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ED50 of plain oxytocin and ergometrine-oxytocin combinations for uterine contraction following elective cesarean delivery.
Abstract Number: TW-4
Abstract Type: Original Research
Introduction: Obstetric hemorrhage is an important cause of maternal death and severe morbidity. Although tocotonic drugs are routinely used, they have important side effects. Oxytocin causes profound vasodilatation, arterial hypotension and ECG changes. Ergometrine, on the other hand, causes systemic vasoconstriction and arterial hypertension, impairing cardiac output. There is some rational for combining these drugs to achieve tocotonicity with fewer hemodynamic side effects. This study is a preliminary step to identify equipotential doses of oxytocin when co-administered with zero, low (1 mcg/kg/hr), medium (2 mcg/kg/hr) and high (3 mcg/kg/hr) doses of ergometrine.
Methods: Inclusion: 57 healthy women presenting for elective repeat cesarean delivery under regional anesthesia, ASA class 1-2, aged 18-45, body weight 60-100 kg. Exclusion: Pre-eclampsia, chronic hypertension, cardiac or neurological disorder, abnormalities of placentation, multiple gestation, preterm delivery, > 3 previous cesareans, history of obstetric hemorrhage or cord blood collection for storage. ED50 for intravenous oxytocin for tocotonicity following cesarean delivery was determined using up-down sequential analysis based on the success or failure of previously allocated subjects. Success was determined by both the subjective (VAS 0-10) assessment of the operating obstetrician (who was blinded to drug doses used) and by a 20% rise in the uterine tone measured externally by a uterine Shore durometer (Schmidt Control Instruments, Waldkraiburg, Germany; size OO, 3/32" spherical tip). Success or failure of the oxytocin infusion was determined within 5 minutes of drug administration. The ED50 was determined by exact logistic regression and is presented as mean (95% CI).
Results: There was a clear dose-dependent tocotonic effect for oxytocin (p=0.0068). The up-down sequential plots for success and failure are shown for the four dose regimes used (Fig 1). The ED50 for plain oxytocin was 0.029 IU/kg/hr (0.000-0.065). The ED50 for oxytocin when co-administered with low, medium and high doses of ergometrine was 0.023 IU/kg/hr (0.000-0.055), 0.017 IU/kg/hr (0.000-0.055), and 0.013 IU/kg/hr (0.000-0.047).
Conclusions: These data provide equipotential doses of oxytocin and ergometrine combinations, an essential pre-requisite for ongoing studies of hemodynamic side effects of oxytocin-ergometrine combinations for tocotonicity following cesarean delivery.