premature atrial complex (PAC), an atrial depolarization that occurs earlier than expected. It is indicated electrocardiographically by an early P′ wave followed by a normal QRS complex. PACs may be the result of atrial enlargement or ischemia or may be caused by stress, caffeine, or nicotine. They may occur occasionally or in a regular pattern. Occasional PACs usually have no significance, but frequent PACs may lead to atrial fibrillation or to paroxysmal supraventricular tachycardia. Also called atrial extrasystole, atrial premature complex, atrial premature beat.
