[Two to one atrioventricular block in intranodal re-entry reciprocating tachycardia. Description of 2 cases].

Atrioventricular nodal reentry tachycardia (AVNRT) is a common form of paroxysmal supraventricular tachyarrhythmia. In this tachycardia, the atrium and ventricle are not necessary links of the reentry circuit, so that the arrhythmia may persist in spite of the occurrence of 2:1 AV ratio or AV dissociation. Only a few examples of 2:1 AV block during AVNRT have been described. We report on 2 patients with a history of paroxysmal supraventricular tachycardia in whom 2:1 AV block with persistence of the arrhythmia was documented. Transesophageal electrophysiologic study was performed after pharmacologic wash-out in both patients. During definition of refractory periods, reciprocating tachycardia was initiated when a critical lengthening of the Stimulus-R interval was reached. Tachycardia showed narrow QRS complexes at a rate of 200 (patient 1) and 180 (patient 2) bpm, with the ventriculo-atrial interval (VA) of 45 and 70 ms, respectively. During tachycardia, sustained episodes of 2:1 AV block, without termination of the arrhythmia, occurred in both patients. The tachycardia could be reproducibly terminated by means of extrastimulus technique, rapid burst pacing as well as intravenous injection of adenosine-5'-triphosphate (ATP) at doses of 0.15-0.20 mg/kg. Initiation of tachycardia after a critical lengthening of the Stimulus-R interval and the effectiveness of either rapid burst pacing or ATP injection in the interruption of the arrhythmia, suggested a reentry circuit involving the AV node. The unusual finding of 2:1 AV block during reciprocating tachycardia with a retrograde time conduction (VA interval) equal to or shorter than 70 ms suggested the presence of an intranodal reentry as the substrate of the tachycardia, and excluded the presence of an accessory AV pathway.