The relation of activation boundary size to body surface QRS potential sum

The relations of QRS amplitude sums to activation boundary size at two instants was evaluated in five normal subjects. In each subject, QRS amplitude sums from 192 simultaneously recorded torso electrocardiograms and from 35 leads from the precordial area only were obtained 10 and 15 msec after the QRS onset. Activation boundary size at these instants was taken from published studies of ventricular excitation sequence in the human heart. The relative size of electrocargiographically effective boundaries at 10 and 15 msec and the measured QRS amplitude sum at 10 msec were used to predict expected QRS amplitude sums at 15 msec. Differences between the predicted and measured QRS amplitude sums at 15 msec constitute an estimate of error in determining cardiac source size from its expressions in the body surface electrocardiogram (ECG). Results demonstrated a more consistent relation of activation boundary size to QRS sums from the 192 than from the 35 electrode array. Indirectly the results suggest summed ST segment displacement from the larger array of electrodes would be more consistently related to the extent or severity of myocardial injury than an ST segment displacement sum based on 35 precordial leads. Finally, the findings suggested that the absolute sum of an electrocardiographic measurement such as ST segment displacement would provide a better index of lesion size than ST elevation or depression only.