JPH0349500B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a period measuring device that measures the period of an electrical signal representing a biological signal, particularly a fetal heartbeat signal, using an autocorrelation method.

[Prior art] The autocorrelation method is to sample a heartbeat signal at an appropriate sampling period, calculate an autocorrelation function of the heartbeat signal based on the sampled data, and calculate the heartbeat signal from the calculated autocorrelation function. This method measures the The autocorrelation function indicates how similar the wave number of a heartbeat signal at a certain time is to the waveform at a time shifted by a certain time from that time. In other words, it indicates the degree of similarity between repetitive waveforms of heartbeat signals.

To explain this with reference to Figure 1, ``If the period is T, then the portion M <sub>1</sub> that is repeated in that period T,
It can be expressed as ``If it is moved by time (period) T on the time axis, it overlaps with the next succeeding portion _M2 with the highest precision.''

By the way, if a biological signal is expressed as f(t), the autocorrelation function φ(τ) is φ(τ)=lim ^T-- 1/2T ^T _-T f(t)・f(t+τ)dt...(1) It can be found by

If the data obtained by sampling the signal to be measured is f(k) (k=1, 2,...n), then the above formula
(1) is expressed as φ(τ)=1/n _o 〓 ^K=1 f(k)f(k+τ)...(2).

n is the number of times a heartbeat signal represented by an electrical signal is sampled in one sampling cycle, and k is a sampling ordinal number. One sampling cycle refers to the process of calculating one autocorrelation function value at a certain value of the phase difference variable τ by sampling n times.

When formula (2) is expanded, it becomes as follows.

φ(τ) = 1/n {f(1)f(1+τ)+f(2)f(2 +τ)+...+f(n)f(n+τ)}...(3) In other words, the time deviates by the phase difference τ It is expressed as the sum of the products of two data in .

In equation (1), T indicates the period of the signal.

In equations (1), (2), and (3), τ represents the time between a certain time of the heartbeat signal and a certain time that is shifted by a certain amount of time from that time. That is, τ is a variable that gives a phase difference to the biological signal f(t), and changes within a range that can be considered as one cycle of the signal.

By the way, if we consider the case of measuring the period of a fetal heartbeat signal using an autocorrelation method, we first start by sampling the heartbeat signal at a predetermined sampling period. As known from clinical experiments, the period of the fetal heartbeat signal is extremely wide, ranging from approximately 300ms to 1500ms. Therefore, conventionally, when measuring τ
was varied in the range of 300ms to 1500ms.
In reality, in the sampling method, if the sampling period is T _s , τ/T _s is used as τ, so
Since it is used as τ, τ is 300/T _s or
It will be varied within the range of 1500/T _s . The autocorrelation function obtained in this range has a peak when τ is the period T of the heartbeat signal and times 2T, 3T, etc., which are integral multiples thereof, so by detecting the peak corresponding to the period T, the heartbeat signal The period of can be found.

[Problems to be solved by the invention] However, when calculating the autocorrelation function with the same sampling period over a wide range, unnecessary calculations are also performed to calculate the period, which increases the time required for signal processing, and the real time This is not preferable in periodic measurements where processing is strongly desired. moreover,
Furthermore, by measuring over a wide range, there is a possibility that the measurement will be affected by noise.

This invention was made in view of the above-mentioned circumstances, and its purpose is to provide a period measuring device that is not affected by noise, eliminates unnecessary calculations, and can perform processing almost in real time. The goal is to provide the following.

Another object of the present invention is to detect peaks at each sampling point, limit the range in which the autocorrelation function is calculated, and change the sampling period depending on the periodic region to be calculated, thereby eliminating unnecessary calculations. , the purpose of the present invention is to propose a period measuring device in which the capacity of the correlation memory for storing the calculation results is reduced.

[Means for Solving the Problems] The present invention focuses on the fact that the predicted maximum number of changes in the fetal heartbeat signal is approximately within ±15 BPM, and calculates the autocorrelation function calculation range of the heartbeat signal per unit time. The sampling period was limited to the period corresponding to the latest heart rate±predicted maximum heart rate change, and the sampling period was changed according to the region to which the period belonged.

