Method and device for eliminating electrocardiogram signal isoline drift

FIELD: medicine; medical engineering.

SUBSTANCE: method involves selecting reference point in every cardiac cycle on TP-segment. Values of neighboring N=2n+1 reference points also belonging to TP-segment are recorded, n=1,2,…, beginning from the first reference point. Other reference points are set to zero. The central reference point value is left without changes in a group of 2n+1 member. Reference point values of each of n pairs of reference points symmetrically arranged relative to the central reference point are scaled relative to condition Uj=U0Kj, where U0 is the central reference point amplitude, Uj is amplitude of j-th reference point pair, j=1,2,…,n is the number of each reference point pair relative to the central reference point, Kj is the scaling coefficients determined from received signal suppression condition of the first n spectral zones in spectrum. The so formed electrocardiogram signal reference point groups sequence is let pass through lower frequency filter with isoline drift signal being obtained being produced on output. The signal is amplified and subtracted from the initial electrocardiogram signal that is preliminarily delayed for lower frequency filter delay time. Device has the first lower frequency filter, discretization unit and unit for selecting anchor reference points connected in series, as well as subtraction unit, unit for saving N reference points, scaling unit, the second lower frequency filter, amplifier and delay unit. Output of the unit for selecting anchor reference points is connected to the first input of memory unit the second input of which is connected to discretization unit output. Each of N memory unit outputs is connected to one of N inputs of scaling units. Scaling unit output is connected to the second lower frequency filter input which output is connected to amplifier input. Amplifier output is connected to the first input of subtraction unit, the second output of subtraction unit is connected to delay unit output. Its input is connected to output of the first lower frequency filter. Subtraction unit output is the device output.

EFFECT: reliable removal of isoline drift.

2 cl, 8 dwg

 

The invention relates to medicine, in particular to electrocardiography, and can be used when processing electrocardiogram (EX).

Drift contours can be caused by the action of the FORMER additive low-frequency noise (polarization of the electrodes, the influence of breathing artifacts, the time drift and the like).

The most common currently Troubleshooting drift contour-based filtering low frequency noise and interpolation restoring signal drift contour with its subsequent subtraction from the original EX.

Low-frequency noise reduction by using high-pass filters leads to a distortion of low-amplitude elements of the FORMER, such as ST-segment. In addition, the effect of artifacts leads to the transition process, which lasts the longer, the lower the cutoff frequency of the filter, i.e. results in a drift of the contour.

There is a method of compensation of the drift contours [1], namely, that in every cardiac cycle release point, take for isoelectric. The parameters of the amplitude of the investigated elements EX (R-wave, ST-segment, and so on) is defined as the difference between the measured amplitude at a particular point and amplitude points adopted for isoelectric.

This method has the following disadvantage. Full compensation of the drift contour POS is on just in case when this drift is constant within a single cardiac cycle offset above or below the actual contour. Usually drift contour real EX has the form of a sine wave (respiratory waves) or the exponent (transition), therefore, the described method cannot be compensated for completely.

There is a method of compensation of the drift contours [2], taking into account the change of signal drift contours within the cardiac cycle. In this way the EX discretizing, in each cardiac cycle emit a reference point (counts), located on the PQ segment, on these points receive approximating functions, including the angular coefficients of the lines drawn between adjacent P-Q points. Next, the offset value y(i) EX, free from the effects of signal drift contour lines are in accordance with the formula y(i)=x(i)-m-k(i-ipq), where i is the number of discrete reference EX; x(i) is the i-th reference; m - P-Q points of the current cardiac cycle; k is the slope of a line connecting adjacent P-Q points; (i-ipq) is the distance of the i-th starting from the corresponding P-Q point.

This method has the following disadvantages:

1) even when sinus rhythm is not always in the FORMER there is an explicit segment PQ, lying on the contour lines, which complicates the selection of reference points;

2) the proposed method can be used to compensate for drift contours only very low on the alignment with the heart rate (HR) frequency when the fair replacement value of the function value of its argument.

Closest to the proposed method (prototype) is the solution of the drift contours [3], which consists in the fact that the source of the FORMER, representing a mixture of (amount) directly cardiomegaly and low-frequency noise (drift contours), distinguish, as in the method [2], reference point, located on the PQ segment is measured at these points the values of the mixture of the FORMER and low-frequency noise (drift contours)obtained values of calculated parameters (coefficients) of the interpolating spline functions and build this function at all points of the processed EX, then subtract the values of the spline function from the original EX.

