[Mobitz type II suprahisian atrioventricular block : block in the subnodal-suprahisian (NH) zone?]

The electrophysiological properties of the atrioventricular node differ from those of the His bundle. Nodal conduction is always decremential; this is its principal physiological characteristic, and accounts for the fact that almost the only form of block found at this level is Wenckebach block. The His bundle is characterised by an all-or-nothing response and usually blocks in the Mobitz II mode. Wenckebach phenomena have been described in the His Purkinje system, but Mobitz II block has not been reported in the atrioventricular node. Similarly, phase IV paradoxical block is found in the His Purkinje system but has not been described in the atrioventricular node in the absence of vagal phenomena. In addition, the atrioventricular node is very sensitive to the influence of the autonomic nervous system, the His bundle much less so. The first patient had a normal resting electrocardiogram showing sinus rhythm; second degree atrioventricular block was observed when the atrial rhythm was increased by exercise, atropine or atrial pacing. His bundle recordings showed the block to be suprahisian; the blocked atrial potentials were not followed by a His potential, whilst the conducted atrial activity was followed by a normal His potential and a normal HV interval. However, this atrioventricular block was of the Mobitz II and not the expected Mobitz I type, conduction to the ventricles suddenly blocking (with a 3/2 or 2/1 response) when the atrial rate was increased, without obvious lengthening of the PR or AH intervals before the blocked atria.(ABSTRACT TRUNCATED AT 250 WORDS)