PACs and Wandering Atrial Pacemaker

An early beat arising from an ectopic atrial focus before the sinus node's next scheduled beat; also called atrial premature beat (APB).

1) Early beat (short R-R before it), 2) abnormal P wave (P′) — flattened, notched, biphasic, inverted, or hidden in the prior T wave, 3) usually narrow QRS.

The PAC travels retrograde into the sinus node and resets it, so the interval from the beat before to the beat after the PAC is LESS than two normal R-R intervals.

A P′ arrives while the AV node is refractory, so no QRS follows — a P wave buried in a T wave then a pause. It is the most common cause of an unexpected pause on an EKG.

Early impulse finds one bundle branch (usually the right) refractory, giving a wide, bizarre QRS (often RBBB/rSR′ in V1) that mimics a PVC — but a premature P′ precedes it.

Bigeminy = PAC every other beat; trigeminy = PAC every third beat; couplet = two PACs in a row; ≥3 in a row at >100 bpm = atrial tachycardia run.

≥3 distinct P wave morphologies in one lead, varying PR intervals, irregularly irregular R-R intervals, and rate <100 bpm.

Same criteria as WAP but rate >100 bpm — 'WAP with a fever'; often seen in pulmonary/critically ill patients.

In AF there are NO discrete P waves, just a fibrillatory baseline; in WAP/MAT you can find real, countable but differently shaped P waves before every QRS.

Very common and usually benign — found in most healthy adults on 24-hour monitoring; triggers include caffeine, alcohol, nicotine, stress, and sleep deprivation.