This method has the following disadvantages:

1) as well as in the method [2], not always in the FORMER there is an explicit segment PQ, the sequence of which are required to build a spline function;

2) with increasing frequency drift contour degrade the accuracy of the recovery signal drift contour spline function, and when you reach half of the heart rate recovery drift contour becomes impossible and, therefore, it is impossible elimination;

3) the delay of the recovered signal drift contour relative to the original signal EX didn't kompensirovat the ü drift contours EX.

The proposed method allows to eliminate the mentioned disadvantages.

The technical result of the invention is to extend the functionality of the method and of the device by the reliable elimination of drift contour lines at frequencies up to the frequency of cardiac contractions and even above it.

Analysis of electrocardiograms with various deviations from the norm have shown that when the electrocardiogram in conditions of clinic or hospital, that is, when the quiescent condition of the patient, the most stable site electrocardiograma is part of the contour between the teeth T and R. In the FORMER emit [4] two other areas that lie on the contour: the segment PQ is a line segment from the end, and P wave to the beginning of the Q wave; the ST segment is cut from the end of the ORS complex before the start of the prong So the Segment PQ reflects the physiological delay transmission of the excitation pulse and the norm can be isoelectric or lobotrachelini. The ST segment characterizes the period of full coverage excitation of the ventricles, so on electrocardiogram at this point is logged contour. Abnormalities in the myocardium shifts the segment above or below the contour. Period of TR corresponds to the electrical diastole of the heart. If the FORMER applies low-frequency additive disturbance, manifested in the form of drift contours, interval TR is present only signal this is amechi.

The essence of the proposed method consists in the following. Purified from high-frequency noise and interference frequency electrocardiogram, which is the initial signal for further processing, discretizing, in each cardiac cycle on the interval TR EX allocate the reference count and, starting from this reference, remember the value of N=2n+1 samples, n=1, 2,..., lying on the interval TR, other times within the cardiac cycle equal to zero, the value of the center of reference thus formed group of (2n+1) times, leave unchanged, and the values of samples of each of the n pairs of samples arranged symmetrically about the Central reference scale in accordance with the condition Uj=U0Kjwhere U0the amplitude of the Central reference Ujthe amplitude of the samples of the j-th pair of samples, j=1, 2,..., n is the number of each pair of samples relative to the Central referencej- scale factor, defined as the solution of a system of equations

where τ - the duration of the countdown EX,

T is the repetition period of the reference samples.

Next, the resulting sequence of groups of samples is passed through Phil is Tr the lower frequencies. The output signal from the lowpass filter drift amplify the receiving signal contours, this signal is subtracted from the original signal FORMER, pre-delay on time delay of the lowpass filter.

The proposed method allows to eliminate the drift contours EX when changing frequency last from zero to frequencies comparable with heart rate and even higher, which improves conditions for processing cardiomegaly (calculation of time and amplitude parameters of individual elements of cardiomegaly).

Explain the principle of achieving a technical result by performing the above proposed actions electrocardiogram. Any reference mixture of the FORMER and low-frequency noise in the form of drift contours (Fig 1,a), selected on the interval TR of cardiomegaly is a reference interference, because the period of TR corresponds to the electrical diastole of the heart. Thus, we get the amplitude (pulse width modulation) signal interference (figure 1,b). The spectrum of the interference signal with amplitude and pulse modulation provided V.A. Kotelnikov theorem, i.e. the repetition frequency of samples Fo=1/T, where T is the sampling period, more than twice the maximum frequency interference Fi, shown in Fig 2,and. In the spectrum contains components at frequency Fi interference and spectral zone at repetition frequencies from the free period of Fo 2Fo, 3Fo, etc. Each spectral zone contains side components kFo±Fi, where k=1, 2,... is the number of spectral zone. With increasing signal frequency drift contour component at the frequency of the interference shifts in the spectrum to the right along the frequency axis, and the left-side component of the first spectral zone shifts to the left. Violation of the conditions of theorem V.A. Kotelnikov frequency left-side component becomes smaller component at frequency interference (figure 2,b), and the selection signal drift contour methods spline approximation becomes impossible.

Let's show how we can distinguish the signal of low-frequency additive noise (drift contours) interference from a mixture of FORMER and interference even when the excess frequency noise half of the heart rate, which in this case determines the sampling frequency interference.

