Procainamide administration during electrophysiology study--utility as a provocative test for intermittent atrioventricular block.

The utility of procainamide, up to 10 mg/kg IV, as a provocative test for intermittent high degree atrioventricular (AV) block was evaluated in a total of 89 patients. Forty two patients had resting 1:1 AV conduction but had bifascicular block and a history of syncope. High degree AV block had not been documented in anyone. Before procainamide, the HV interval was greater than 60 ms in 17 of the 42 patients but no patient developed infra-Hisian block with fixed rate atrial pacing or following programmed atrial extrastimuli. Procainamide administration lengthened the mean HV interval by 11.9 ms and in seven (14%) the HV increment was marked, 15-75 ms. Furthermore, four (9.5%) of these 42 patients developed second or third degree infra-Hisian block and in two of these four patients, the HV prior to procainamide administration was normal or only mildly prolonged (less than 60 ms). The findings were compared to those in three "control" groups. Among four patients with bifascicular block, previously documented transient AV block but 1:1 AV conduction at the time of study, three developed high degree AV block following procainamide. Among five patients with bifascicular block but without syncope nor documented high degree AV block, the mean HV interval lengthened by 18.8 ms and in three the HV increment was 24-30 ms. In another 38 patients with neither syncope nor an intraventricular conduction defect, the mean HV interval lengthened by 5.3 ms and in two cases by 20-25 ms. Most importantly, high degree AV block was never observed in the latter two groups. During follow-up of up to 10 years (mean 46 months), three of the seven patients in whom procainamide provoked high degree AV block have subsequently progressed to fixed complete AV block. Although the incidence of provocation of AV block was relatively low, it was concluded that, among patients with possible intermittent AV block, administration of procainamide as a test of distal conduction has limited value but is still useful, and may provide information additional to that obtained from mere assessment of the HV interval.