Premature Atrial Contraction (PAC)

Premature atrial contractions (PACs), also known as atrial premature complexes (APC) or atrial premature beats (APB), are a common cardiac dysrhythmia characterized by premature heartbeats originating in the atria. While the sinoatrial node typically regulates the heartbeat during normal sinus rhythm, PACs occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat. The exact cause of PACs is unclear; while several predisposing conditions exist, PACs commonly occur in healthy young and elderly people without heart disease, and by themselves are not considered an abnormal finding. PACs found in healthy young are more likely to lead to other heart conditions or even death. Elderly people that get PACs usually don't need any further attention besides follow ups due to unclear evidence. PACs are often completely asymptomatic and may be noted only with Holter monitoring, but occasionally they can be perceived as a skipped beat or a jolt in the chest. In most cases, no treatment other than reassurance is needed for PACs, although medications such as beta blockers can reduce the frequency of symptomatic PACs.

On an electrocardiogram (ECG), PACs are characterized by an abnormally shaped P wave. Since the premature beat initiates outside the sinoatrial node, the associated P wave appears different from those seen in normal sinus rhythm. Typically, the atrial impulse propagates normally through the atrioventricular node and into the cardiac ventricles, resulting in a normal, narrow QRS complex. However, if the atrial beat is premature enough, it may reach the atrioventricular node during its refractory period, in which case it will not be conducted to the ventricle and there will be no QRS complex following the P wave.