Fasciculoventricular accessory pathway unmasked by a pseudo gap phenomenon.

A 28 year old female with manifest preexcitation underwent electrophysiology study for intermittent palpitation. During progressively premature atrial extrastimuli, bypass tract was blocked before relative refractory period of AV node, making it unfeasible to observe the change of H-V interval and QRS morphology during decremental nodal conduction. However, dual AV node physiology, presented as a marked increase of A-H interval, occurred when a short-coupled extrastimulus was delivered, followed by a preexcited QRS with an H-V interval identical to that in sinus rhythm. This was an example of gap phenomenon involving accessory pathway and AV node. Diagnosis of fasciculoventricular bypass tract was made and ablation was therefore not performed.