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Myometrial contractility after oxytocin pre-exposure in women with advanced maternal age and morbid obesity
Abstract Number: F-11
Abstract Type: Original Research
Women with advanced maternal age (AMA) and morbid obesity (MO) are at a greater risk for postpartum hemorrhage (PPH). Oxytocin is the first line drug in the treatment of PPH. Prolonged exposure to oxytocin can result in desensitization of the oxytocin receptors , which may result in poor uterine tone after delivery with attenuated response to oxytocin. It is not known if the higher incidence of PPH seen in these women is due to poor uterine contractility. Further it is not known if oxytocin desensitization specifically affects contractility in AMA and MO women when compared to younger or normal weight women. We aimed to investigate the effect of oxytocin on myometrial strips of AMA and MO women in-vitro.
The in-vitro study was conducted after REB approval and written informed consent from women undergoing elective cesarean deliveries. Three groups of patients were studied: control (≤35 yr, BMI 20–24.9 kg/m2), AMA (≥40 yr, BMI 20–24.9 kg/m2), and MO (≤35 yr, BMI≥40 kg/m2). Myometrial tissue obtained from the uterine incision was dissected into six strips. Each strip was mounted in a single organ bath with physiological salt solution (PSS) and then pretreated with oxytocin 10-5M (desensitization model) or left in PSS (untreated) for 2 hours. This was followed by a dose-response testing to oxytocin 10-10M to 10-5M. The primary outcome was motility index (MI; amplitude x frequency) of myometrial contractions. Data was analyzed using the % response during the dose response relative to the baseline contractions.
So far 126 experiments have been performed (required n=168) with samples from 33 women: control (n=56), AMA (n=48), MO (n=22). The MI, calculated as a cumulative dose-response average, was higher in the control group (457%) compared to the AMA (414%) and MO (321%) groups in samples not pretreated with oxytocin. In the oxytocin-pretreated samples, the MI was lower in the control group (111%) compared to the AMA (158%) and MO (281%) groups (Fig 1). We plan to complete this study by March 15, following recruitment of 7 more patients.
Our results suggest that women with AMA and particularly those with MO may exhibit poor intrinsic uterine contractility as compared to younger and normal weight women. Furthermore their uterine contractility is further impaired by pre-exposure to oxytocin.
References: 1) Am J Obstet Gynecol 2003; 188:497-502; 2) Anesthesiology 2013; 119: 552-561