Antiarrhythmic effect of disopyramide injected into the sinus nodal artery

In a anaesthetized dogs, the antiarrhythmic drug disopyramide was injected directly into the sinus nodal artery in doses that ranged from 10 to 200 microgram/ml. Lower concentrations of the drug induced a non-significant increase in sinus rate of atropinic origin and were incapable of blocking atrial fibrillation induced by 10 microgram/ml of acetylcholine injected into the sinus modal artery. Only concentrations higher than 100 microgram/ml of the drug, which caused bradycardia, proved to be effective in preventing the arrhythmia. It is concluded that the antidyshythmic effect of disopyramide in sinus arrhythmia is a consequence of a direct negative chronotropic effect opposite to its known atropinic effect.