Performance of a closed-loop feedback computer-controlled infusion system for maintaining blood pressure during spinal anaesthesia for caesarean section: a randomized controlled comparison of norepinephrine versus phenylephrine

Closed-loop feedback computer-controlled vasopressor infusion has been previously described for maintaining blood pressure during spinal anaesthesia for caesarean section but there are limited data available comparing the relative performance of different vasopressors. The aim of this study was to compare the performance of norepinephrine versus phenylephrine in this system. Data from a randomized, two-arm parallel group, double-blinded controlled trial were reanalyzed. 104 patients scheduled for elective caesarean section under spinal anaesthesia were randomized to receive computer-controlled closed-loop infusion of either norepinephrine 5 µg ml⁻¹ or phenylephrine 100 µg ml⁻¹. This was started immediately after induction of spinal anaesthesia and used an algorithm designed to maintain systolic blood pressure near baseline until fetal delivery. Performance error calculations were used to compare the performance of the two vasopressors. The primary outcome was defined as the median absolute performance error. Median performance error, wobble and divergence were also compared. Median absolute performance error was smaller in the norepinephrine group (median 3.79 [interquartile range 2.82–5.17] %) versus the phenylephrine group (4.70 [3.23–6.57] %, P = 0.028). In addition, median performance error was smaller (0.75 [−1.56–2.52] %) versus 2.61 [0.83–4.57] %, P = 0.002) and wobble was smaller (2.85 [2.07–5.17] %) versus 3.39 [2.62–4.90] %, P = 0.028) in the norepinephrine group versus the phenylephrine group. Divergence was similar between groups. The precision of the control of blood pressure was greater with norepinephrine compared with phenylephrine at the drug concentrations used.