Prophylactic IV infusion of phenylephrine has been recommended to prevent hypotension during spinal anesthesia for cesarean delivery. However, the optimal infusion dose is unknown. This study aimed to determine the infusion dose of phenylephrine that would be effective in preventing hypotension in 50% (ED50) and 90% (ED90) of patients when administered as a prophylactic infusion at a fixed rate based on the individual body weight.
Eighty parturients scheduled for elective cesarean delivery were randomly allocated to receive IV infusion of prophylactic phenylephrine at 0.25, 0.375, 0.5, or 0.625 µg/kg/min (n = 20 per group) started immediately after intrathecal injection of 10 mg hyperbaric bupivacaine and 5 µg sufentanil using a combined spinal–epidural technique. An effective dose was defined by the occurrence of no hypotension (defined as a decrease in systolic blood pressure by ≥20% below baseline and to <90 mm Hg) during the interval from the initiation of spinal anesthesia to delivery of the infant. Values for ED50 and ED90 of prophylactic phenylephrine were calculated using probit analysis.
Hypotension occurred in 13/20, 8/20, 2/20, and 1/20 patients in the groups that received phenylephrine infusion at 0.25, 0.375, 0.5, or 0.625 µg/kg/min, respectively. The calculated values for ED50 and ED90 were 0.31 (95% CI, 0.24–0.36) and 0.54 (95% CI, 0.46–0.76) µg/kg/min, respectively. No difference was found in the incidence of adverse effects and neonatal outcomes among groups.
Under the conditions of this study, when phenylephrine was given as a fixed-rate prophylactic infusion during spinal anesthesia for cesarean delivery to prevent hypotension, the values for ED50 and ED90 were 0.31 (95% CI, 0.24–0.36) and 0.54 (95% CI, 0.46–0.76) µg/kg/min, respectively.