Determination of the 90% effective dose (ED90) of phenylephrine for hypotension during elective cesarean delivery using a continual reassessment method.

OBJECTIVE: The purpose of this study was to determine, by continual reassessment, the 90% effective dose (ED90) of phenylephrine for hypotension after combined spinal-epidural anesthesia.

STUDY DESIGN: Term pregnant women scheduled for elective cesarean delivery received combined spinal epidural anesthesia. Subjects received phenylephrine at one of 6 incremental doses ranging from 60 to 160μg (n=3 for each dose). While the first cohort received a conservative, predetermined dose of 60μg, subsequent cohorts received phenylephrine doses determined using Bayesian-based software. One of the predetermined bolus doses of phenylephrine was given in the event of both hypotension [defined as systolic blood pressure (SBP)<80% of baseline or below 100mmHg] and tachycardia [defined as heart rate >120% of baseline or >100beatsmin(-1)]. Treatment was considered successful if SBP returned to within 80% of the baseline or ≥100mmHg within 2min.

RESULTS: Twenty-four subjects with hypotension and tachycardia were included. T6 block was achieved within 15min in 20 patients and after additional epidural chloroprocaine in the remaining four. The estimated ED90 was 100μg, with a response probability of 90.7% (95% CI 74.1-99.5%). Treatment was successful in 20 patients. Probability of success at each bolus dose (in μg) was as follows: 60, 58.9%; 80, 80.3%; 100, 90.7%; 120, 95.5%; 140, 98.3%; and 160, 99.2%.

CONCLUSIONS: The ED90 of a phenylephrine bolus dose for hypotension in term pregnant women is approximately 100μg, based on continual reassessment.