Cardiac monitoring in ambulant subjects

Miniature magnetic tape recorders permit physiological data to be collected from ambulant subjects. During 24 hours the signals from some 100,000 heart beats can be registered but the analysis of this amount of information poses technological problems. Monitoring of an oscilloscope display of the signal replayed from tape was an early method. A record-replay speed ratio of 25:1 enabled a tape to be scrutinised in one hour but was a tedious task. Digital computers were being used to analyse standard ECGs of short duration but were expensive and complex. It appeared that a comparatively simple semi-automatic analyser based on analogue techniques would be practical, given the limited objective of identifying ventricular and supraventricular extrasystoles, ventricular and supraventricular tachycardias, bradycardia, gaps or short periods of asystole and change in QRS wave-form. Commercial systems using variations in width or area of the QRS complex as criteria for classifying ventricular extrasystoles proved to be unsuccessful in real time analysis and seemed unlikely to be suitable for adapting to high speed operation. It appeared possible that the QRS wave-forms from ventricular extrasystoles might contain a higher proportion of low frequency components than those from contractions arising in the atrium and that they might be segregated by a method based on this hypothesis. No information could be found on this subject in the literature, so measurements were made which showed that ventricular extrasystoles contain a high proportion of frequencies around 2.8 to 3.8 Hz, whilst for sinus beats the predominant frequencies are near 13 Hz. This being so, an analyser using analogue filters was developed to exploit the discovery. In validation tests on 51 recordings the incidence of false positive or false negative detection of premature beats ranged from 0% to 5.8%, whilst misclassification of ventricular and supraventricular premature beats occurred with a frequency of 0% to 1.5%. No errors occurred in the classification of tachycardias, bradycardia or gaps. Subsequently the analyser has been used in a survey of the 24-hour ECG of 100 normal subjects, in a study of the diurnal variation in the incidence of ventricular extrasystoles and in several drug trials.