The incidence of spinal anesthesia-induced hypotension in pregnant women undergoing a cesarean section is high. A preoperative fluid bolus of undetermined volume is frequently administered to lower the incidence of maternal hypotension, with a somewhat poor efficacy. Recently, several investigations have shown that the use of a phenylephrine infusion after the induction of spinal anesthesia results in a significant reduction in hypotensive episodes. Given the high efficacy of this therapy (incidence of hypotension around 20%), it is possible to determine the effective volume of fluid which would prevent hypotension in 50% of the patients studied (ED50).~Healthy term pregnant women undergoing elective cesarean section under spinal anesthesia will be recruited in this trial. The spinal anesthesia regimen will be standardized and all subjects will receive a phenylephrine infusion. The fluid investigated is hydroxyethylstarch (HES) 130/0.4 (Volulyte(R)). The ED50 will be determined using an up-down sequential allocation method initially described by Dixon & Massey. The determination of the HES ED50 will help the anesthesiologist in further treating maternal hypotension appropriately.
The purpose of this trial is to determine the effective volume of hydroxyethylstarch 130/0.4 which would prevent the occurence of maternal hypotension in 50% of healthy pregnant women undergoing a cesarean section under spinal anesthesia.