RU2219828C2 - Method and device for detecting cardiac cycle onset - Google Patents

Abstract

FIELD: medicine; medical engineering. SUBSTANCE: method involves twice differentiating electrocardiogram signal filtered from high frequency noise and comparing the obtained signal at two threshold levels. One level is set Δ higher than the zero line and the other one Δ lower than the zero line. Double- differentiated signal amplitude being found between the threshold levels, clock pulses counting begins. Given number N being reached, position of the last pulse from the counted ones (that is number N) on time axis is to be taken as the next cardiac cycle onset. The double-differentiated signal amplitude being found beyond the threshold levels earlier than the number N is achieved, counting is started once more. The number N is selected in a way that when counting the value could be reached only on electrocardiogram signal interval between T and P teeth. The device has in series connected filter and two differentiating units, two threshold level sources, two comparator units, clock pulse oscillator, the first, the second, the third AND- gate and pulse counter. Filter input is device input. EFFECT: high reliability in determining cardiac cycle onset notwithstanding the cardiac signal teeth deviations from norm. 2 cl, 2 dwg

Description

 The invention relates to medicine, in particular to electrocardiography, and can be used to determine the beginning of the next cardiocycle in the analysis of time parameters of an electrocardiogram (EX), performed by both hardware and software.

 Currently, the most common methods for isolating the beginning of a cardiocycle are based on the detection of a QRS complex (allocation of R-R intervals).

 A known method of allocating R-R intervals [1], which consists in performing the following sequence of actions. The EKS are filtered, differentiated, two-half-wave rectification is performed to eliminate the omission of the QRS complex with a negative R wave, are detected using a peak detector, the comparison threshold is formed from the detected signal, with which the EKS is compared, the moment of comparison of the EKS with the threshold level is taken as the beginning of the next RR interval ( cardiocycle).

This method has the following disadvantages:
1) in some cases, the amplitude of the R wave of the QRS complex can be comparable with the amplitude of the T wave (this was detected in the first standard lead even in patients with a normal electrocardiogram), which complicates reliable isolation of the QRS complex,
2) in cardiograms with a split R wave, the reliability of isolating the beginning of the cardiocycle is reduced.

 Closest to the proposed method (prototype) is a method for determining the beginning of a cardiocycle. implemented in the device [2], namely, that the ECS is differentiated to sharpen the QRS complex, a threshold level is formed from the received signal, and the differentiated signal is compared for the first time with a threshold level and for the second time with the result of the first comparison. According to the results of comparisons, a “time window” is formed, which allows to exclude false selections of the QRS complex.

This method has the following disadvantages:
1) reliable selection of the QRS complex can be achieved only with reliable selection of the first (starting) QRS complex;
2) the effect of low-frequency additive interference on the ECS (polarization of electrodes, respiratory waves, artifacts, etc.) reduces the reliability of distinguishing the beginning of a cardiocycle; an attempt to remove the low-frequency additive interference with a high-pass filter will not give a result, since the cutoff frequency of such a filter should be low (0.05 Hz in accordance with the standards for electrocardiographs), and the action of artifacts leads to a transient process that lasts longer the lower the cutoff frequency of the filter, during the transition process, the reliability of the allocation of the QRS complex decreases.

 The proposed method allows to eliminate these disadvantages and to ensure reliable selection of the beginning of the next cardiocycle.

 Analysis of electrocardiograms with various deviations from the norm showed that when taking an electrocardiogram in a clinic or hospital, that is, when the patient is at rest, the most stable section of the electrocardiogram is the part of the contour between the T and P waves. Two other areas lying on the contour can be distinguished on the ECS : segment PQ - segment from the end of tooth P to the beginning of tooth Q, segment ST - segment from the end of the QRS complex to the beginning of tooth T. Comparison of the durations of these segments shows that in a calm state ienta duration TP segment substantially exceeds the length of the segment PQ and the segment ST. The indicated ratios are preserved under the effect of additive low-frequency noise on the EX. This circumstance is the basis of the proposed method for isolating the beginning of the cardiocycle.

