The use of phenylephrine to obtund oxytocin-induced hypotension and tachycardia during caesarean section

Abstract

BackgroundOxytocin causes clinically significant hypotension and tachycardia. This study examined whether prior administration of phenylephrine obtunds these unwanted haemodynamic effects. MethodsForty pregnant women undergoing elective caesarean section under spinal anaesthesia were randomised to receive either an intravenous 50μg bolus of phenylephrine (Group P) or saline (Group S) immediately before oxytocin (3U over 15s). Systolic blood pressure, diastolic blood pressure, mean arterial pressure and heart rate were recorded using a continuous non-invasive arterial pressure device. Baseline values were averaged for 20s post-delivery. Between-group comparisons were made of the mean peak changes in blood pressure and heart rate, and the mean percentage changes from baseline, during the 150s after oxytocin administration. ResultsThe mean±SD peak percentage change in systolic blood pressure was −16.9±2% in Group P, and −19.0±1.9% in Group S and the estimated mean difference was 2.1% (95% CI −3.5% to 7.8%; P=0.44); corresponding changes in heart rate were 13.5±2.3% and 14.0±1.5% and the mean estimated difference was 0.5% (95% CI −6.0% to 5%; P=0.87). The mean percentage change from the baseline measurements during the 150s period of measurement was greater for Group S than Group P: systolic blood pressure −5.9% vs −3.4% (P=0.149); diastolic blood pressure −7.2% vs −1.5% (P=0.014); mean arterial pressure −6.8% vs −1.5% (P=0.007); heart rate 2.1% vs −2.4% (P=0.033). ConclusionIntravenous phenylephrine 50μg immediately before 3U oxytocin during elective caesarean section does not prevent maternal hypotension and tachycardia.