Premature atrial complex (PAC)

ECG: Atrial activity originates from the sinus node with a normal duration of the PR interval and QRS complex; on this tracing there are 2 premature atrial contractions (PACs) arising from the lower aspect of the atrium (probably in proximity to the coronary sinus) given the negative atrial activity in the inferior leads; the QRS complexes following these PACs are narrow and identical to the sinus complexes; the P’R is relatively short, the ectopic focus being located near the atrioventricular junction; partially compensated pause during the cycle following the PAC;

Comments: A PAC (P’ wave) corresponds to ectopic atrial activity, meaning that it occurs early relative to the sinus activity (P) and originates from outside the sinus node. The PP’ cycle is therefore shorter than the PP cycle and the morphology of the P’ wave (variable depending on the origin) differs from that of the sinus P wave. On this tracing, the PAC originates in the lower aspect of the right atrium near the atrioventricular node, which explains the negative deflection in the inferior leads and the relatively short P’R pattern.

Sometimes PACs are interpolated (not altering the PP cycle) while in the majority of cases, the PAC modifies the timing of the following P wave. The atrial ectopic depolarization is transmitted to the sinus node which it penetrates and recycles, causing a post-extrasystolic pause. The P’R interval which follows the premature atrial beat generally exceeds 120 ms, which allows to differentiate from a premature junctional contraction, which is associated with a relatively simultaneous occurrence of the P wave and the QRS. The P’R interval is often prolonged due to its prematurity since the extrasystole finds the node in a relative refractory period.

In absece of conduction disease, the extrasystolic impulse is conducted by the atrioventricular conduction pathways, with near simultaneous ventricular depolarization in a physiological manner by the two bundle branches. The QRS complex which follows the PAC is therefore generally narrow and unaltered. The presence of a narrow QRS is however not a prerequisite for the diagnosis of atrial extrasystole. Indeed, the QRS complexes may be broad and unmodified in patients with a pre-existing bundle branch block. More importantly, an aberrant conduction with a functional bundle branch block, due to an anticipation of the impulse finding a branch or hemibranch in a refractory period, can occur when the extrasystole is very premature and the previous cycle is slow and conduction has not been slowed at the atrioventricular node. Similarly, the PAC can be blocked (not followed by a QRS complex), when the ectopic activation wave encounters the atrioventricular node in an absolute refractory period.

Take-home message: A premature atrial complex corresponds to an ectopic and premature atrial electrical activity. Usually, the following QRS is narrow but the QRS may be broad in the case of a pre-existing bundle branch block or due to aberrant