Differential effect of intravenous procainamide on anterograde and retrograde accessory pathway refractoriness

Abstract

Although procainamide may markedly impair or abolish anterograde conduction over an accessory atrioventricular (AV) pathway, orthodromic AV reentry may remain inducible. This difference may be related to a systemic differential effect of procainamide on anterograde and retrograde accessory pathway refractoriness. To examine this phenomenon, an infusion of procainamide producing five incremental Mood levels over 75 min was administered to 15 patients with the Wolff-Parkinson-White syndrome. At each procainamide level, accessory pathway effective refractory period and accessory pathway block cycle length were determined in the anterograde and retrograde directions.At baseline, there were no significant differences between anterograde and retrograde accessory pathway effective refractory periods (282 ± 7 vs. 266 ± 9 ms, p = 0.08) and block cycle lengths (288 ± 15 vs. 283 ± 9 ms, p = 0.66). The concentration of procainamide resulting in 50% prolongation of accessory pathway refractoriness was less in the anterograde direction than in the retrograde direction (27.5 [log concentration −4.56 ± SE 0.13] vs. 64.6[−4.19 ± 0.11] μmol/liler, p = 0.02). Similarly, the concentration of procainamide resulting in 50% prolongation of accessory pathway block cycle length in the anterograde direction (25.1 [−4.60 ± 0.13) μmol/liter) was less than in the retrograde direction (52.5 [−4.28 ± 0.07] μmol/liter, p = 0.01).The probability of persistence of accessory pathway conduction in the anterograde direction was less than in the retrograde direction by Kaplan-Meier analysis (p = 0.04). By the 3rd procainamide dose (mean procainamide level 32.3 ± 1.36 μmol/ liter) anterograde pre-excitation was no longer present in 6 of the 15 patients, whereas retrograde conduction over the accessory pathway did not disappear in any patient. These results demonstrate that procainamide has more marked effects on anterograde than on retrograde accessory pathway refractoriness. Thus, it may be more efficacious for arrhythmias dependent on anterograde accessory pathway conduction than for arrhythmias dependent on retrograde accessory pathway conduction.