///2014 Abstract Details
2014 Abstract Details2019-07-18T14:34:47+00:00

Determination of the potency of phenylephrine and metaraminol infusions to prevent hypotension during elective cesarean delivery under spinal anesthesia

Abstract Number: F-18
Abstract Type: Original Research

Michael J Paech FRCA.FANZCA,FFPMANZCA,FRANZCOG,DM1 ; Nolan J McDonnell FANZCA2; Neil Muchatuta FRCA3; Richard W Parsons PhD4; Kaiyi Phua -5

Introduction: Vasopressors are the mainstay for prevention and treatment of spinal anesthesia-induced maternal hypotension at cesarean delivery. Phenylephrine and metaraminol have not been compared. The aim of this study was to determine their ED95 and estimate the ED50, so as to estimate a potency ratio for prophylactic infusion.

Methods: A sequential up-down allocation study of women having elective cesarean delivery under spinal anesthesia. Participants randomized in blinded fashion to prophylactic intravenous infusion of phenylephrine (group P) or metaraminol (group M), the first patient starting at 150 mcg/mL (dose rate 75 mcg/min) or 600 mcg/mL (300 mcg/min) respectively. A successful infusion maintained systolic blood pressure within 80% of baseline. The concentration allocated to the next patient (incremental interval 20 mcg/mL for group P and 80 mcg/mL for group M) was based on the response of the preceding patient, using the Dixon and Mood method based on biased coin design methodology and a variation of the Narayana rule. The ED95 and confidence intervals for each drug were calculated and the ED50 estimated from the probability of being effective at each dose using pooled-adjacent-violators algorithm (PAVA) probabilities.

Results: Fifty nine participants with similar baseline characteristics – 29 group P and 30 group M - completed the study. The estimated ED95 was 106 (95% CI 50-110) mcg/min (group P) and 651 (95% CI 589-676) mcg/min (group M). The estimated ED50 of phenylephrine was 30 (95% CI 21-50) mcg/min and of metaraminol 224 (95% CI 200-360) mcg/min. Maternal side effects and neonatal acid-base status were not significantly different between groups.

Conclusion: This study demonstrated a potency ratio of 7.4, when comparing phenylephrine with metaraminol as an infusion to maintain systolic blood pressure after spinal anesthesia for cesarean delivery.

SOAP 2014