The effect of intravenous procainamide on the HV interval at electrophysiologic study

The His bundle electrogram recorded at electrophysiologic study clearly dif ferentiates atrioventricular (AV) node disease from distal conduction syste m disease. The distal conduction system may be tested further by infusing p rocainamide (10-15 mg/kg) intravenously. High-grade distal AV block or prol ongation of the HV interval >80 ms was defined as an abnormal response to t his test. We retrospectively reviewed the medical records of 79 patients wh o underwent electrophysiologic study with intravenous procainamide. An abno rmal response to procainamide was observed in only 3% of 37 patients with a normal baseline HV (less than or equal to 55 ms), in 48% of 27 patients wi th mild HV prolongation (56 to 70 ms), and in all 15 patients with moderate HV prolongation (>70 ms) (P < 0.0001 for the trend). Procainamide induced high-grade AV block in 4 of 28 patients (14%) studied for syncope and in 1 of 51 patients (2%) studied for ventricular tachycardia. Syncope as the ind ication for electrophysiologic study (P = 0.05) and left bundle branch bloc k morphology (P = 0.03) were predictors of high-grade AV block; baseline HV and QTc intervals were significantly prolonged in patients who developed A V block with procainamide. We identified a strong linear correlation (R = 0 .85) between post-drug and baseline HV intervals, with a regression slope o f 1.17 +/- 0.09 and an intercept (+/- standard error) of 5.8 +/- 5.0 ms. Th is linear response to procainamide and published prospective studies suppor t pacing syncope patients with baseline HV >70 ms. Therefore, procainamide infusion during the electrophysiologic study of patients with undifferentia ted syncope should be reserved for those with mild HV prolongation from app roximately 55 to 70 ms.