If the reference signal interference, located on a segment of TR EX, add a couple of samples, one of which is adjacent to the original reference on the left, and the other symmetrically at right), the spectrum of this new signal, each sample of which now consists of three pulses, it is possible to suppress any spectral zone when the condition

where k is the number of suppressed spectral zone

τ - the pulse width of the reference and additional impulses

T is the repetition period atsche the s.

For practical reasons it is advisable to set convenient to implement value τ (usually this value to an integer times smaller than the sampling period T) and solve the equation for U1.

If you want to suppress another spectral zone, for example, with m, then you want to add to the existing three pulses a couple of pulses, one of which is adjacent to the left of the leftmost pulse, and the other is adjacent to the right to the far right impulse. Each reference signal is now represented by a group of five times. To suppress this signal spectral zones with numbers k and m, it is necessary to solve a system of two equations

Is the Central reference U0, which is the original, known, so the system is solved on the basis of the above considerations U1and U2.

In the General case the number of equations in the system is equal to the number of suppressed spectral zones, for example n, the number of pulses representing each signal sample is equal to 2n+1, and the number of terms in each equation is n+1. If the suppressed spectral zones numbered 1, 2,..., n, then the system of equations takes the form

As in the previous cases, the system is solved in terms of Uj, j=1, 2,..., n, and denotes the number of newly added pairs of samples arranged symmetrically about the Central reference. In the spectrum of such a signal will be suppressed n first spectral zones, but saved zero spectral area containing component interference (figure 2). The number n is suppressed spectral zones is selected so that the frequency of the left-side component of the (n+1)-th spectral area was larger than the frequency of the interference. Under this condition component at the frequency of the interference can be distinguished from the total signal using a lowpass filter. Because processing electrocardiograma to change the values of its samples is possible only with the help of scaling operations, we introduce the concept of scale factor Kj=Uj/U0let write the above system of equations in the form

This system of equations is solved relative to Kj.

Figure 3 shows the structural diagram of a device for the implementation of the proposed solution drift contours of electrocardiograma, figs.4 - var the ant execution unit 3, figure 5 - performance of the comparison circuit 13, 6 and 7 of the notes to the block 3, and Fig - time diagrams explaining the operation of the device.

To achieve the technical result and the implementation of the proposed method in a device containing connected in series lowpass filter, whose input is an input of the block sample rate and block allocation reference counts, and the subtraction unit, put the unit memorizing values of N=2n+1, n=1, 2,..., counts, a scaler, a second low pass filter, an amplifier and a delay unit. The output of block allocation reference counts connected to the first input (Manager) of the memory block, the second input is (information) is connected to the output of block sampling, each of the N outputs of the memory block is connected to one of N inputs of block scaling, the output of the scaler is connected to the input of the second low pass filter, the output of which is connected to the input of the amplifier, the amplifier output is connected to the first input of the subtraction unit, the second input of the subtraction unit is connected to the output of the delay unit, the input of which is connected to the input sample rate, the output unit subtracting an output device.

A device for the implementation of the proposed solution drift contour contains the first low pass filter 1, the block sampling 2, BL is to highlight the reference count 3, block memory 4, a scaler 5, the second low pass filter 6, the amplifier 7, the subtraction unit 8 and the delay unit 9.

To the input of the filter 1, which is the input device is electrocardiogram. The output of the filter is connected to the input of the sample 2 and the input of the delay unit 9, the output of block sampling 2 is connected to the input of the block selecting anchor reference 3 and with the second input of the memory 4, the first input of which is connected to the output of the block selection reference reference 3, each of the N outputs of the block memory 4 is connected to one of N inputs of the scaler 5, the output side of the scale 5 is connected to the input of the second low pass filter 6, the output of which is connected to the input of the amplifier 7, the output of the amplifier 7 is connected to the first input of the subtraction unit 8, the second input of the subtraction unit 8 is connected to the output of the delay unit 9, the input of which is connected to the input sample rate, the output of the subtractor 8 is an output device.

