Changing electrocardiographic recording technology and diagnostic accuracy of myocardial infarction criteria. Improved standards for evaluation of ECG measurement precision.

An analytical study was performed to estimate the magnitude of the visual Q wave duration bias produced by pressurized ink round stylus electrocardiographic (ECG) recorders. With a paper speed of 25 mm/sec and the ECG tracing width of 0.25 mm, the visually measured Q waves are on the average 8 msec too short. The corresponding error with the older type flat stylus recorder is less than 2 msec. Considerable differences can thus be anticipated in the frequency of observed ECG abnormalities in studies which use different types of electrocardiographs. The effect of the visual Q wave duration measurement bias on the diagnostic ECG classification was investigated in a group of 237 patients with old myocardial infarction and 299 subjects with no clinical evidence of infarction. An 8 msec measurement bias toward too short Q wave duration was observed to result in a potential loss of diagnostic accuracy of about 25% in some ECG coding categories. As a corrective procedure, it is recommended that the baseline width produced by a round recording stylus of uniform thickness irrespective of the vertical deflection velocity should be less.