The action of droperidol on cardiac vagal efferents was studied in anaesthetized dogs. Droperidol in cumulative doses (0.125, 0.25 and 0.5 mg kg-1 i.v.) inhibited cardiac vagal discharge, shifted pressure-response curves to lower activities, increased heart rate, and decreased arterial pressure slightly. The effects on vagal discharge and heart rate reached their maxima at 0.25 mg kg-1 as a further increase in the dose had no additional effect. The vagal inhibition and the tachycardia were independent of arterial pressure as indicated by the shift in the pressure-response curves and by the fact that they also occurred when the decrease in arterial pressure was prevented. After pretreatment with atropine, droperidol had no effect on heart rate. Thus, droperidol inhibits central vagal drive independently of arterial pressure. This central vagolytic action seems to be the main cause for the positive chronotropic effect of droperidol.