The device operates as follows. Electrocardiogram, purified by the filter 1 from the high-frequency and network interference, but containing low-frequency noise in the form of drift contour lines (shown by the solid line in Fig 4,a and denoted by EX), is converted to the unit of sampling in discrete samples, which represent a sequence of rectangular pulses of duration C4; , modulated in amplitude in accordance with the type of electrocardiogram, following with sampling period T. This sequence of pulse counts to the input of the block selecting anchor reference. One of the possible embodiments of block 3 is shown in figure 4. It contains the block 10 forming the differences of the second order, a source of positive 11 and negative 12 threshold level, the comparison circuit 13, the generator 14 clock pulses, the first schema And 15, the pulse counter 16 and the second scheme And 17.

With unit output sample rate 2 discrete counts electrocardiogram (EX) is fed to the input of the shaper differences of the second order 10, which is the input unit 3 selection of the reference samples. The output of block 10 are signals of the differences of the second order, generated from three consecutive samples EX

ddUi=Ui-2Ui-1+Ui-2,

where i is the number of samples involved in the formation of ordinary differential second order,

U is the amplitude of the corresponding reference.

The received signals of the differences of the second order are compared by the comparison circuit 13 with two threshold levels, one of which is positive +Uop (output of block 11), and the second negative-Uop (exit block 12). The comparison circuit 13 can be performed (figure 5) on the basis of two Comparators 18 and 19 and diagrams OR 20.

RAB is the unit 3 selection of the reference samples, made in schemes 7 and 4 simulated using circuit simulation MicroCAP5. When modeling was used 6-bit pulse counter. The simulation results presented in Fig.6 and 7.

When the signal difference of the second order is between the threshold levels (6), the signal at the output of the comparison circuit 13 has a potential of high level (Fig.7,a), which is fed to the input R of the counter 16, allowing his work in the counting mode, and one of the inputs of the first circuit And 15, allowing the passage at its output received at the other input of clock pulses from generator 14. The repetition frequency of clock pulses equal to the sampling frequency of electrocardiograma.

Clock pulses from the output of the circuit 15 And arrive at the counting input of the counter 16 (Fig.7,b), which provides an account of these pulses. The respective bit outputs of the counter 16 (Fig.7,b,...,C) are connected to the corresponding inputs of the second circuit And 17. The signal in the form of a rectangular pulse of positive polarity (Fig.7,and appears at the output of the second circuit And 17 only when the counter 16 will count Q of consecutive clock pulses. Discrete counting the FORMER coinciding with the position on the time axis of the signal output circuit And 17 is taken as the reference count in the cardiac cycle, and the signal from the output of the circuit And 17, an output signal is crimson unit 3 selection of the reference samples, supplied to the first input of the block memory 4, allowing memorizing the reference count, followed by 2n times the FORMER.

Thus, the output signal from the unit 3 selection of the reference samples manages the process started memorizing groups of N=2n+l times the FORMER, and the first count in this group, coinciding in time with the mentioned signal is taken as the reference count in the cardiac cycle.

The number Q is chosen so that it could be achieved at the expense of consecutive clock pulses only on TR-segment electrocardiograma 7. The longest after TR-segment is the ST-segment. On the time interval TSTequal to the duration of ST-segment fit qst=Tst/t time intervals equal to the sampling period T. Thus, if we choose Q>QST+1, the counter can count Q of consecutive clock pulses only on the TP segment. On all other segments (PQ, ST) cardiomegaly timing differences of the second order will go beyond the threshold level before the counter 16 will count to Q. at the output of the comparison circuit 13, a signal will appear low level (Fig 7,a), which sets the counter for the input R to the zero state and prohibits the passage of clock pulses through the first circuit And 15. When the next input signal difference of the second order in the area between the threshold levels, the counter starts counting clock pulses from the beginning. In this way made the device described in the patent of the Russian Federation on the application number 2002101968/14.

