Accuracy of QRS detection in relation to the analysis of high-frequency components in the electrocardiogram

The detection of high-frequency components in the QRS complex by means of coherent signal averaging is affected by inaccuracy in the time reference. Jitter of the time reference or trigger, which is derived from the low-frequency QRS complex, will be caused by the noise in the QRS complex. A theory is developed by which the trigger jitter can be predicted from the properties of signal and noise for a system consisting of a filter and a single-level, dual-level or peak detector. The theory is applicable when the noise is additive and, under certain conditions, also when the noise is multiplicative. Using this theory the trigger jitter of a given filter-level detection system is compared with that of an optimal detection system consisting of a matched filter and peak detector. The theoretical trigger jitter of the above-mentioned detectors has been computer for e.c.g. recordings of 23 individuals, as a function of different filter settings and and with the assumption that the noise was additive. This resulted in an average trigger jitter of 0·2±0·1 ms for the optimal system, while for the peak detector and the dual level detector the jitter was slightly higher provided that the QRS complexes were symmetrical after filtering. With the effects of ventilation taken into account (multiplicative noise) it is shown that dual-level detection is considerably more accurate than single-level detection. A description of the dual-level detector is also presented.

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