Maternal blood pressure targets and neonatal outcomes at caesarean birth

The trial by Chen et al. comparing maternal systolic blood pressure targets during caesarean birth concludes that maintaining a higher maternal blood pressure protects against fetal acidaemia [1]. Few would dispute that maternal hypotension is undesirable or that there are good reasons to maintain uterine perfusion pressure. However, whether this trial supports the specific claim being made about neonatal outcomes deserves consideration. The trial was powered for its primary outcome of mean umbilical artery pH, which was identical between groups. The concluding claim regarding difference in fetal acidaemia is from a secondary outcome, for which the study was not powered. Only 16 events occurred across 1183 patients. With event counts this low, a difference of a handful of cases could abolish or reverse the finding. Base excess was identical between groups and Apgar scores were also comparable. The data therefore do not show that the higher blood pressure target conferred meaningful neonatal benefit, even if the underlying physiological principle remains sound. The maternal outcomes show that targeting 90% of baseline blood pressure reduced the incidence of hypotension, severe hypotension and nausea and vomiting, consistent with previous work [2]. These are worthwhile findings and provide a reasonable justification for targeting a higher blood pressure, but they are distinct from the claim that higher targets improve neonatal outcomes. A 2018 international consensus statement recommended prophylactic vasopressor infusions [3]. A reactive bolus strategy as used by Chen et al. is inherently less able to prevent hypotension than a prophylactic infusion, which is reflected in the striking incidence of hypotension in the control group. Many anaesthetists would consider this incidence avoidable with a prophylactic regimen, and it is unclear whether the observed differences reflect the blood pressure targets themselves or the limitations of reactive bolus dosing. Chen et al. have conducted a large multicentre trial and the reductions in maternal hypotension and nausea are clinically relevant. Maintaining maternal blood pressure during caesarean birth is good practice. However, the concluding message that higher blood pressure targets protect against fetal acidaemia should be interpreted with caution.