Supernormal Excitability and Conduction in the His-Purkinje System of the Dog

The electrophysiological characteristics of the period of supernormal excitability and supernormal conduction were investigated in the isolated canine His-Purkinje system. Strength-interval curves were determined as the minimum transmembrane current required to bring the impaled fiber to threshold potential following a conducted action potential. During the period of supernormal excitability, 17.0 ± 4.6% (SD) less current than that required during diastole was needed to reexcite fibers throughout the left and right bundle branch-Purkinje system. A period of supernormal excitability was not found in the His bundle proximal to its pseudobifurcation or in ventricular muscle. The period of supernormal excitability was voltage dependent in the bundle branch-Purkinje system; it began during phase 3 at full repolarization (88.8 ± 5.6 [SD] mv) and reached minimum current requirements at about 74.3 ± 5.8 mv. Action potentials evoked during this period were conducted faster than they were during diastole. The maximum rates of depolarization of these supernormally conducted action potentials were not greatly depressed compared with control rates. A period of supernormal conduction was not observed in the His bundle. When the external potassium concentration was increased from 2.7 mM to 5.0 um or 7.5 mM, both the supernormal period of excitability and the period of supernormal conduction were eliminated in Purkinje fibers.