Differences between proximal left and right bundle branch block action potential durations and refractoriness in the dog heart.

To date the electrophysiological mechanism responsible for aberrant intraventricular conduction of critically timed premature supraventricular impulses has not been documented. Microelectrode techniques were used to measure in vitro action potential and refractory period durations of the canine proximal right and left bundle branches equidistant from the distal bundle of His. Both measurements in the right bundle branch were statistically significantly longer than these parameters of the left bundle branch. Transection of the bundle branches immediately distal to the distalmost recording sites effected no change in the proximal right bundle action potential but caused marked prolongation of proximal left bundle branch action potential and refractory period durations. We conclude that functional right bundle branch aberrancy is most likely due to the longer proximal right bundle action potential duration and refractoriness. Our data also suggest that the shorter proximal left bundle branch action potential durations and refractory periods may be due to the proximity of the low ohmic resistance Purkinje fiber-muscle junctions on the left septal surface, effecting electrotonic foreshortening of these proximal left bundle branch parameters.