A hospitalized patient with atrial fibrillation shows occasional wide QRS complexes with RBBB morphology on telemetry. Each wide beat follows a pause and then a shorter preceding cycle (a long-short sequence), and no fixed coupling is present. What is the mechanism of these wide beats?

• A Ventricular bigeminy from a single focus
• B Aberrant ventricular conduction due to cycle-length dependent refractoriness of a bundle branch (Ashman phenomenon) ✓
• C Intermittent pre-excitation through an accessory pathway
• D Rate-related left bundle branch block

Explanation

Ashman phenomenon is functional aberrancy triggered by a long-short cycle sequence: the long cycle lengthens refractory period in one bundle branch (usually right), so the early beat following the short cycle conducts with RBBB morphology. In atrial fibrillation the irregular RR intervals provide exactly this sequence, and the lack of fixed coupling argues against ventricular ectopy. Pre-excitation would show delta waves in sinus rhythm, not a long-short pattern.

Reference: Goldberger's Clinical Electrocardiography, 9th ed.

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Written and medically reviewed by the StethoPrep medical team.