Carbetocin vs. Oxytocin: In-vitro Myometrial Contractions With and Without Oxytocin Pre-treatment

The purpose of this study is to compare the ability of an isolated sample of uterine muscle tissue (in a tissue bath) to contract in the presence of various drugs. The drugs studied--uterotonics--are typically used to contract the uterus when a pregnant patient continues to bleed after delivery. Oxytocin is an old standard, but seems to suffer from a desensitization phenomenon. Carbetocin, a similar drug, has recently been suggested to clinicians as a replacement for oxytocin directly after certain types of Cesarean section. The investigators will be testing isolated uterine muscle samples after pre-treatment with oxytocin OR nothing (control) to increasing concentrations of oxytocin OR carbetocin. Contractile measures will be measured and compared between all groups. The investigators hypothesize that oxytocin pre-treatment will reduce contractions in both oxytocin- and carbetocin-induced contractions, and oxytocin and carbetocin will induce different patterns of contractions.

Postpartum hemorrhage (PPH) is a major cause of maternal mortality and morbidity. In 80% cases of PPH, the primary cause is failure of the uterus to contract after delivery of the baby, which then requires further treatment of the mother with uterotonic drugs (drugs used to contract uterus and thus prevent bleeding). Patients participating in this study will be asked to donate a very small sample of uterine tissue during Cesarean section, which will be tested in the laboratory for the ability to contract in response to the uterotonics oxytocin and carbetocin. We will seek to better understand the effects of these drugs by comparing their contractile capability in isolated uterine tissue.

A control sample from each patient (no uterotonic drug will be applied during pre-treatment) will be measured concurrently with samples pre-treated with oxytocin. Controls will undergo the same dose response of oxytocin or carbetocin after pre-treatment period as the groups given oxytocin for pre-treatment.

A sample from each patient will be pre-treated with oxytocin 10-5mol/L. These samples will undergo the same dose response of oxytocin or carbetocin after pre-treatment period as the groups given no drug for pre-treatment (controls).

Oxytocin, 10-10mol/L to 10-5mol/L; solution made in double-distilled autoclaved water and then tested in physiological salt solution.

Carbetocin, 10-10mol/L to 10-5mol/L; solution made in double-distilled autoclaved water and then tested in physiological salt solution.

Inclusion Criteria: Gestational age 37-41 weeks Non-laboring patients, not exposed to exogenous oxytocin Patients requiring primary or first repeat Cesarean section Cesarean section under spinal anesthesia Exclusion Criteria: Patients who require general anesthesia Patient who had previous myometrial surgery or more than one previous Cesarean section Patients with placental anomalies Emergency Cesarean section in labor Patients with multiple pregnancy (twins, etc.)