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 act ivity in gravid women undergoing labor induction. STUDY DESIGN: Ten wo men receiving oxytocin for labor induction and agreeing to participate had blood sampled before initiation of oxytocin and at different leve ls of uterine pressure. Samples were analyzed with 200 mu l extracts f rom 1 ml of plasma with an oxytocin radioimmunoassay. The intraassay c oefficient of variation was <3%. Sensitivity of the assay was 1.5 pg/m l. Pharmacokinetic parameters including plasma levels and metabolic cl earance rates were calculated Data were analyzed with the paired t tes t and linear and logistic regression. RESULTS: Mean oxytocin levels an d metabolic clearance rates were 26.6 pg/ml and 7.97 ml/min. There was no correlation between changes in oxytocin level and metabolic cleara nce rate. Increases in infusion rates were correlated with increases i n oxytocin levels (r = 0.71, p < 0.001). Cervical dilatation and uteri ne contraction pressures did not correlate with oxytocin levels. CONCL USION: 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.