To compare the median effective dose (ED₅₀) of phenylephrine for prophylactic continuous infusion in parturients with different body mass indices (BMIs) during combined spinal-epidural anaesthesia for caesarean section and to investigate the impact of maternal BMI on the prophylactic dose of phenylephrine.

Parturients receiving combined spinal-epidural anaesthesia for elective caesarean section were divided into a standard group (Group S, BMI < 30 kg/m²) and an obesity group (Group O, BMI > 30 kg/m²), each with 30 patients. A sequential allocation design was used to administer the prophylactic infusion of phenylephrine after the completion of a spinal anaesthetic injection to prevent hypotension (defined as a reduction of systolic blood pressure ≥ 20% of the baseline value or systolic blood pressure < 90 mmHg), with an initial infusion rate of 50 μg/min for the first parturient subsequent adjusted up or down by 10 μg/min depending on whether the previous parturient developed hypotension or not during the study period. The Dixon and Massey method and the isotonic regression method were used to calculate and compare the ED₅₀ and 95% confidence interval (CI) of phenylephrine between the two groups.

The results were 21.92 μg/min (95% CI, 14.90–28.94 μg/min) for Group S and 42.14 μg/min (95% CI, 24.58–59.70 μg/min) for Group O. The ratio of relative potency of Group O to Group S is 1.92 (95% CI 1.09–3.14), P = 0.034.

The dose of phenylephrine for the prevention of hypotension after spinal anaesthesia for caesarean section is dependent on maternal BMI. Therefore, a weight-based phenylephrine dose is reasonable.