Roleofvagal activity inthecardiovascular responses tophenylephrine inman

Theeffects ofphenylephrine on blood pressure(BP), left ventricular function andtotal peripheral resistance (TPR) were evaluated insixnormotensive malevolunteers before andafter vagal blockade withatropine. Beforeatropine, phenylephrine dose-related increased TPR,systolic anddiastolic BP anddecreased cardiac output bydecreases in bothstroke volumeandheart rate.Post-atropine, theBP (but notTPR)responses to phenylephrine were markedly potentiated, related toan increase instrokevolume despite thehigher afterload. Thusvagal tonemay regulate cardiac output viaeffects on bothheart rateandvenousreturn.