ED50/ED90 of Phenylephrine Infusion for Preventing Spinal Anesthesia-Induced Hypotension: Supine vs Left-Tilt 15°

The purpose of this study is to determine the 50% and 90% effective doses (ED50 and ED90) of prophylactic phenylephrine infusion for preventing spinal anesthesia-induced hypotension during elective cesarean delivery, in the supine position and in the left-lateral-tilt 15° position, respectively. Two hundred parturients with singleton term pregnancies will be randomized 1:1 to the supine group or the left-tilt 15° group, and within each position 1:1:1:1 to one of four fixed phenylephrine infusion rates (0.2, 0.4, 0.6, or 0.8 μg/kg/min; 25 per group), with the rate allocation masked by concentration blinding (identical pump rate, different solution concentrations). Spinal anesthesia consists of hyperbaric bupivacaine 12 mg plus morphine 0.15 mg. The primary outcome is the incidence of hypotension (systolic blood pressure \< 80% of baseline or \< 90 mmHg) from completion of the intrathecal injection to delivery. ED50 and ED90 will be estimated by probit regression within each position.