of heartbeat time is obtained, and an index of heartbeat variability can be obtained from this time-series data.

  As shown in FIG. 6, by providing an FIR (Finite Impulse Response) filter 5 that performs high-pass filter processing on the sampling data string of the ECG waveform output from the electrocardiograph 1, the baseline of the ECG waveform is provided. It is also possible to remove the peristaltic motion and improve the reliability of heartbeat detection.

The heartbeat detection method of the present embodiment can be remarkably effective when applied to ECG induction that provides a large R wave and a deep S wave, for example, an ECG waveform of V3 to V5 induction. In addition, it is particularly preferable to apply to an ECG waveform of CC5 or similar guidance that is often used when taking an ECG waveform in daily life.
According to the heartbeat detection method of the present embodiment, an accurate heartbeat time data string can be obtained with high time resolution, and a highly accurate heartbeat fluctuation index can be obtained based on the data string.

  The storage unit 2, the detection unit 3, the heartbeat time calculation unit 4, and the FIR filter 5 described in this embodiment are a computer having a CPU (Central Processing Unit), a storage device, and an interface, and hardware resources thereof. It can be realized by a program to be controlled. The CPU executes the processing described in the present embodiment in accordance with a program stored in the storage device.

  The present invention can be applied to a technique for detecting a heartbeat of a living body.

  DESCRIPTION OF SYMBOLS 1 ... Electrocardiograph, 2 ... Memory | storage part, 3 ... Detection part, 4 ... Heartbeat time calculation part, 5 ... FIR filter.

Claims (8)

A detection step of detecting a point where the decrease width of the potential between two consecutive sampling data exceeds the decrease width threshold and becomes a maximum value from the sampling data of the electrocardiogram waveform of the living body;
The sampling data at the two consecutive points when the potential decrease width exceeds the decrease width threshold and reaches a maximum value, and the two points of sampling data representing the change from the R wave to the S wave of the electrocardiogram waveform. And a heart rate time calculating step for calculating a time at which a straight line connecting the two points of data on the electrocardiogram waveform intersects with a predetermined potential for determining the heart time as a heart rate time. Method. The heartbeat detection method according to claim 1,
The heart rate detection method according to claim 1, wherein the detecting step includes a step of updating the decrease width threshold based on an average of maximum values of the decrease width of the potential between the two consecutive data points. The heartbeat detecting method according to claim 1 or 2,
In the heartbeat time calculating step, when calculating a time at which a straight line connecting the two points of data on the electrocardiogram waveform intersects the predetermined potential, it is determined whether or not this time and the immediately preceding heartbeat time are more than a certain time apart. A heart rate detection method characterized by determining and not adopting the calculated time as the heartbeat time when it is not more than a certain time away from the immediately preceding heartbeat time. The heartbeat detecting method according to claim 1 or 2,
In the heartbeat time calculating step, when a time at which a straight line connecting the two points of data on the electrocardiogram waveform intersects the predetermined potential is calculated, a heartbeat interval when this time is regarded as a heartbeat time is a previous heartbeat A heart rate detection method characterized in that it is determined whether or not a certain rate has increased from the interval, and the calculated time is not adopted as the heart rate time when the heart rate interval has increased by a certain rate or more. Detecting means for detecting a point where the decrease width of the potential between two consecutive sampling data exceeds the decrease width threshold and becomes a maximum value from the sampling data of the electrocardiogram waveform of the living body;
The sampling data at the two consecutive points when the potential decrease width exceeds the decrease width threshold and reaches a maximum value, and the two points of sampling data representing the change from the R wave to the S wave of the electrocardiogram waveform. And a heart rate detection unit that calculates a time at which a straight line connecting the two points of data on the electrocardiogram waveform intersects with a predetermined potential for determining a heart rate as a heart rate time. apparatus. The heartbeat detecting device according to claim 5, wherein
The heart rate detection device, wherein the detection means updates the decrease width threshold based on an average of the maximum value of the decrease width of the potential between the two consecutive data points. The heartbeat detecting device according to claim 5 or 6,
The heartbeat time calculating means calculates whether or not a time when a straight line connecting the two points of data on the electrocardiogram waveform intersects the predetermined potential is longer than a predetermined time. A heart rate detection device characterized in that, when it is determined that the calculated time is not more than a predetermined time apart from the previous heart rate, the calculated time is not adopted as the heart rate. The heartbeat detecting device according to claim 5 or 6,
The heartbeat time calculating means calculates a time when a straight line connecting the two points of data on the electrocardiogram waveform intersects the predetermined potential, and a heartbeat interval when this time is regarded as a heartbeat time is a previous heartbeat A heart rate detection device that determines whether or not a certain rate has increased from an interval, and does not adopt the calculated time as the heart rate time when the heart rate interval has increased by a certain rate or more.

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