Background

Nearly everyone has premature atrial contractions. What makes them worth recognising is the shorter pause that tells them from a ventricular ectopic, and that a high burden of them predicts atrial fibrillation.

The pause, in arithmetic

A ventricular ectopic usually cannot get back through the AV node, so the sinus node never learns it happened and keeps its own time. The next sinus beat lands exactly where it was always going to, and the span across the ectopic comes to two full cycles: the compensatory pause.

An atrial ectopic does reach the sinus node and resets it, so the return cycle is one sinus cycle plus the small delay of getting there and restarting. The span is therefore less than two cycles. On this strip it is 1438 ms against 1667, about 86%. The margin is what you are looking for, so a pause that measures 95% of compensatory is telling you the beat was probably ventricular.

What the P wave shape says

The further the focus sits from the sinus node, the less the P looks like a sinus P. A low atrial focus activates the atria from the bottom upward and inverts the P inferiorly, as here. A focus near the sinus node produces a P almost indistinguishable from sinus, and then only the timing gives it away.

Frequency is the prognosis

Occasional PACs are normal and need nothing. A high burden is different: a meta-analysis of twelve cohorts and about 110,000 people found frequent premature atrial complexes carried roughly a threefold increase in new atrial fibrillation, with a pooled risk ratio of 2.76. The ectopic beats are not causing the fibrillation so much as revealing an atrium already disposed to it.

When a PAC does not look like this

Two things change the picture without changing the mechanism. If the beat arrives while the AV node is still refractory it is not conducted at all, so there is a P and no QRS. If it arrives while a bundle branch is still refractory it conducts with aberrancy, so there is a P and a wide QRS. Both have their own article.