The effect of morphine and naloxone administration on plasma oxytocin concentrations in the first stage of labour

Abstract

OBJECTIVE: We investigated the effect of an opiate (morphine) and an opiate antagonist (naloxone) on the maternal secretion of oxytocin in the first stage of labour. DESIGN: Patients were randomized to receive either morphine 5 mg (n = 9), naloxone 1.2 mg (n = 10) or sterile water (n = 9) which was injected intravenously. PATIENTS: Healthy women in the first stage of labour between 3 and 6 cm dilated with no prior analgesia or oxytocin administration were recruited for the study. MEASUREMENTS: Peripheral maternal oxytocin levels were measured by radioimmunoassay for 15 minutes before and 15 minutes after administration of the assigned substance. Sampling was at 2.5 minute intervals. RESULTS: Significant reduction in the mean oxytocin concentration was found in the patients who received morphine (-2.62 pmol/l/sample) but no change was found in the naloxone group (+0.57 pmol/l/sample) when compared with controls (+0.64 pmol/l/sample). CONCLUSION: Maternal oxytocin secretion is inhibited by exogenous opiates in the first stage of labour while an effect of opiate antagonism was not demonstrated.