A 62-year-old man with AF and a variable ventricular response has an ECG showing one wide QRS complex with typical right bundle branch block morphology. This beat follows the longest RR interval of the tracing, which is itself followed by a short RR interval. The wide beat disappears when the rate slows. What is the mechanism?
- A Aberrant conduction due to incomplete recovery of the right bundle branch refractory period ✓
- B Ventricular ectopy arising from the right ventricular outflow tract
- C Pre-excited conduction over an accessory pathway
- D Rate-related left bundle branch block
Correct answer: A. Aberrant conduction due to incomplete recovery of the right bundle branch refractory period
Explanation
This is the Ashman phenomenon: a long RR interval lengthens the refractory period of the bundle branches, so the next early supraventricular impulse finds one branch still refractory and conducts with aberrancy, classically with right bundle branch block morphology because the right bundle recovers more slowly. The long-short cycle sequence is the diagnostic clue. Ventricular ectopy lacks this relationship to cycle length, pre-excitation produces delta waves during sinus rhythm, and the described morphology is right-sided, not left.
Reference: Goldberger's Clinical Electrocardiography: A Simplified Approach, 9th ed.
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Written and medically reviewed by the StethoPrep medical team.