Polymorphic ventricular tachycardia may degenerate into ventricular fibrillation, the most common cause of sudden cardiac death. The ECG shown here is from a 79 year old male patient with an ischemic cardiomyopathy and congestive heart failure. It demonstrates the development of ventricular fibrillation initiated be a ventricular premature beat that arises from the downslope of the preceding T wave. The interval for the apex to the end of the T wave is referred to as the āvulnerable periodā because of the propensity of premature beats occurring in this interval to initiate more sustained and more malignant arrhythmias such as ventricular fibrillation.