The pharmacokinetics of oxytocin as they apply to labor induction

OBJECTIVE: Our purpose was to determine the relationship among plasma oxytocin levels, metabolic clearance rate of oxytocin, and uterine activity in gravid women undergoing labor induction. STUDY DESIGN: Ten women receiving oxytocin for labor induction and agreeing to participate had blood sampled before initiation of oxytocin and at different levels of uterine pressure. Samples were analyzed with 200 μl extracts from 1 ml of plasma with an oxytocin radioimmunoassay. The intraassay coefficient of variation was <3%. Sensitivity of the assay was 1.5 pg/ml. Pharmacokinetic parameters including plasma levels and metabolic clearance rates were calculated. Data were analyzed with the paired t test and linear and logistic regression. RESULTS: Mean oxytocin levels and metabolic clearance rates were 26.6 pg/ml and 7.97 ml/min. There was no correlation between changes in oxytocin level and metabolic clearance rate. Increases in infusion rates were correlated with increases in oxytocin levels (r = 0.71, p < 0.001). Cervical dilatation and uterine contraction pressures did not correlate with oxytocin levels. CONCLUSION: Peripheral plasma levels of oxytocin may not accurately reflect uterine activity or progress in labor. Plasma levels of oxytocin may merely reflect the rate of oxytocin infusion.