According to this invention, there is provided a sampling means for sampling an electrical signal representing a heartbeat signal at a predetermined sampling period, and an autocorrelation of the heartbeat signal using data obtained by the sampling means for each sampling. an autocorrelation function calculating means for calculating a function by sequentially changing a phase difference variable that gives a phase difference to the electrical signal over a predetermined change range; peak detection means for detecting a peak from a function; period calculation means for calculating a period of a heartbeat signal from an autocorrelation function of the peak detected by the peak detection means; and a heart rate corresponding to the period previously calculated by the period calculation means. is the latest heart rate per unit time, and the predetermined change range of the phase difference variable that is calculated by the autocorrelation function calculating means this time is the period corresponding to the latest heart rate per unit time±predicted maximum number of heart rate changes). calculation range setting means for setting a range; periodic region detection means for detecting to which region of pre-divided periodic regions each phase difference variable within the set calculation range belongs; There is provided a period measuring device comprising: sampling period changing means for changing the sampling period.

When the heartbeat signal is that of a fetus, the predicted maximum heartbeat change rate is usually ±15 PMB, but since it may vary more than that, it is preferable to set it to ±20 BPM with some margin.

However, the normal maximum value is approximately ±
Even if set to 15 BPM, it is possible to obtain sufficient accuracy for practical use, and in this way it is possible to further reduce calculation time, so if particularly high accuracy is not required, the maximum variation range is ±
You may also select 15BPM.

Further, it is preferable that the rate of change between each change stage of the sampling period be a constant ratio.

[Embodiments] Examples of the present invention will be described below with reference to FIGS. 2 to 4.

FIG. 2 is for explaining the case where the period measuring device according to the present invention is used to measure the heartbeat signal period of a fetus. The horizontal axis shows the heartbeat signal period, and the range indicated by the arrow is the heartbeat signal period. This shows the autocorrelation function calculation range in the signal period, that is, the range in which the phase difference variable τ is changed.

From the viewpoint of real-time processing, it is desirable to set the autocorrelation function calculation range of not only fetal heartbeat signals but also biological signals in general as narrow as possible without substantially reducing the accuracy of measurement data. In other words, it is desirable to define only the signal range that substantially affects the measurement results as the calculation range, and perform calculation processing only within this range in real time without substantially reducing the accuracy of the measurement data. It will be done.

Furthermore, if the calculation range is set unnecessarily wide, there is a possibility that it will be affected by noise. From this point of view as well, it is desirable to control the autocorrelation function calculation range so that the calculation is performed within a range that substantially affects the measurement results.

With this idea in mind, the present inventor has determined that the maximum number of changes in the fetal heart rate per unit time is within a certain predictable range of changes as described above, and therefore the range of changes in the phase difference variable τ. In other words, the autocorrelation function calculation range is (latest heart rate per unit time ±
This invention has devised a period measuring device that is provided with a control means for controlling the period within a range corresponding to the predicted maximum heart rate change rate. In other words, by varying this phase difference variable τ within the above time range and calculating this autocorrelation function, it is possible to process almost in real time without substantially reducing measurement accuracy. . To explain with reference to Figure 2, the horizontal axis shows the fetal heartbeat signal period, and the range indicated by the arrow shows the autocorrelation function calculation range, which is the calculation range in the period range of 300ms to 1500ms. is the time range corresponding to (the latest heart rate per minute ±20 BPM) as shown by the arrow range. In other words, the range in which the phase difference variable τ is changed is limited to the above-mentioned time range. As seen in this example, the predicted maximum number of changes in heart rate per minute is ± the maximum value of data obtained in clinical experiments.
It is preferable to select, for example, about ±20 BPM, which is a little larger than 15 BPM with some margin.
This makes it possible to prevent the accuracy of measurement data from decreasing due to calculation omissions.

In the embodiment shown in FIG. 2, in other words, the counting range of the autocorrelation function as described above is the phase difference variable τ
By controlling the range of change in calculation to be limited to the range that substantially affects the calculated cycle, there is no need to process a large amount of virtually meaningless data, and the correlation method cycle This greatly contributes to real-time processing, which is strongly desired from a practical viewpoint of measurement methods, and also greatly reduces the possibility of being affected by noise.

The period measurement method based on the correlation method as shown in FIG. 2 is achieved, for example, by a period measurement device having a configuration as shown in FIG.

