Interactions between digoxin and brief vagal bursts influencing atrioventricular conduction

The interaction between intravenous injections of digoxin (20 microgram/kg every 15 minutes) and brief electrical bursts of vagal stimulation was determined in chloralose-anesthetized dogs. Vagal effect curves were generated to characterize the effect of brief vagal stimulus bursts on atrioventricular conduction. These curves were fit with an analytic expression from which the following parameters were derived as the experimental observations: 1) the maximal change in atrioventricular conduction (deltaPRmax), 2) the time after the stimulus at which atrioventricular conduction was maximally inhibited (Tmax) and 3) the width of the vagal effect curve at one-half the maximal amplitude (TD). Digoxin administration significantly (P less than .05) increased deltaPRmax, Tmax and TD by 21.6 +/- 4.3, 50.0 +/- 16.1 and 125.5 +/- 42.4 msec, respectively, before the disruption of sinus rhythm. Diluent or saline administration did not alter deltaPRmax, Tmax or TD. In addition, digoxin produced dose-dependent increases in deltaPRmax. These results suggest that digoxin vagal interactions not only affect the PR interval but also the time to the maximum delay in atrioventricular conduction and the length of time for depressed conduction after brief bursts of vagal activity.