Effect of procainamide on the postrepolarization refractoriness in cardiacmuscle: Evaluation using the block coupling interval in the artificial isthmus model in the canine right atrium

The post-repolarization refractoriness (PRR) is an important factor to dete rmine the conduction block in cardiac muscle. Recently, we proposed the blo ck coupling interval (BCI) as an useful electrophysiological index for eval uating the PRR. In the present study, the effect of procainamide on PRR was evaluated using the BCI and the effective refractory period (ERP). In five beagle dogs, radiofrequency linear ablation was performed on the right atr ial surface parallel to the AV groove, forming an artificial isthmus (8-10 mm width and 15-20 mm length). Bipolar recordings were performed in the ist hmus at a resolution of 1.2 mm and single extrastimuli with eight basic dri ve trains were delivered to cause conduction blocks in the isthmus. When a conduction block occurred, the recorded coupling interval at the recording site just proximal to the site of block was defined as BCI. At the site of the block, the ERP and duration of the monophasic action potential (MAP) at each drive cycle length was measured. The PRR was calculated using two dif ferent formulas. (1) [ERP - MAP] and (2) [BCI - MAP]. Procainamide was admi nistrated intravenously at a dose of 15 mg/kg after the control study and t he whole study protocol was repeated. The site of the block in an individua l dog was always the same. BCI, ERP, and MAP were all shortened in accordan ce with the shortening of the basic drive cycle length, and the BCI was alw ays the longest, ERP the middle, and the MAP was the shortest. The administ ration of procainamide prolonged each parameter, but the order of BCI > ERP > MAP remained unchanged. The PRR calculated as [BCI - MAP] was prolonged from 15 +/- 10 ms to 29 +/- 8 ms by the administration of procainamide (P = 0.048), but [ERP - MAP] was unchanged (8 +/- 10 ms vs 8 +/- 4 ms). In the conduction block model in the canine right atrium, procainamide prol onged the [BCI - MAP], but did not change the [ERP - MAP]. The procainamide effect of prolonging the PRR might be expressed better by the change in th e BCT than the ERP.