The transducer 2 is placed, for example, on the abdomen W of a woman, detects the heartbeat signal of the fetus and generates a corresponding electrical signal. The electrical signal representing the heartbeat signal output from the transducer 2 is shaped into a waveform by the preprocessing circuit 3 connected thereto, and then sampled at a set sampling period by the sampling circuit 4 and converted into a digital signal. Analog-to-digital conversion (A-D conversion) is performed. The sampled data is stored in a data memory 6 connected to the sampling circuit 4. The data memory 6 is composed of a plurality of shift registers, and always stores the latest N shift registers, for example.
It stores 256 pieces of data. data memory 6
A multiplier 8 is connected to the multiplier 8, and an adder 10 is connected to the multiplier 8. The multiplier 8 and the adder 10 substantially calculate the autocorrelation function shown in equation (3) based on the data stored in the data memory 6, and apply the result to the correlation function connected to the adder 10. The data is stored in the memory 12. Therefore, the multiplier 8 and the adder 10 can be considered as an autocorrelation function calculation circuit for heartbeat signals.

A peak detector 14 is connected to the correlation memory 12, and the peak detector 14 detects a peak from the autocorrelation function data stored in the correlation memory 12.

A period calculation circuit 16 is connected to the peak detector 14, and the period calculation circuit 16 receives the peak detection signal from the peak detection circuit 14 and calculates the period of the heartbeat signal. The period T is expressed as a time corresponding to the value of the phase difference variable γ determined by the position of the obtained peak on the time axis, that is, T=γ×t (t _s is the sampling period). A heart rate calculation circuit 18 is connected to the period calculation circuit 16, and the heart rate calculation circuit 18 calculates the heart rate based on a signal indicating the period from the period calculation circuit 16. Heart rate calculation circuit 18 is connected to control circuit 20 . A display 22 comprising, for example, a light emitting diode (LED) is connected to the control circuit 20. The display 22 displays the heart rate of the heartbeat signal based on the signal output from the heart rate calculation circuit 18 via the control circuit 20. At this time, if the signal from the heart rate calculation circuit 18 contains a noise component or if the probe has become dislodged, the control circuit 20 will not input the signal from the heart rate calculation circuit 18 to the display 22. It is preferable to include an auxiliary means to control the heart rate in such a way as to prevent false heart rate display. However, since this is not directly related to this invention, detailed explanation will be omitted. The control circuit 20 is further connected to a calculation range setting circuit 24 that sets the change range of the phase difference γ, that is, the calculation range of the autocorrelation function. The calculation range setting circuit 24 is further connected to the multiplier 8 and the adder 10. A reference level detector 26 is further connected to the control circuit 20, and the reference level detector 2
6 is connected to the sampling circuit 4.

In the above configuration, the calculation range setting circuit 2
4 receives a signal representing the heart rate from the heart rate calculation circuit 18 and calculates the time corresponding to (the heart rate ±20 BPM). Then, a signal regarding the phase difference variable γ within this time range is output to the multiplier 8. In this case, the control circuit 20 controls the signal from the heart rate calculation circuit 18 to be within the calculation range when the signal from the heart rate calculation circuit 18 contains a noise component or when the probe is misaligned. Control is performed to prevent input to the setting circuit 24. The multiplier 8 reads two pieces of sampling data separated by the value of the phase difference variable γ indicated by the signal input from the calculation range setting circuit 24 from the data memory 6 and multiplies them. The calculation range setting circuit 24 further outputs a timing signal to the adder 10, and the adder 10 responds to the timing signal by combining the data calculated by the multiplier 8 with the correlation memory 12.
The previous autocorrelation function calculation value for the corresponding phase difference variable γ is read out and calculated. The addition result is stored again at a predetermined address in the correlation memory 12. Such calculations are performed every time data is sampled, and the autocorrelation function of the heartbeat signal is stored in the correlation memory 12.

As described above, the peak detector 14 detects a peak from the autocorrelation function stored in the correlation memory 12, and the period calculation circuit 16 generates a phase difference variable determined by the position of this peak on the time axis. The period of the heartbeat signal expressed by the time corresponding to the value of γ is calculated, and the heart rate calculation circuit 18 calculates the heart rate from the period.

Control circuit 20 further outputs a signal to reference level detector 26 at appropriate time intervals. The reference level detector 26 receives the signal from the control circuit 20 and detects the optimum reference level (zero level) when coding the sampled data. In detail, when the sampled data is coded, the more accurately the positive and negative balance of the data is maintained, the more clearly periodicity is expressed in the autocorrelation function curve, and the reference level detector 26
The optimum value of the reference level is determined by detecting the maximum value, minimum value, or average value of the data during sampling provided for this purpose.

In the embodiment shown in FIG. 3, the calculation range setting circuit 24 sets the change range of the phase difference variable γ, that is, the autocorrelation function range (latest heart rate per minute ±
By controlling the time range to a time range corresponding to 20BPM), it is possible to avoid sampling a large amount of virtually meaningless data and unnecessarily increasing calculation time, and also to prevent a substantial decrease in data accuracy. An unobtrusive periodic measurement is obtained.

