Introduction: Postpartum hemorrhage (PPH) is a major cause of maternal morbidity and mortality and the commonest cause of PPH is uterine atony. Oxytocin as a uterotonic is used to prevent uterine atony during cesarean section as well as vaginal delivery but higher doses of oxytocin can cause serious maternal side effects.There is significant variability in dose when we compare weight-based versus non weight-based approach and the adverse effects of oxytocin gets unnecessarily amplified in case of the excess dose while its uterotonic effect gets compromised for a deficient one.Hence weight-based oxytocin infusions may help to optimize the amount while also decreasing the side effects.There is no published data regarding weight-based doses of oxytocin infusion. 

Aim: To evaluate the minimum effective dose (ED90) of weight-based fixed-rate oxytocin infusion for achieving and maintaining adequate uterine tone during cesarean section.

Materials and Methods:

Randomization: According to a computer-generated random number table, randomization will be done for allocating patients to 1 of 5 weight-based, fixed-rate prophylactic oxytocin infusions. The various groups would be, group 0.1 (0.1 IU/kg/hr); group 0.15 (0.15 IU/kg/hr); group 0.2 (0.2 IU/kg/hr); group 0.25 (0.25 IU/kg/hr) and group 0.3 (0.3 IU/kg/hr). Concealment of group allocation will be done using sealed opaque envelopes prepared by an uninvolved anesthesiologist.

Blinding: As per group randomization, hourly dose of the weight-based fixed-rate oxytocin infusion will be calculated for each patient and diluted to a volume of 50 ml in appropriate syringe by an uninvolved anesthetist. The infusion will be initiated via a syringe infusion pump at the time of cord clamping at rate of 50 ml/hour. 

Methodology: 

Anaesthetic management: The anaesthetic management including the spinal block for these patients will be standardized as per routine practice. Post-spinal hypotension, prior to initiating oxytocin, will be defined as >20% fall in systolic blood pressure from preoperative baseline value or absolute reading of <90 mmHg whichever is higher. Administration of vasopressor and fluids will be left to the discretion of attending anaesthesiologist.

Upon initiation of oxytocin, blood pressure will be measured every 1 minute for 10 minutes and then as per standard clinical practice; The oxytocin infusion, with dose as per allocated group, will be initiated upon clamping of umbilical cord. The uterine tone will be assessed and graded as adequate or inadequate by the obstetrician, 4 minutes after the initiation of the oxytocin infusion. If uterine tone is deemed adequate at 4 minutes and there is no incidence of an inadequate tone reported subsequently till end of surgery the designated dose of oxytocin will be categorized as a “success”. In case of an unsatisfactory uterine tone at 4 minutes or at any time thereafter during intraoperative period, dose will be considered as a “failure”. For inadequate uterine tone at the 4 minutes observation or at any time thereafter, a rescue bolus of 1 U of oxytocin over 15 seconds will be administered. If uterine tone does not improve after another 3 minutes an additional uterotonic agent will be administered. In all patients, the predetermined weight-based infusion as per group allocation will be continued till end of surgery, irrespective of the outcome of the dose. 

Maintenance oxytocin infusion will be administered at 3 U/hr after the surgery till patient is in the recovery room. 

For calculation of the dose of oxytocin, weight of the patient will be taken on day of admission to hospital or just prior to surgery if same is not available. 

After initiation of oxytocin infusion, associated side effects including hypotension, tachycardia (>10 bpm), ST-T changes, nausea/vomiting, flushing and chest pain will be observed and questioned for, till end of surgery or prior to rescue bolus whichever is earlier. Hypotension will be defined as a fall in systolic blood pressure of >20% below baseline or <90 mmHg whichever is higher. The fall will be calculated from the blood pressure recorded just prior to initiation of oxytocin. 

Statistical Analysis: SPSS-20 statistical software will be used for statistical analysis. The ED50 and ED90 for an effective prophylactic oxytocin infusion dose will be determined using regression.