Atrioventricular block masked by posteroseptal bypass tract located in coronary sinus aneurysm

Atrial fibrillation with concurrent ventricular preexcitation identifies a high-risk arrhythmic substrate and usually results in catheter ablation of the atrioventricular bypass tract. Electrocardiography can only approximate the anatomical location of an accessory pathway. Here we report a case where a bypass tract was localised to a coronary sinus aneurysm and antegrade atrioventricular conduction masked underlying atrioventricular nodal block.