The stored values of the N samples from the outputs of the block memory 4 are received at the respective inputs of the scaler 5. The Central point of the group of 2n+1 samples are transferred to the output of the scaler 5 factor 1, and splitting each of the n pairs of samples arranged symmetrically about the Central reference transmitted coefficients Kj, j=1,2,..., n. At the output of the scaler 5 is formed a signal consisting of a sequence adjacent to each other 2n+1 pulses, the Central pulse has an amplitude equal to the value of interference at the same time, and each pair of pulses on either side of the center, the amplitude equal to the value of interference at the relevant time multiplied by a scale factor of Kj. An example of such a signal, the Jot for n=3 is shown in Fig 4,b. Figure 4,e one of the groups of the signal of the Notes shown on a larger scale. As noted earlier, the spectrum of this signal contains a component at frequency interference and does not contain the first three spectral zone. In the example, the frequency of the interference is equal to 0.8 Hz and 0.8 from heart rate adopted is equal to 60 beats/minute, a frequency of the left-side component of the (n+1)-th, i.e. the fourth, the spectral area equal to 3.2 Hz (smpeg,in). The output signal from the scaler 5 is fed to the input of the lowpass filter 6, the output of which components are allocated zero spectral zone, in our case highlights the signal proportional to the drift contours. Because of the suppression of spectral zones decreases energy components in the zero spectral zone, the output signal of the lowpass filter 6 is supplied to the amplifier 7. To restore the amplitude of the selected signal drift contours to the amplitude of the original signal Uiz drift contours, the gain Kusamplifier 7 must be chosen from the condition

where Kf- gain lowpass filter at zero frequency.

The signal at the output of the amplifier 7 Uiz shown in Fig 4B. This signal is applied to one input of the subtraction unit 8. To the second input of the subtraction signal a mixture of FORMER and drift contours delayed in the delay unit 9 at the time equal to the time delay of the lowpass filter 6 (signal AX Fig 4G). In the subtraction unit 8 is subtracting the selected signal drift contours of the original signal, and the output of this block is the signal EX, purified from signal drift contours (Exo in Fig. 4 d).

Technical and economic effect of the proposed method and device for its implementation Zack is udaetsya is to eliminate the drift contours EX when changing frequency last from zero to a frequency comparable to the frequency of cardiac contractions and even above it. This contributes to the improvement of the conditions for processing cardiomegaly (calculation of time and amplitude parameters of individual elements of cardiomegaly).

Sources of information

1. The US patent No. 6381493 Ischemia detection during nonstandard xardiac excitation patterns.

2. Publication No. 2001121142, And 61 In 5/0444, 20030520.

3. The heart monitor. Equipment for continuous monitoring of ECG/ Alabakovski, Aielines, Laanila and other edited Alabakovski and Aphemia. -M.: Radio and communication, 1993. S...200.

4. Makolkin VI, Podzolkov VI, Samoilenko CENTURIES ECG: analysis and interpretation. -M.: Publishing House. Home “GEOTAR-MED, 2001. P.27-30.

5. RF patent №2195164, And 61 In 5/02. The way to select the beginning of the cardiac cycle and the device for its implementation, Mai/BI 2002. No. 36.

1. The solution of the drift contours of electrocardiograma, namely, that the original electrocardiogram (EX) is filtered from the action of high-frequency noise and interference frequency, discretizing, in each cardiac cycle emit the reference count, restore based on a series of reference samples using spline approximation signal drift contours and subtract this signal from the original signal EX, characterized in that the reference is a reference to allocate TR-segment in each cardiac cycle, from this reference, remember the values of the neighboring N=2n+1 times, where n=1, 2,..., also lying on the interval TR, other times within the cardiac cycle equal to zero, the value of the center of reference of the group of (2n+1) times, leave unchanged, and the values of samples of each of the n pairs of samples arranged symmetrically about the Central reference scale in accordance with the condition Uj=U0Kjwhere U0the amplitude of the Central reference Ujthe amplitude of the samples of the j-th pair of samples, j=1, 2,..., n is the number of each pair of samples relative to the Central reference frame, Kj- scale factor, defined as the solution of a system of equations

where τ - the duration of the countdown EX,

T is the sampling period,

the resulting sequence of groups of samples EX is passed through a lowpass filter and then amplify the received signal drift contour subtraction from the original signal FORMER, pre-delay on time delay of the lowpass filter.

2. Device for eliminating drift contours of electrocardiograma containing the follower is on the United lowpass filter, block sample rate and block allocation reference counts, and the subtraction unit, characterized in that it additionally introduced unit storing values of N samples, a scaler, a second low pass filter, the amplifier and the delay unit, and output unit selection reference reference connected to the first input of the memory block, the second input is connected to the output of block sampling, each of the N outputs of the memory block is connected to one of N inputs of block scaling, the output of the scaler is connected to the input of the second low pass filter and the input of the latter is connected to the input of the amplifier, the output of which is connected with the first input of the subtraction unit, the second input of the subtraction unit is connected to the output of the delay unit, the input of which is connected to the output of the first lowpass filter, the output unit subtracting an output device.



 

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