Comparison of phenylephrine infusion regimens for maintaining maternal blood pressure during spinal anaesthesia for Caesarean section

Background. During spinal anaesthesia for Caesarean section, the optimal phenylephrine regimen and the optimal blood pressure (BP) to which it should be titrated are undetermined. The ideal regimen would balance efficacy for maintaining uteroplacental perfusion pressure against potential for uteroplacental vasoconstriction, both of which may affect fetal acid-base status. We compared phenylephrine infusion regimens based on three different BP thresholds. Methods. After intrathecal injection, we infused phenylephrine 100 mug min(-1) for 2 min. Then, until delivery, we infused phenylephrine whenever systolic BP (SBP), measured every 1 min, was below a randomly assigned percentage of baseline: 100% (Group 100, n=25), 90% (Group 90, n=25) or 80% (Group 80, n=24). We compared umbilical blood gases, Apgar scores and maternal haemodynamics and symptoms. Results. Patients in Group 100 had fewer episodes [median 0 (range 0-8)] of hypotension (SBP <80% baseline) compared with Group 80 [5 (0-18)] and Group 90 [2 (0-7)] (P<0.001 in each instance). Total dose of phenylephrine was greater in Group 100 [median 1520 mug (interquartile range 1250-2130 mug)] compared with Group 90 [1070 (890-1360) mug] and Group 80 [790 (590-950) mug]. Umbilical arterial pH was greater in Group 100 [mean 7.32 (95% confidence interval 7.31-7.34)] than in Group 80 [7.30 (7.28-7.31)] (P=0.034). No patient had umbilical arterial pH <7.2. In Group 100, 1/24 (4%) patients had nausea or vomiting compared with 4/25 (16%) in Group 90 and 10/25 (40%) in Group 80 (P=0.006). Conclusions. For optimal management, phenylephrine should be titrated to maintain maternal BP at near-baseline values.