Method and Apparatus for Enhancing the Signal-to-Noise Ratio of ECG Signals

1. Field of the Invention

The present invention relates to a method and a device for enhancing the signal-to-noise ratio of ECG signals.

2. Description of the Prior Art

ECG signals recorded on the exterior of the body always contain extensive noise originating from muscular activity, digitization of the recorded analog ECG signal, the recording instruments and influence from the power supply. Using modern technology, however, the two latter sources of noise can be largely eliminated. Noise from the muscular activity of the patient and the digitization procedure, however, often drowns out low-amplitude signal components in the ECG signal.

In an effort to reduce noise, and thus to improve the signal-to-noise ratio for these low-amplitude signal components, it is known to subject the ECG signals to different kinds of averaging procedures. Time-varying Wiener filtration has also been attempted in order to enhance the signal-to-noise ratio, see Weerd and Martens: "Theory and Practice of A Posteriori Wiener Filtering of Average Evoked Potentials", Biol. Cybern., 30, pp. 81-94, (1978), and Weerd and Kap.: "A Posteriori TimeVarying Filtering of Averaged Evoked Potentials", Biol. Cybern., 41, pp. 223-234, (1981). This technique is based on the estimated signal and noise effects determined for different frequencies. In this process, some spectral loss is unavoidable in the Fourier transform procedure utilized. With low signal-to-noise ratios, this method also results in a consistently erroneous identification of the sought signal contained in the noise.

An object of the present invention is to provide a method and apparatus which achieve additional enhancement of the signal-to-noise ratio which, in turn, permits identification of low-amplitude signal components in the ECG signal, such as so-called "late potentials", without use of Fourier transform with all its disadvantages.

The above object is achieved in accordance with the principles of the present invention in a method and apparatus wherein incoming ECG signals from a plurality of cardiac cycles are analog-to-digital converted and the digital signals from the respective cardiac cycles are time-synchronized to form a signal ensemble, and wherein a correlation between the signals comprising the ensemble is determined in time intervals, the time intervals including at least two samples, during at least a part of the cardiac cycles.

The correlation can then be used to determine a filter characteristic for use in a filter for filtering the ECG signals, which is, at any given time, an optimum characteristic.

In the present invention representative parts of ECG signals from a plurality of cardiac cycles are thus filtered in a filter, the characteristics of which are related to the prevailing signal-to-noise ratio.

Thus according to the invention, the signals from a plurality of cardiac cycles are time-synchronized to form a signal ensemble, and the correlation across the signal ensemble is determined, although not the time correlation. In this way the size of the resulting cross-correlation matrix is reduced such that determination of the optimum filter characteristic at every point of time is made possible.

According to an advantageous embodiment of the apparatus of the invention, the correlation between the signals in the signal ensemble is determined in successive, overlapping intervals with the aid of a window unit. The optimum filter characteristic at any given moment is thus determined though short-period analysis. In this way time distortion (smearing) is avoided.

FIG. 1 is an overview block diagram of the device according to the invention.

FIG. 2 is a block diagram showing the signal processing and averaging stage of the device in FIG. 1 in greater detail.

FIG. 3 shows curves illustrating the technique utilized by the invention.

As shown in FIG. 1, ECG signals obtained from an ECG recorder 1 are converted into digital form in an A/D converter 2, and QRS complexes in the digital signal are detected in a detector 4 and the detected complexes are supplied to a signal processing and averaging stage 7. Averaged digital signal values are formed in an averager 6 (FIG. 2), and an interval or time window is determined in a window unit 8 (FIG. 2), the time window is used for enhancing the signal-to-noise ratio (S/N) of the ECG signal during at least a part of each cardiac cycle in an S/N enhancer 10.

The units 6, 8 and 10 are described in greater detail with reference to FIG. 2.

Recorded ECG signals from a plurality of cardiac cycles are time-synchronized so they form an ensemble of signals. The elimination of low-frequency components, which is performed by a high-pass filter with a low cut-off frequency, is essential in the determination of the optimum filter at any given moment. This high-pass filtering, with other signal processing including the aforementioned time-synchronization, takes place in a signal processing unit 5.

In the averager 6, average values over the signal ensemble are formed. Averaging need not be performed as a "pure" averaging of all the signals in the ensemble, but can be carried out in different other ways in order to "improve" the average value and provide a representative signal cycle. So, e.g., strongly deviating signals, in the ensemble could be omitted in the averaging, and/or the averaging could be performed, e.g., in successive, overlapping intervals determined by the window unit 8. Averaging could also consist of median determination.

In the correlation determination unit 12 the correlation between the individual signals in a time window in the ensemble of signals, set by the time window unit 8, is determined according to a Maximum Likelihood method. The ensemble correlation .mu.ml thereby obtained is given by the following equation: ##EQU1## in which z.sub.i denotes the i.sup.th filtered ECG signal, N is the number of cardiac cycles considered and

M is the number of samples in the interval or time window.

The ensemble correlation thus determined is utilized for determining the optimum characteristics of a filter 14 for filtering the mean signal value obtained from the averager 6 in order to get as large a signal-to-noise ratio as possible. The optimum filter characteristic is determined for each successive interval or time window, thus the resulting filter 14 becomes time-dependent. The filter consequently has a certain weighting for each observation interval or sample. The filtration in the filter 14 is thereby reduced to multiplication by a weight function h.sub.n obtained from the correlation determination unit 12. This is illustrated in FIG. 3 which shows N signals in the signal ensemble and the resulting weighting function h.sub.n, which determines the characteristic of the filter 14.

The momentarily optimum characteristic for the filter 14 achieved in this way is a non-frequency-selective filter, and the magnitude of the signal-to-noise ratio is used for determining the filter weightings. Since the signal-to-noise ratio is a function of the number of available cardiac cycles, the momentarily optimum filter characteristic, and accordingly the output signal, become a function of this parameter. This filter characteristic can also easily vary considerably if the number of cardiac cycles is low, however, this variation decreases as the number of utilized cardiac cycles increases.

With an increasing number of cardiac cycles N, the noise in the averaged signal decreases, a circumstance which contributes to enhancement of the signal-to-noise ratio at the same time as the momentarily optimum filter characteristic is improved through the access to more information, as mentioned above. Both these effects consequently contribute to a reduction of fluctuations in the output signal from the filter as the number of cardiac cycles increases.

Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.