Optimal Infusion Rate of Norepinephrine for Prevention of Spinal Hypotension for Cesarean Delivery: A Randomized Controlled Trial, Using Up-Down Sequential Allocation.

Background: Norepinephrine has recently been suggested to be as effective as phenylephrine for the prevention of hypotension after spinal anesthesia for cesarean delivery. Moreover, compared to phenylephrine, norepinephrine may be superior in maintaining heart rate (HR) and consequently, cardiac output (CO). A recent study demonstrated that norepinephrine given as a single intravenous bolus is approximately 13 times more potent than phenylephrine. However, it is uncertain whether this finding can be applied when these vasopressors are administered as infusions. Therefore, the optimum infusion rate of norepinephrine remains unknown. We aimed to determine the median effective dose (ED50; defined as the rate of vasopressor infusion required to prevent spinal hypotension in 50% of subjects) of both drugs needed to maintain maternal systolic blood pressure within 20% of the baseline after spinal anesthesia for cesarean delivery and to derive the relative potency ratio.

Methods: Sixty healthy patients undergoing elective cesarean delivery with standardized spinal anesthesia were randomized into 2 groups. The first patient in group 1 received phenylephrine 1200 µg in normal saline 0.9% w/v 60 mL at 60 mL/h infusion rate (20 µg.min-1). The first patient in group 2 received norepinephrine 96 µg in normal saline 0.9% w/v 60 mL at 60 mL/h infusion rate (1.6 µg.min-1). Using up-down sequential allocation technique, the vasopressor dose for every subsequent patient was determined by the response in the previous patient. If effective, the next patient received a dose reduced by 150 µg of phenylephrine (2.5 µg.min-1) or 12 µg (0.2 µg.min-1) of norepinephrine. If ineffective, the dose for the next patient was increased by the same amount. The ED50s were determined according to the Dixon-Massey formula. Stroke volume (SV), HR, and CO were also measured.

Results: The ED50 was 12.7 µg.min-1 (95% CI, 10.5-14.9) for phenylephrine and 1.01 µg.min-1 (95% CI, 0.84-1.18) for norepinephrine, giving a potency ratio of 12.6 (95% CI, 9.92-15.9). HR, SV, and CO did not differ between the groups.

Conclusions: Norepinephrine is more potent than phenylephrine by a factor of approximately 13 when administered as infusion for equivalent maternal blood pressure control. Based on these findings, we recommend a variable rate prophylactic infusion of norepinephrine to be initiated at 1.9 to 3.8 µg.min-1 for the management of hypotension during cesarean delivery under spinal anesthesia.