EFFECT OF IV LOW-DOSE ADRENALINE AND PHENYLEPHRINE INFUSIONS ON PLASMA-CONCENTRATIONS OF BUPIVACAINE AFTER LUMBAR EXTRADURAL ANESTHESIA IN ELDERLY PATIENTS

Thirty patients undergoing primary total hip replacement under lumbar extradural anaesthesia with 0.75 % bupivacaine 25 ml were allocated randomly to receive either low-dose adrenaline or phenylephrine infusions i.v. throughout surgery. Haemodynamic measurements and arterial blood samples were obtained before the extradural injection and at 10, 20, 30, 40, 50, 60 and 90 min thereafter. Peak arterial plasma concentrations of bupivacaine were observed 10 min after extradural anaesthesia and were significantly lower in patients receiving adrenaline infusions. Cardiac output was significantly greater in patients receiving adrenaline infusions (P < 0.01). It is postulated that the smaller circulating concentrations of bupivacaine observed in patients receiving adrenaline were caused by increased cardiac output and a greater volume of distribution than in patients receiving phenylephrine.