PubMed1992
Twenty episodes of progression of 2:1 atrioventricular (AV) block were identified during incremental atrial stimulation in 7 patients with documented (2-level) block in the AV node and His-Purkinje system. All occurred at cycle lengths shorter than those at which stable 2:1 HV block had been detected. Thirteen episodes were typical since 2:1 increased to 3:1 AV block when an atrio-His (AH) Wenckebach period was completed with an atrial impulse that otherwise would have been conducted. These episodes occurred with dynamic A(M): V(N) ratios similar to those seen at the AV node. Seven atypical episodes were identified (while AH Wenckebach periods were occurring): (1) 2:1 increasing to 3:1 AV block and then to 4:1 AV block resulting from prolonged refractoriness in the His-Purkinje system subsequently followed by concealed conduction in the latter structure; (2) conversion of 3:2 directly into 3:1 AV block due to block of the next-to-last atrial impulse in the His-Purkinje system with completion of AH Wenckebach period with the following atrial impulse; and (3) 4:2 AV block presumably due to supernormal conduction in a transversely dissociated His-Purkinje system. These episodes occurred with A(M): V(N) ratios, which in other structures would have been indicative of different degrees of AV block. In conclusion, progression of 2:1 AV block during documented 2 level conduction disturbances (1) can be explained by mechanisms different than those currently known, and (2) has rich, but different dynamics from those observed exclusively in the AV node and exclusively in the His-Purkinje system.