Fast and slow idioventricular rhythms in the canine heart: a study of their mechanism using antiarrhythmic drugs and electrophysiologic testing.

Complete heart block in dogs was induced by injecting the His bundle with formalin. For the 1st few days after surgery, a fast idioventricular rhythm (cycle length 471 +/- 37 ms, mean +/- standard error of the mean) occurred either as the only ventricular rhythm or interspersed with a slow idioventricular rhythm (cycle length 1,307 +/- 17 ms). The response to cardiac pacing, lidocaine, ethmozin and verapamil in conscious dogs was studied 1 day and 1 to 2 weeks after surgery. The fast idioventricular rhythm could not be suppressed by overdrive pacing, and at times its rate actually increased after pacing. The basic length of the fast idioventricular rhythm was prolonged by ethmozin but not by verapamil or lidocaine. The recovery cycle length ( that is, that of the first beat after cessation of overdrive pacing) was prolonged by ethmozin and verapamil, but not lidocaine. The slow rhythm was suppressed by overdrive pacing and its rate was decreased by lidocaine, unaltered by ethmozin and increased by verapamil. The recovery cycle length of the slow rhythm also was prolonged by lidocaine, unaltered by ethmozin and decreased by verapamil. The results are consistent with the slow rhythm resulting from normal automaticity (that is, that which occurs at high levels of membrane potential and is overdrive-suppressed) and the fast rhythm resulting from an abnormal automatic mechanism (that which occurs at low membrane potentials and is not overdrive-suppressed). The results obtained with verapamil are consistent with drug-induced catecholamine release.