Electrical threshold for defibrillation of canine ventricles following myocardial infarction

The minimum current and energy (threshold values) required for direct ventricular defibrillation in dogs was measured before and after acute myocardial infarction produced by ligation of the interventricular descending branch of the left coronary artery. The current waveform employed was a single 8.33 msec. duration halfsinusoidal pulse. Myocardial infarction markedly increased the strength of electric countershock required for ventricular defibrillation in each of the fourteen subjects. The average current threshold increase was 55 per cent, whereas the average energy threshold increase was 155 per cent. The maximum increase in current and energy occurred during the first 30 minutes after infarction and gradually decreased toward control values over the following 90 minutes.