PVC vs. PAC with Aberrancy (Ashman Phenomenon)

When confronted with a wide QRS complex that appears unexpectedly on an ECG, one of the key clinical challenges is to determine whether it's due to a premature ventricular contraction (PVC) or a premature atrial contraction (PAC) with aberrant conduction, also known as the Ashman phenomenon. Though both result in wide beats, their origin, mechanism, and morphology differ substantially—and recognizing the distinction can prevent misdiagnosis and inappropriate treatment.

This article compares the electrophysiological mechanisms and ECG morphology of PVCs and PACs with aberrancy, helping you confidently distinguish them in practice.

1. Origin and Mechanism

Premature Ventricular Contraction (PVC)

• Origin: Arises from an ectopic focus within the ventricles, outside the His-Purkinje system

• Conduction: Does not use the normal conduction system; depolarization spreads cell-to-cell through the myocardium

• Result: A completely abnormal ventricular activation sequence, leading to a wide and bizarre QRS complex

PAC with Aberrancy (Ashman Phenomenon)

• Origin: A supraventricular impulse (atrial origin), typically occurring early

• Conduction: Uses the normal AV node but encounters a partially refractory bundle branch, most often the right bundle

• Result: The impulse is conducted with a delay through one bundle, usually producing a right bundle branch block (RBBB) pattern, despite the impulse being supraventricular in origin

2. Ashman Phenomenon: Why It Happens

Ashman phenomenon is a rate-related functional bundle branch block, most common during atrial fibrillation or irregular rhythms. It follows this classic sequence:

• A long cycle (RR interval) allows both bundle branches to fully repolarize

• This is followed by a short cycle (early beat) that catches one of the bundle branches—typically the right—still in its refractory period

• The result is a supraventricular beat with aberrant conduction, mimicking a PVC

Think: "Long–short rule" = Ashman beat 

3. Clinical Context and Clues

PVC

• More likely in structurally abnormal hearts

• May cause palpitations or compensatory pauses felt by the patient

• Benign if isolated, but frequent PVCs may warrant further evaluation

PAC with Aberrancy

• More likely in atrial fibrillation, or during rapid supraventricular rhythms

• The key clue is the preceding long–short RR interval and RBBB morphology

• Important not to mistake for VT or PVCs, especially when irregularly irregular rhythms are involved

Clinical Takeaway

Not all wide QRS complexes are ventricular in origin. PVCs and PACs with aberrant conduction may appear similar, but their mechanisms and implications are vastly different. Recognizing the morphological patterns, P wave associations, and preceding RR intervals can help you correctly identify Ashman phenomenon and avoid misdiagnosis of ventricular arrhythmia.