Minimum Effective Dose of Prophylactic Oxytocin Infusion During Cesarean Delivery in Preterm and Term Pregnancy: A Sequential Allocation Dose Finding Study

BACKGROUND: There is a paucity of uterine oxytocin receptors during preterm gestation. Whether this affects the requirement of oxytocin dose for uterine contraction in patients with preterm gestation is not researched. We compared effective dose in 90% of target population (ED90) of oxytocin infusion for satisfactory uterine tone during cesarean delivery in patients with preterm and term pregnancy. METHODS: This biased coin sequential allocation, dose finding study, with triple blinding to dose allocation included nonlaboring women >18 years posted for cesarean delivery under spinal block, into either term or preterm group (n = 30 each; completed or <37-week gestation, respectively). Oxytocin infusion was initiated at 13 IU·h −1 in the first patient in both groups. Dose in subsequent cases was determined by response to oxytocin in previous patient of a particular group (dosing interval = 2 IU·h −1 ). Uterine tone was assessed using the one-finger palpation method by the surgeon. Myometrial oxytocin receptor expression was also evaluated on tissue obtained during surgery, using immunohistochemistry (IHC). RESULTS: The ED90 of oxytocin infusion to prevent intraoperative uterine atony was 1.5 times greater in the preterm group (25.7 IU·h −1 [95% confidence interval {CI}, 16.4–35.1]) as compared to the term group (16.2 IU·h −1 [95% CI, 14.8–17.7]). Intraoperative oxytocin amount was significantly greater (14.3 [11.7–17.5] vs 12.8 [10.4–14.7] IU; P = .048), and the need of additional uterotonic was clinically higher (16% vs 10%; effect size = 0.5 [95% CI, 0.1–2.5]; P = .448) for the preterm group. IHC showed increased oxytocin receptor expression for term versus preterm group ( P = .040). Incidence of oxytocin-associated hypotension was greater for preterm group (50% vs 13%; P = .002). CONCLUSIONS: During cesarean delivery, oxytocin requirement is almost 1.5 times greater for preterm as compared to term pregnancy. This was supported by decreased expression of the myometrial oxytocin receptor upon IHC.