NARROW COMPLEX TACHYCARDIA WITH VA BLOCK - DIAGNOSTIC AND THERAPEUTICIMPLICATIONS

To review our experience with cases of narrow complex tachycardia with VA block, highlighting the difficulties in the differential diagnosis , and the therapeutic implications. Prior reports of patients with nar row complex tachycardia with VA block consist of isolated case reports . The differential diagnosis of this disorder includes: automatic junc tional tachycardia, AV nodal reentry with final upper common pathway b lock, concealed nodofascicular (ventricular) pathway, and intra-Hissia n reentry. Between June 1994 and January 1996, six patients with narro w complex tachycardia with episodes of ventriculoatrial block were ref erred for evaluation. All six patients underwent attempted radiofreque ncy ablation of the putative arrhythmic site. Three of six patients ha d evidence suggestive of a nodofascicular tract. Intermittent antegrad e conduction over a left-sided nodofascicular tract was present in two patients and the diagnosis of a concealed nodofascicular was made in the third patient after ruling out other tachycardia mechanisms. Two p atients had automatic junctional tachycardia, and one patient had atro ventricular nodal reentry with proximal common pathway block. Attempte d ablation in the posterior and mid-septum was unsuccessful in patient s with nodofascicular tachycardia. In contrast, those with atrioventri cular nodal reentry and automatic junctional tachycardia readily respo nded to ablation. The presence of a nodofascicular tachycardia should be suspected if: (1) intermittent antegrade preexcitation is recorded, (2) the tachycardia can be initiated with a single atrial premature p roducing two ventricular complexes, and (3) a single ventricular extra stimulus initiates SVT without a retrograde His deflection. The presen ce of a nodofascicular pathway is common in patients with narrow compl ex tachycardia and VA block. Unlike AV nodal reentry and automatic jun ctional tachycardia, the response to ablation is poor.