Effects of clonidine on vulnerability to fibrillation in the normal and ischemic canine ventricle.

Clonidine infusion (10 microgram/kg, i.v.) elicited a 30% increase in repetitive extrasystole (RE) threshold in 6 chloralose-anesthetized dogs. A reduction in heart rate and arterial blood pressure accompanied the increased threshold. Intracisternal injection of clonidine (2 microgram/kg) in 6 dogs caused similar alterations in these parameters. Bilateral vagotomy, performed in 6 dogs prior to intravenous clonidine, prevented the increase in RE threshold but did not prevent the drug-induced bradycardia. Atropine (0.2 and 0.6 mg/kg), however, did not attenuate the effect of clonidine on RE threshold. Clonidine administration did not prevent the reduction in ventricular fibrillation threshold associated with a 10 min occlusion of the left anterior descending coronary artery or following reperfusion. We conclude that: (1) clonidine reduces ventricular vulnerability in the normal but not the ischemic heart, and (2) its protective effect is mediated by enhanced afferent vagal input to midbrain cardiovascular regulatory centers. This central nervous system action causes a reduction in sympathetic tone to the heart.