Maintenance Infusion of Oxytocin Following Elective Cesarean Deliveries

Maintenance Infusion of Oxytocin Following Elective Cesarean Deliveries: an Up-down Sequential Allocation Dose-response Study

This study is designed to determine the minimal effective oxytocin maintenance dose required during cesarean delivery to achieve the best effect. Oxytocin is a drug which is routinely used to help contract the uterus and keep it contracted after the delivery of the baby and placenta. The aim of oxytocin is to reduce the amount of blood that might be lost. In order to determine the minimal effective dose, the investigators will conduct a dose-finding study. The first patient will receive a set oxytocin infusion. If the uterus contracts well, this is considered satisfactory and the next patient will receive the same dose. If the uterus does not contract well, this is considered unsatisfactory and the next patient will receive a higher dose. The dose for the next patient will be determined based on the results of the uterine contraction of the previous patient. The investigators hypothesize that the ED90 of an oxytocin infusion rate to maintain a satisfactory uterine tone during a cesarean delivery in non-laboring women, would be lower than that found in previous studies without an initial bolus (less than 16 IU/h).

This study will be conducted as a prospective, double blinded clinical trial (patient, anesthesiologist and obstetrician blinded to the oxytocin dose), in an up-down sequential allocation fashion. The objective of the study is to determine the minimum effective dose of oxytocin infusion required to produce appropriate uterine contraction during and after cesarean delivery under neuraxial anesthesia in non-laboring women. For the purpose of this study, the minimum effective dose is defined to be that at which adequate response occurs in 90% of patients, i.e. ED90.

Uterine tone will be evaluated as satisfactory or unsatisfactory by the obstetrician throughout the surgery, starting at 5 minutes after the oxytocin infusion. It will also be measured at 10 minutes following the oxytocin infusion and every 10 minutes following that for the duration of the surgery. Unsatisfactory uterine tone at any time until discharge from the PACU will be considered a failure of the maintenance infusion.

Estimated blood loss will be calculated using the difference in hematocrit values prior to surgery and 24 hours post surgery, using the following formula: Calculated blood loss (mL) = EBV ((Pre-op Htc-Post-op Htc)/pre-op Htc) EBV (estimated blood volume) in ml: patient's weight in kg x 85.

A request made by the obstetrician performing the cesarean delivery for additional uterotonic medication, due to bleeding or poor uterine tone.

Requirement for additional uterotonic medication from discharge from the PACU until 24 hours postpartum.

Any uterotonic medication administered after the patient is discharged from PACU until 24 hours postpartum.

Postoperative uterine position (below or above the umbilicus) as determined by the PACU nurse. This is measured every 15 minutes for the first hour and then hourly until discharge to the postpartum unit.

Presence of ventricular tachycardia as recorded by ECG, from drug administration until end of surgery

The presence of nausea and number of episodes, from drug administration until end of surgery, as reported by the patient

Any presence of chest pain, from drug administration until end of surgery, as reported by the patient

Any presence of shortness of breath, from drug administration until end of surgery, as reported by the patient

Inclusion Criteria: Elective uncomplicated cesarean delivery under neuraxial anesthesia >18 year-old with ASA status II or III, with a singleton gestation and a gestational age between 37 to 42 weeks Informed consent to participate in this study Exclusion Criteria: Refusal to give written informed consent Allergy or hypersensitivity to oxytocin Body mass index ≥ 40 kg/m2on the day of admission A history of hypertension and/ or severe cardiac disease(s) Contra-indications for neuraxial anesthesia Conditions that predispose to uterine atony and postpartum hemorrhage such as placenta previa, multiple gestation, preeclampsia, macrosomia (≥ 4 kg), polyhydramnios, uterine fibroids in and near operating field, previous history of uterine atony and postpartum bleeding, or bleeding diathesis.