5.1 Premature atrial contractions, wandering atrial pacemaker & MAT

Premature Atrial Contractions (PACs)

A premature atrial contraction (PAC) — also called an atrial premature beat or APB — is a single ectopic beat that arises from an irritable focus somewhere in the atria outside the SA node and fires earlier than the next expected sinus beat. On the CRAT exam, the defining feature is timing: the beat interrupts the underlying regular rhythm early. Because the impulse originates away from the SA node, the P wave of a PAC looks different from the surrounding sinus P waves — it may be flattened, notched, peaked, biphasic, or even hidden inside the preceding T wave (making that T wave look taller or oddly shaped). This early, abnormal P wave is your single best clue.

After the ectopic P wave, the impulse usually travels down the normal AV node and His–Purkinje pathway, so the QRS is typically narrow (≤ 0.12 s) and identical to the sinus beats. The PR interval of the PAC may be normal, slightly longer, or shorter depending on where the focus sits. If the PAC arrives so early that the AV node or bundle branches are still partly refractory, two things can happen: the beat may be non-conducted (blocked PAC) — an early P wave with no QRS after it, a classic cause of an unexpected pause — or it may conduct with aberrancy, producing a wide, bundle-branch-block–shaped QRS that can be mistaken for a PVC. The presence of a premature P wave points to a PAC rather than a PVC.

The pause after a PAC

A hallmark of PACs is the non-compensatory (incomplete) pause. The premature atrial impulse usually travels back into the SA node and resets its timing. Because the sinus node has to restart its cycle, the pause after the PAC is shorter than a full compensatory pause. Practically: measure two normal R–R intervals, then measure across the PAC (from the beat before to the beat after) — that span is less than twice a normal R–R. This contrasts with PVCs, which usually produce a fully compensatory pause because they do not reset the sinus node.

PACs are extremely common and usually benign, occurring even in healthy hearts. Triggers include caffeine, nicotine, alcohol, stress, fatigue, sympathetic stimulation, electrolyte disturbances, and stimulant medications. Frequent PACs can be described by pattern — atrial bigeminy (every other beat is a PAC), trigeminy (every third beat), or couplets (two in a row). Frequent PACs sometimes herald the onset of atrial fibrillation or flutter, so the CRAT technician should note and document them.

Wandering Atrial Pacemaker (WAP)

A wandering atrial pacemaker occurs when the site of impulse formation shifts back and forth among the SA node, ectopic atrial foci, and sometimes the AV junction. Because the pacing site moves from beat to beat, the P-wave morphology changes — the exam definition requires at least three different P-wave shapes in the same lead. The PR interval also varies (different foci sit different distances from the AV node), and the rhythm is slightly irregular. The defining rate criterion is that the overall rate is normal or slow — less than 100/min. The QRS stays narrow because conduction below the atria is normal.

WAP is usually benign and is often seen in the young, in athletes, and in older adults; it is frequently related to shifts in vagal (parasympathetic) tone. It generally requires no treatment.

Multifocal Atrial Tachycardia (MAT)

Multifocal atrial tachycardia is essentially WAP that has sped up. It shares the same signature — at least three distinct P-wave morphologies with varying PR and P–P intervals and an irregularly irregular rhythm — but the atrial (and ventricular) rate exceeds 100/min (typically 100–180). Because it is irregularly irregular with a busy atrial picture, MAT is a classic mimic of atrial fibrillation; the key distinction is that in MAT you can still identify discrete, if varied, P waves before each QRS, whereas atrial fibrillation has no organized P waves at all.

MAT is strongly associated with severe pulmonary disease, especially COPD exacerbations, and with hypoxemia, theophylline toxicity, and electrolyte abnormalities (low potassium/magnesium). Management centers on treating the underlying lung disease and correcting hypoxia and electrolytes rather than on cardioversion.

How these three rhythms relate

Think of them as a spectrum of multiple atrial pacemakers: a single early ectopic beat is a PAC; multiple competing atrial foci at a slow-to-normal rate is WAP; and those same multiple foci firing rapidly is MAT. All three keep a narrow QRS because conduction from the atria downward is normal — the abnormality is where in the atria the impulse starts, not how it travels to the ventricles.