title: Spontaneous transition of 2:1 atrioventricular block to 1:1 atrioventricular conduction during atrioventricular nodal reentrant tachycardia: evidence supporting the intra-Hisian or infra-Hisian area as the site of block.
abstract: BACKGROUND
  The incidence of spontaneous transition of 2:1 AV block to 1:1 AV conduction during AV nodal reentrant tachycardia has not been well reported. Among previous studies, controversy also existed about the site of the 2:1 AV block during AV nodal reentrant tachycardia.
  
  
  RESULTS
  In patients with 2:1 AV block during AV nodal reentrant tachycardia, the incidence of spontaneous transition of 2:1 AV block to 1:1 AV conduction and change of electrophysiologic properties during spontaneous transition were analyzed. Among the 20 patients with 2:1 AV block during AV nodal reentrant tachycardia, a His-bundle potential was absent in blocked beats during 2:1 AV block in 8 patients, and the maximal amplitude of the His-bundle potential in the blocked beats was the same as that in the conducted beats in 4 patients and was significantly smaller than that in the conducted beats in 8 patients (0.49 +/- 0.25 mV vs 0.16 +/- 0.07 mV, P = 0.007). Spontaneous transition of 2:1 AV block to 1:1 AV conduction occurred in 15 (75%) of 20 patients with 2:1 AV block during AV nodal reentrant tachycardia. Spontaneous transition of 2:1 AV block to 1:1 AV conduction was associated with transient right and/or left bundle branch block. The 1:1 AV conduction with transient bundle branch block was associated with significant His-ventricular (HV) interval prolongation (66 +/- 19 ms) compared with 2:1 AV block (44 +/- 6 ms, P < 0.01) and 1:1 AV conduction without bundle branch block (43 +/- 6 ms, P < 0.01).
  
  
  CONCLUSIONS
  The 2:1 AV block during AV nodal reentrant tachycardia is functional; the level of block is demonstrated to be within or below the His bundle in a majority of patients with 2:1 AV block during AV nodal reentrant tachycardia, and a minority are possibly high in the junction between the AV node and His bundle.
authors.affiliation: Division of Cardiology, Department of Medicine, National Yang-Ming University, Veterans General Hospital-Taipei, Taiwan, Republic of China. ufae0061@ms7.hinet.net
authors.forename: Shih-Huang
authors.initials: SH
authors.lastname: Lee
authors.forename: Ching-Tai
authors.initials: CT
authors.lastname: Tai
authors.forename: Chern-En
authors.initials: CE
authors.lastname: Chiang
authors.forename: Wen-Chung
authors.initials: WC
authors.lastname: Yu
authors.forename: Jun-Jack
authors.initials: JJ
authors.lastname: Cheng
authors.forename: Yu-An
authors.initials: YA
authors.lastname: Ding
authors.forename: Mau-Song
authors.initials: MS
authors.lastname: Chang
authors.forename: Shih-Ann
authors.initials: SA
authors.lastname: Chen
mesh_terms: D000328:Adult; D000368:Aged; D002037:Bundle-Branch Block / Q000150:complications / Q000503:physiopathology*; D017115:Catheter Ablation; D022062:Electrophysiologic Techniques, Cardiac; D005260:Female; D006327:Heart Block / Q000150:complications / Q000503:physiopathology*; D006329:Heart Conduction System / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D013611:Tachycardia, Atrioventricular Nodal Reentry / Q000150:complications / Q000503:physiopathology*