 The essence of the proposed method is as follows. Purified from high-frequency interference and interference of industrial frequency, the electrocardiogram is differentiated twice and the received signal is compared with two threshold levels. One level is set at Δ above the zero line, and the other at Δ below the zero line. Given the temporal and amplitude relationships between the various parts of the electrocardiogram and choosing the corresponding differentiation constants, the level values can be set equal to the amplitude of the P wave. If the amplitude of the twice differentiated signal is between threshold levels, they start counting clock pulses, the repetition rate of which can be chosen equal to the sampling frequency generally accepted in electrocardiography 500 imp / s. When the specified number N is reached as a result of counting, the position on the time axis of the last of the counted, that is, the N-th pulse, is taken as the beginning of the next cardiocycle. If the amplitude of the twice differentiated ECS goes beyond the threshold levels earlier than when the number N is reached, then the counting starts anew.

 The number N is chosen such that the values of N in the calculation could be achieved only on the segment of the EX between the T and P teeth.

 The proposed method allows you to reliably distinguish the beginning of each cardiocycle regardless of possible deviations from the norm of the parameters (shape, amplitude, duration) of the teeth of the cardiosignal, in particular the QRS complex, and the isoline drift due to the action of additive low-frequency interference on the ECS (the effect of breathing, artifacts, time drift etc.), which helps to improve the conditions of the subsequent processing of the cardiosignal (calculation of the time parameters of individual elements of the cardiosignal, calculation of the duration of cardiac cycles, etc.).

 In FIG. 1 is a structural diagram of a device for implementing the proposed method for isolating the beginning of a cardiocycle, and FIG. 2 is a timing diagram explaining the operation of the device.

 To achieve a technical result and implement the proposed method into a device containing a series-connected filter, the input of which is the input of the device, and a differentiating unit, a threshold level source and two comparators, the first inputs of which are connected to the output of the differentiating unit, and the second input of the first comparator is connected to the output a threshold level source, a second differentiating block, a second threshold level source, a clock generator, the first, second and third circuits And, with a pulse counter, and the input of the second differentiating unit is connected to the output of the first differentiating unit, and the output is connected to the inverting input of the first comparator and to the non-inverting input of the second comparator, the first threshold level source + Δ is connected to the non-inverting input of the first comparator, and connected to the inverting input of the second comparator the second source of the reference level is Δ, the output of the first comparator is connected to the first input of the first circuit And, and the output of the second comparator is connected to the second input of this circuit, the output the circuit And is connected to the first input of the second circuit And and to the input "Zero setting" (R) of the pulse counter, the second input of the second circuit And is connected to the output of the clock generator, the output of the second circuit And is connected to the input "Count" (C) of the pulse counter whose bit outputs are connected to the corresponding inputs of the third AND circuit, the output of which is the output of the device.

 A device for implementing the proposed method for isolating the beginning of the cardiocycle contains a filter 1, first 2 and second 3 differentiating blocks, first 4 and second 5 comparators, first 6 and second 7 threshold level sources, 8 clock pulse generator (GTI), pulse counter 9, first 10 , second 11 and third 12 of scheme I.

 The input of the filter 1, which is the input of the device, receives an electrocardiogram. The output of the filter is connected to the input of the first differentiating unit 2, the output of which is connected to the input of the second differentiating unit 3, the output of which, in turn, is connected to the inverting input of the first comparator 4 and to the non-inverting input of the second comparator 5, and the source is connected to the non-inverting input of the first comparator 4 6 threshold level + Δ, and the source 7 of the threshold level -Δ is connected to the inverting input of the second comparator 5, the output of the first comparator 4 is connected to the first input of the first circuit And 10, and the output of the second comparator RA 5 - with the second input of this circuit, the output of the first circuit And 10 is connected to the first input of the second circuit And 11 and to the input "Setting zero" (R) of the counter 9, the second input of the circuit And 11 is connected to the output of the GTI 8, the output of the circuit And 11 connected to the input "Account" (C) of the counter 9, the bit outputs of the counter 9 are connected to the corresponding inputs of the third circuit And 12, the output of which is the output of the device.

