Unidirectional complete heart block.

Forty-two patients with complete heart block were subjected to electrophysiological studies wherein apart from localization of the site of the conduction defect, ventricular pacing was done to assess ventriculo-atrial (VA) conduction and concealed ventriculo-nodal (VN) conduction. There was evidence of retrograde conduction in the presence of orthograde CHB in 22 patients (52.4 per cent). Fifteen patients (35.7 per cent) had VA conduction and seven (16.6 per cent) had concealed VN conduction. In patients with supra-Hisian CHB, three of the nine patients had VA conduction while of the 11 patients with intra-Hisian CHB, six had retrograde conduction (four with VA and two with concealed VN conduction). In the infra-Hisian CHB group, of the 22 patients, eight had VA conduction and five had concealed conduction. Incremental ventricular pacing induced VA Wenckebach periods at VPR from 110 to 133/minute with a VA interval of 110 to 130 msec. In view of the induction of Wenckebach VA periods, the recording of retrograde H potentials in some cases, and relatively long VA conduction time, it is surmised that retrograde conduction in the presence of orthograde CHB takes place through the AV conduction system.