Effects of antiarrhythmic drugs on the chronic pacing threshold and the endocardial R wave amplitude in the conscious dog.

To determine the effects of antiarrhythmic drugs on the chronic pacing threshold and the endocardial R wave amplitude, we tested seven drugs (representative of Vaughn Williams' antiarrhythmic class-I-IV) on four to six dogs after the implanted ventricular endocardial bipolar leads had reached chronic stable thresholds. The seven antiarrhythmic drugs tested were procainamide (oral [p.o.] and intravenous [IV], class IA), lidocaine (IV, class IB), tocainide (p.o., class IB), flecainide (p.o. and IV, class IC), propranolol (p.o. and IV, class II), amiodarone (p.o. and IV, class III), and verapamil (p.o. and IV, class IC), propranolol (p.o. and IV, class II), amiodarone (p.o. and IV, class III), and verapamil (p.o. and IV, class IV). The drugs were administered in sufficient doses to achieve plasma levels associated with antiarrhythmic efficacy in humans. All oral drugs were given for sufficient time to achieve expected steady state levels. The pacing thresholds, both voltage and current, and the endocardial R wave amplitude were measured in the conscious dog with a Medtronic pacing system analyzer at a pulse width of 0.1, 0.5, 1.0, and 2.0 ms at intervals following IV or p.o. administration of each antiarrhythmic drug. We found that there were no significant effects of the tested seven antiarrhythmic drugs on the chronic pacing threshold and the endocardial R wave amplitude at any of the pulse widths. We conclude that in the healthy dog antiarrhythmic drugs do not alter the pacing threshold and the endocardial R wave amplitude.