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Relative potency of noradrenaline vs. phenylephrine infusions in the prevention of hypotension after spinal anaesthesia for caesarean delivery
Abstract Number: GM-05
Abstract Type: Original Research
Introduction: Noradrenaline has recently been introduced for preventing hypotension after spinal anesthesia for caesarean delivery. Compared to phenylephrine, noradrenaline is effective in maintaining blood pressure (BP), but may be superior at maintaining heart rate (HR) and cardiac output (CO). In a comparative dose-response study, noradrenaline given as a single IV bolus was 13 times more potent than phenylephrine. It is uncertain if this potency ratio also applies to infusions. We aimed to determine the minimum infusion rates (MIR) (median effective rate, ER50) of both drugs needed to maintain maternal systolic BP within 20% of the baseline after spinal anesthesia for caesarean delivery and to derive the relative potency ratio.
Methods: Sixty ASA physical status 2 patients with a normal singleton pregnancy beyond 36 weeks gestation undergoing elective caesarean delivery with standardized spinal anesthesia (hyperbaric bupivacaine 0.5% w/v 12.5mg, with Fentanyl 15mcg) were randomized into two groups. The first patient in Group 1 received phenylephrine 1200mcg in normal saline 0.9% w/v 60 ml at 60ml/h infusion rate (20mcg/min). The first patient in Group 2 received noradrenaline 96mcg in normal saline 0.9% w/v 60ml at 60ml/h infusion rate (1.6mcg/min). The vasopressor dose for every subsequent patient was determined by the efficacy of the dose in preventing hypotension in the previous patient. If effective, the next patient received a dose reduced by 150mcg of phenylephrine or 12 mcg of noradrenaline. If ineffective, the dose for the next patient was increased by the same amount. The MIRs were determined according to the Dixon–Massey formula. Continuous stroke volume (SV), HR and CO were also measured using non-invasive hemodynamic monitoring system.
Results: The MIR was 12.7 mcg/min (95%CI 10.4 to 14.9) for phenylephrine and 1.0 mcg/min (95%CI 0.8 to 1.2) for noradrenaline, giving a potency ratio of 12.6 (95%CI 9.9 to 15.9). There was no difference in HR, SV and CO between the groups.
Conclusion: We determined MIR of phenylephrine and noradrenaline in prevention of hypotension after spinal anesthesia for caesarean delivery and confirmed a potency ratio of 12.6 (95%CI 9.9 to 15.9) for equivalence in blood pressure control. At the MIR, noradrenaline does not provide benefits of greater HR and CO.
1. Ngan Kee WD. A Random-allocation Graded Dose-Response Study of Norepinephrine and Phenylephrine for Treating Hypotension during Spinal Anesthesia for Cesarean Delivery. Anesthesiology, 2017;127:934-941