Role of vagal activity in the cardiovascular responses to phenylephrine in man.

The effects of phenylephrine on blood pressure (BP), left ventricular function and total peripheral resistance (TPR) were evaluated in six normotensive male volunteers before and after vagal blockade with atropine. Before atropine, phenylephrine dose-related increased TPR, systolic and diastolic BP and decreased cardiac output by decreases in both stroke volume and heart rate. Post-atropine, the BP (but not TPR) responses to phenylephrine were markedly potentiated, related to an increase in stroke volume despite the higher afterload. Thus vagal tone may regulate cardiac output via effects on both heart rate and venous return.