Non-synchronized direct-current countershock in cardiac arrhythmias.

One hundred and thirty-two direct-current shocks were administered to 82 patients in an attempt to convert supraventricular tachyarrhythmias. Sinus rhythm was restored in 74 patients. In at least seven instances the shock was delivered during the T wave. Ventricular fibrillation did not occur. One episode of ventricular tachycardia and one of third-degree heart block followed countershock and terminated spontaneously. Excessive digitalis was considered to be a contributing factor in these two arrhythmias. Non-synchronized countershock is considered a safe and effective method of converting atrial fibrillation and flutter.