Effect of clonidine on excitability and refractoriness of normal cat myocardium.

Increased activity of sympathetic nervous system enhances cardiac susceptibility to arrhythmias. Clonidine through its central action decrease sympathetic activity to the heart. The aim of this study was to investigate whether Clonidine influences excitability and refractoriness of the heart in such a way that cardiac susceptibility to arrhythmias is reduced. The effect of Clonidine (5 microgram/kg i.v.) on diastolic threshold and refractory period was investigated in the normal cat heart in situ paced at a constant rate. Clonidine increased diastolic threshold up to 63.5 +/- 24.0% (mean +/- SE) of initial value and prolonged refractory period up to 13.2 +/- 2.4% of the control throughout 120 min of observation. These data suggest that Clonidine might be of value as an antiarrhythmic agent in patients with chronic angina in whom ventricular arrhythmias are often associated with excessive sympathetic activity.