2:1 Atrioventricular Block During Atrioventricular Node Reentrant Tachycardi

Objectives. The purpose of this study was to determine the incidence and to clarify the mechanism of 2:1 atrioventricular (AV) block during AV node reentrant tachycardi induced in the electrophysiology laboratory. Background. In patients with 2:1 AV block during AV node reentrant tachycardia, the absence of His bundle potential in the blocked beats has been considered evidence of intranodal, lower common pathway block. Methods. In consecutive patients with AV node reentrant tachycardia, the incidence of 2:1 AV block and the response to atropine and single ventricular extrastimulus was observed. Results. Persistent 2:1 AV block occurred in 13 of 139 patients with AV node reentrant tachycardia. His bundle deflection was present in the blocked beats in eight patients and absent in five. Patients with 2:1 AV block had shorter tachycardi cycle length than did patients without such block (mean ± SD 312 ± 32 vs. 353 ± 55 ms, p < 0.01). Atropine did not alter the 2:1 block in any patient. In every patient, single ventricular extrastimulus introduced during the tachycardi converted the 2:1 block to 1:1 conduction. Conclusions. The incidence of induced 2:1 AV block during AV node reentrant tachycardi is not, vert, similar10%. The lack of response to atropine and the consistent conversion of 2:1 block to 1:1 conduction by ventricular extrastimulus indicate that, regardless of the presence or absence of His bundle potential in blocked beats, 2:1 block during AV node reentrant tachycardi is due to functional infranodal block.