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

Abstract

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. 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. 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. 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. 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). Maternal oxytocin secretion is inhibited by exogenous opiates in the first stage of labour while an effect of opiate antagonism was not demonstrated.