Distortions in the ECG caused by instruments for ambulatory electrocardiography

The frequency response and speed stability of a widely used tape recorder (Oxford Medilog) and arrhythmia computer (Reynolds Pathfinder) was evaluated and measurement in time and amplitude on the ambulatory ECG compared with correspondent measurement in the conventional ECG. The overall bandwidth was 0.15--33Hz (3 dB down) and the mean speed error +2% +/- 1.8%. There was no statistically significant difference between P-Q and Q-T intervals measured by the 2 methods, but a significant reduction in wave amplitudes in the ambulatory ECG. The demarcation of QRS complexes was often blurred in the ambulatory ECG and borderline complexes difficult to classify as normal or abnormal on QRS width alone. Artifical J point depression was a common finding in the ambulatory ECT, whereas horizontal or downsloping ST segment depression was faithfully reproduced.