Electrocardiogram (ECG) strips showing a blocked atrial premature beat (APB)

Simultaneous electrocardiographic leads V1, II, and V5 show an atrial premature beat (red arrow) that is blocked within the atrioventricular node and not conducted to the ventricle. The P wave of the atrial premature beats alters the terminal portion of preceding T wave, seen in leads V1 and II, but not in lead V5.