By the way, from the perspective of wanting real-time processing, almost all
It is not desirable to sample with a uniform constant sampling period over the entire fetal heartbeat signal period of 300 ms to 1500 ms. In the heartbeat signal region with a short period, it is desirable to set a short sampling period and perform dense data detection from the viewpoint of achieving high-precision measurement, but on the other hand, in the heartbeat signal region with a long period, it is desirable to Since signal changes are not so sudden, setting a long sampling period does not substantially reduce the accuracy of the measured data; rather, the sampling period is set to the same period as the sampling period of the short-period heartbeat signal. In this case, a large amount of data is sampled unnecessarily, and the number of calculations increases meaninglessly, which greatly impedes real-time measurement. Furthermore, it may be affected by noise.

From this point of view, in addition to limiting the autocorrelation function calculation range, as shown in Figure 4, the sampling period is changed in stages in response to changes in the period of the heartbeat signal, which makes it virtually meaningless. It is even more preferable to eliminate data arithmetic processing.

Another reason for changing the sampling period in stages in response to changes in the period of the heartbeat signal is as follows. That is, the period is inversely proportional to the heart rate, so for example, as the heart rate decreases, the period becomes wider. For this reason, the time range corresponding to ±20 BPM becomes wider, and the sampling period must also be longer. In this way, since the time range corresponding to ±20 BPM also changes in response to changes in the heartbeat signal period, it is preferable to change the sampling period in response to changes in the heartbeat signal period.

As a specific means for this purpose, for example, a wide range of heartbeat signal regions is divided into several regions, and a sampling period of a corresponding size is determined according to the magnitude of the heartbeat signal in each region. A short sampling period is set for a region where the heartbeat signal period is small, and a long sampling period is set for a region where the heartbeat rate is low, that is, a region where the heartbeat signal period is large.

An example of setting the sampling period is explained with reference to Fig. 4. Two thresholds are set in the heartbeat signal period region.
TH ₁ and TH ₂ are determined, the fetal heartbeat signal period region is divided into three regions, and a different sampling period is set for each region. threshold
For example, TH ₁ and TH ₂ are each 600ms,
1000ms is defined, in this case the area,,
The ranges are 300−600ms, 600−1000ms, respectively.
It will be 1000−1500ms.

Assuming that the sampling periods in these regions are T _s −, T _s −, and T _s −, respectively, the relationship between the sampling periods must be as follows.

T _s −<T _s −<T _s − The setting of the sampling period T _s −・T _s − differs depending on the form of division of the region, , but if the region, , is set as in the above example,
300ms−600ms, 600ms−1000ms, 1000ms−
If set to 1500ms, the sampling period
For example, T _s − and T _s − are 5 ms and 7.5 ms, respectively.
It can be determined as 11.25ms.

In addition, when a change in the area occurs, when using the sampling data obtained from measurements in the previous area by correcting it to data with a period corresponding to the sampling period set in the new area,
In order to facilitate correction calculations, it is desirable that the rate of change in the sampling period between adjacent areas be constant. In particular, this fixed ratio is preferably a fixed ratio expressed as a fractional ratio, such as 3/2, 4/3, etc.

Note that although the number of sampling period change areas can be set arbitrarily, increasing the number too much is not preferable because it only becomes complicated. measurement target,
In consideration of accuracy, shortening of calculation speed, etc., it is appropriate to define, for example, about three regions as shown in the embodiment.

In the embodiment shown in FIG. 3, a region setting circuit 28 is provided in order to divide the entire period range of the heartbeat signal into, for example, three regions and to appropriately change the regions in response to changes in the period of the heartbeat signal. There is. The area setting circuit 28 is connected to the control circuit 20, the sampling circuit 4, and the period calculation circuit 16.

