The Intraoperative Median Effective Dose of Oxytocin for Preventing Uterine Atony in Parturients with a Prior History of Caesarean Delivery.

While oxytocin is commonly used for the prevention of uterine atony, its pharmacology may be affected by a prior history of caesarean delivery. The objective of this study was to determine the 50% effective dose (ED50) of bolus oxytocin after caesarean delivery in parturients with and without prior caesarean delivery. This was a parallel-group, double-blind, dose-response study using Dixon’s up-and-down sequential allocation method to estimate the ED50 of bolus-administered oxytocin in parturients having caesarean delivery under combined spinal-epidural anaesthesia (CSE). Twenty-seven parturients with a history of prior caesarean delivery (With-PCD group) and 26 parturients with no such history (Without-PCD group) were enrolled. Oxytocin was administered as an intravenous bolus at a starting dose of 0.5 units, which was then increased or decreased by 0.25 units at a time. Uterine tone was assessed by the obstetrician as either ‘adequate’ or ‘inadequate’ 3 min after delivery of the fetus. Adverse effects, administration of additional uterotonic agents, and estimated blood loss were recorded. The ED50 of oxytocin was greater in the With-PCD group than in the Without-PCD group (0.95 units [95% CI 0.82–1.08] vs. 0.55 units [95% CI 0.38–0.73], P < 0.001). The overall incidence of adverse effects was higher in the With-PCD group than in the Without-PCD group (33.3% vs. 7.7%, P = 0.02). The initial bolus dose of oxytocin needed to prevent uterine atony was higher in parturients with prior caesarean delivery than in parturients without prior caesarean delivery. Uterine scarring may contribute to the increased oxytocin requirements of the former group. ChiCTR1900023474; investigator: Wei CN; date of registration: 30 May 2019.