Antidromic AVRT — EECC MCQ

Correct answer: E — Wide QRS complexes (fully pre-excited, resembling VT) with the morphology determined by the location of the accessory pathway — differentiation from VT requires EP study or response to manoeuvres

Antidromic AVRT uses the accessory pathway for antegrade conduction (atrium → ventricle via the bypass tract) and the AV node for retrograde conduction (ventricle → atrium). Because ventricular activation occurs entirely through the accessory pathway (not the normal His-Purkinje system), the QRS is fully pre-excited (maximally wide), resembling ventricular tachycardia. The QRS morphology depends on the pathway location (left-sided pathways produce RBBB-like pattern; right-sided produce LBBB-like). Antidromic AVRT accounts for only ~5% of AVRT (orthodromic, with narrow QRS, accounts for ~95%). The ESC 2019 SVT Guidelines note that antidromic AVRT is a challenging diagnostic scenario as it can be indistinguishable from VT on surface ECG alone. Catheter ablation of the accessory pathway is curative.

Reference: ESC (2019): Guidelines on SVT Management