Maternal kinetics and transplacental passage of pethidine during labour

1 Pethidine is commonly used in single doses as an analgesic in obstetrics. Plasma concentration-time profiles of pethidine after intramuscular administration of 1.5 mg/kg body weight to 16 pregnant women during labour were investigated. There was only a two-fold variation in peak plasma concentration (300-650 ng/ml). The mean (+/- s.d.) value of the apparent plasma half-life of pethidine was 3.4 (+/- 1.0) h which is not different from that in healthy controls. norpethidine plasma levels were not measurable (less than 10 ng/ml). 2 The placental transfer transfer of pethidine was studied at delivery in samples from the umbilical cord vessels and from a maternal peripheral vein. In another 14 patients serial determinations of pethidine concentration were made in foetal scalp blood and maternal venous blood simultaneously during the different stages of labour. The foeto-maternal drug ratio varied between 0.35 and 1.5 with a positive correlation between ratio and dose delivery time interval. The concentration of pethidine in umbilical cord plasma or blood varied between 60 and 400 ng/ml with dose-delivery time intervals of 30 min to 10.5 h. The foetal concentration of pethidine reached a peak-plateau value between 1-5 h after dose.