Electrophysiologic effects of digitalis on A-V transmission

Abstract

Electrophysiologic effects of digitalis glycoside on A-V transmission were studied in isolated, spontaneously beating rabbit hearts. Timing of activation in various A-V junctional fibers was determined utilizing two microelectrodes as well as bipolar arterial and ventricular electrograms. Desacetyl lanatoside C in the concentration of 0.2-0.4 mg/liter significantly decreased the action potential amplitude and the rate of depolarization in the N (nodal) fibers and, to a lesser extent, in the NH (node-His) fibers. Thus, prolongation of the A-V interval (first degree A-V block) was due predominantly to further slowing of conduction in the N region. In second degree A-V block, major decrement of conduction occurred in the N region, resulting in ultimate failure of propagation in the NH region. Various records suggested that inhomogeneous conduction in the N region engendering multiple wave fronts arriving at the NH region was one possible cause of decremental conduction and higher grades of block.