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Effect of magnesium sulphate pre-exposure on oxytocin-induced contractility in desensitized human myometrium – an in vitro study
Abstract Number: F-09
Abstract Type: Original Research
Introduction: Women with oxytocin-augmented labors are at higher risk for postpartum hemorrhage (PPH), possibly due to desensitization of oxytocin receptors (OTR) . Magnesium sulphate (MgSO4) is used for preeclampsia, eclampsia, fetal neuroprotection and as a tocolytic. There are suggestions MgSO4 may lead to increased oxytocin requirements or PPH in preeclampsia . It is known to reduce myometrial contractions in vitro , however, its effect on oxytocin-induced contractility in desensitized myometrium is unknown. The objective of this study was to determine oxytocin-induced myometrial contractility, in desensitized and control samples exposed to MgSO4.
Methods: This prospective in vitro study was conducted with institutional REB approval and the informed consent of women undergoing elective Cesarean section, with no PPH risk factors. A myometrial sample was divided into 6 strips and mounted in separate organ baths with physiological salt solution (PSS) under homeostatic conditions. Samples were allocated to one of 3 pretreatment groups: MgSO4 3.5mM (Mg group), MgSO4 3.5mM + oxytocin 10-5M (Mg-Oxy group) and PSS (control group). Strips were then subjected to dose-response testing with oxytocin (10-10M to 10-5M). The primary outcome was motility index (MI, amplitude x frequency), presented as a % of baseline contractility.
Results: So far we have completed 23 experiments: Mg n=6, Mg-Oxy n=9, control n=8 (required sample size 96 strips, 32/group). We plan to complete the study by March 2016. Results so far indicate MI was higher in the Mg group (489%) and lower in the Mg-Oxy group (202%) as compared to the control group (227%).
Discussion: Pre-treatment with MgSO4 itself does not seem to affect myometrial contractions. The attenuated contractions in the Mg-Oxy group could possibly be due to the desensitization mechanism, as a result of oxytocin pre-treatment. Nonetheless, further samples and experiments are yet to be conducted. Final discussion and conclusion will be presented at the meeting.
References: 1) AJOG 2003;188:497-502. 2) AJOG 1997;176:623-7 3) AJOG 2006;194:1384-90