 The device operates as follows. The electrocardiogram, cleaned by the filter 1 of high-frequency and network noise (signal "EX + LF interference" in Fig. 2), is differentiated twice in series connected blocks 2 and 3 (signal "Second derivative" in Fig.2). Comparators 4 and 5 compare the signal amplitude of the second derivative, respectively, with a positive threshold level and a negative threshold level (levels + Δ and -Δ in figure 2). As noted above, with corresponding differentiation constants, the values of the levels can be set equal to the amplitude of tooth P. For example, those shown in FIG. 2 time diagrams are obtained at differentiation constants τ1 of the first differentiating block 2 and τ2 of the second differentiating block 3 for 30 ms. If the amplitude of the signal of the second derivative is between threshold levels, high level signals are set at the outputs of the comparators, otherwise low level signals (signals "K4" and "K5" in figure 2) are set. The AND 10 circuit is an OR circuit for low level signals, therefore, a low level signal appears at the output of the And 10 circuit every time when the amplitude of the ECS samples exceeds threshold levels (signal "Set 0" in Fig. 2). This signal is fed to the “Zero setting” input of counter 9 and sets the latter to the initial zero state whenever the amplitude of the second derivative signal exceeds threshold levels. When the amplitude of the signal of the second derivative is between threshold levels, a high level signal is present at the output of circuit And 10, which is fed to the first input of circuit And 11 and allows the passage of its clock pulses from the output of the clock generator 8. The pulses transmitted to the output of the circuit And 11 (signals "Count" in figure 2) are considered the counter 9. The corresponding bit outputs of the counter 9 are connected to the inputs of the circuit And 12. In FIG. Figure 2 shows, as an example, the output signals of a four-digit binary counter (signals "Counter outputs"). The inputs of the circuit And 12 are connected to the outputs of the counter 9 so that the signal at the output of this circuit appears only at that moment in time when the counter will count a certain predetermined number N of pulses (in the example in Fig. 2 this number is 12). The number N is selected based on the a priori known clock repetition rate from the output of the generator 8 and the durations of the intervals PQ, ST and TP so that it can be reached by the counter 9 only on the TP interval. The beginning of the pulse at the output of circuit And 12 is taken as the beginning of the next cardiocycle.

 The technical and economic effect of the proposed method and device for its implementation is to increase the reliability of distinguishing the beginning of each cardiocycle, regardless of possible deviations from the norm of the parameters (shape, amplitude, duration) of the teeth of the cardiosignal, in particular the QRS complex, and the drift of the contour due to the action on the ECS additive low-frequency interference (breathing effects, artifacts, temporal drift, etc.). Reliable identification of the beginning of each cardiocycle improves the conditions for subsequent processing of the cardiosignal (calculating the time parameters of individual elements of the cardiosignal, calculating the duration of the cardiocycles, etc.).

Sources of information
1. Podlepetskiy B. I., Torubarov S. V. Analysis of metrological and operational characteristics of microelectronic QRS detectors // Measuring equipment, 1990, 7, p. 50, 51.

 2. Patent 2021753, A 61 B 5/046. Device for highlighting QRS-complexes of an electrocardiogram / B.I. Kruk, N.I. Belkin // BI 1994, 20.

Claims (2)

1. The method of isolating the beginning of the cardiocycle, namely, that the electrocardiogram is filtered, differentiated and the received signal is compared with threshold levels, characterized in that the differentiation is carried out twice, and the comparison of the received signal of the second derivative is carried out with threshold levels separated by a distance of + from the zero line Δ and –Δ, then, provided this signal is located between threshold levels, the clock pulses are allowed to pass and these pulses are counted, if, as a result of counting, the specified of the Nth number, take the position of the Nth clock pulse as the time axis of the next cardiocycle, and if the second derivative signal goes beyond threshold levels before the N number is reached, the counting starts again from zero. 2. A device for isolating the beginning of a cardiocycle containing a series-connected filter, the input of which is the input of the device, and a first differentiating unit, a first threshold level source and two comparators, characterized in that it is equipped with a second differentiating unit, a second threshold level source, a clock pulse generator , the first, second and third circuits And and a pulse counter, and the input of the second differentiating unit is connected to the output of the first differentiating unit, and the output is connected to invert the first input of the first comparator and the non-inverting input of the second comparator, the first source of the threshold level is connected to the non-inverting input of the first comparator, and the second source of the reference level is connected to the inverting input of the second comparator, the outputs of the first and second comparators are connected respectively to the first and second inputs of the first circuit And, the output of which is connected to the first input of the second circuit And and to the input of setting the zero of the pulse counter, the second input of the second circuit And is connected to the output of the clock c, and the output is connected to the input of the pulse counter account, the bit outputs of which are connected to the corresponding inputs of the third AND circuit, the output of which is the output of the device.

Images