Atypical AV nodal reentry with bystander accessory pathway: an unusual mechanism of preexcited tachycardia.

We present an unusual mechanism of preexcited tachycardia--atypical AV nodal reentry with bystander AP. It can be differentiated from other preexcited tachycardias by its variable degree of preexcitation (either spontaneous or in response to atrial pacing), higher degree of preexcitation with pacing near the origin of the AP than during tachycardia, inability to preexcite the tachycardia by either late atrial or ventricular premature beats, the presence of nonpreexcited atypical AV nodal reentry tachycardia following successful AP ablation, and by exclusion of atrial tachycardia.