The area setting circuit 28 receives an area change instruction signal from the control circuit 20 and changes the area. The control circuit 20 receives a signal indicating the heart rate from the heart rate calculation circuit 18, calculates the period corresponding to the heart rate, and outputs a signal indicating the region to which the period belongs. Therefore, when the period of the calculated heartbeat signal exceeds the period range in the currently set area, the control circuit 20 sets a signal indicating a new area of the period range to which the period belongs. Output to circuit 28. For example, the current cycle range is
When a region defined as 300ms-600ms is set and measurement is performed in the region, the period corresponding to the heart rate of the signal obtained from the heart rate calculation circuit 18 changes from, for example, 590ms to 610ms. In such a case, change the measurement area from, for example, an area with a sampling period of 5ms to, for example, 600ms-
Outputs a signal instructing a change to the area that defines the period range of 1000ms. In response to this change instruction signal, the area setting circuit 24 outputs a sampling period change signal to the sampling circuit 4, and changes the sampling period in the sampling circuit 4 to the sampling period preset for the area, for example, 7.5 ms.
Change to In this way, when the period corresponding to the measured heart rate exceeds the predetermined period range in the set area, the area is changed and the sampling period is changed to the preset period in the new area. be done.

The area setting circuit 28 also outputs a signal indicating the sampling period determined in the set area to the period calculating circuit 16. Period calculation circuit 16
receives the peak detection signal from the peak detector 14 and calculates the period T=t×γ of the heartbeat signal expressed by the time corresponding to the value of the phase difference variable γ determined by the position on the time axis of the peak. Find _s (τ _s is the sampling period).

As described above, the sampling period is changed by changing the area, and the period of the heartbeat signal is calculated.

[Effects of the Invention] As described above, according to the present invention, in the period measurement of a heartbeat signal, the change range of the phase difference variable for the current calculation of the autocorrelation function is determined according to the latest period detected previously. , the period that substantially affects the calculated period, for example, the period corresponding to (the latest heart rate per unit time ± 20 BPM), and the period in which the phase difference variable is divided in advance. By changing the sampling period according to the region it belongs to, for example, increasing the sampling period in regions where the phase difference variable is large, it is possible to avoid sampling large amounts of virtually meaningless data, wasteful calculations, and storing correlation memory. This makes it possible to omit capacity, eliminate the risk of being affected by noise, and reduce calculation time without substantially reducing data accuracy, making it virtually possible to process in near real time. We can provide a period measuring device that can

Furthermore, by changing the sampling period in stages at a constant rate, old data can be corrected and used as new data, thereby making continuous measurement possible.
A period measuring device that processes almost in real time can be provided.

[Brief explanation of drawings]

FIG. 1 is a heartbeat signal wave diagram used to explain period measurement using the autocorrelation method, and FIG. 2 is a diagram showing a case where the period measurement device according to the present invention is applied to measurement of the heartbeat signal period of a fetus. 3 is a diagram schematically showing the configuration of an embodiment of the period measuring device of the present invention in the form of a block diagram, and FIG. 4 is a diagram showing changes in the period of the heartbeat signal in the period measurement of the present invention. FIG. 3 is a diagram for explaining a method of changing the sampling period in stages in response to the change in the sampling period. In the figure, 2...transducer, 3...preprocessing circuit, 4...sampling circuit, 6...data memory,
8... Multiplier, 12... Correlation memory, 14... Peak detector, 16... Period calculation circuit, 18... Heart rate calculation circuit, 20... Control circuit, 22... Display, 24... Calculation range setting circuit, 26... Reference level Detector, 28...
Area setting circuit, T-, _Ts- , _Ts -...sampling period.

Claims (1)

[Claims] 1. Sampling means for sampling an electrical signal representing a heartbeat signal at a predetermined sampling period, and an autocorrelation function for the heartbeat signal using data obtained by the sampling means for each sampling. from the autocorrelation function calculated over the predetermined change range of the phase difference variable by sequentially changing a phase difference variable that gives a phase difference to the electrical signal over a predetermined change range; peak detection means for detecting a peak; period calculation means for calculating a period of a heartbeat signal from an autocorrelation function of the peak detected by the peak detection means; and a unit of heart rate corresponding to the period previously calculated by the period calculation means. As the latest heart rate per unit time, the period range corresponding to the predetermined change range of the phase difference variable when the autocorrelation function calculation means calculates (the latest heart rate per unit time ± predicted maximum number of changes in heart rate) calculation range setting means for setting the calculation range, periodic region detection means for detecting to which region of the periodic regions divided in advance each phase difference variable within the set calculation range belongs; A period measuring device comprising: sampling period changing means for changing a sampling period. 2. The period measuring device according to claim 1, wherein the heartbeat signal is a fetal heartbeat signal, and the predicted maximum heartbeat change rate is 20 BPM. 3. The period measuring device according to claim 1, wherein the heartbeat signal is a fetal heartbeat signal, and the predicted maximum heartbeat change rate is 15 BPM. 4. The period measuring device according to claim 1, wherein the rate of change between each change stage of the sampling period is a constant ratio.