The influence of dose-delivery time interval on neonatal plasma pethidine levels

Plasma pethidine levels were determined by RIA during the first 24h of life in 126 neonates whose mothers had received a single intramuscular dose of 100-150 mg pethidine HCl 0.25 to 19.5 h before delivery. The mean (geometric) maternal and cord venous plasma pethidine concentration and the neonatal capillary plasma levels at 0.5, 1.5 and 12-24 h expressed per unit dose were 125, 98, 71, 59 and 23 ng/ml per mg/kg respectively. The mean (+/- S.E.M.) cord: maternal plasma concentration ratio was 0.741 +/- 0.036 (n = 40). The ratio was significantly correlated with the dose-delivery interval, r = 0.42, p less than 0.01. The pethidine concentration in cord and in neonatal capillary plasma was significantly correlated to the weight related dose, r = 0.25 to 0.48, p less than 0.01. Cord venous levels of pethidine rose rapidly after drug administration but neonatal capillary levels increased slowly to reach peak levels at 2 to 5 h before declining over the next 15 to 18 h. The maternal plasma pethidine half-life, 5.59 h was considerably shorter than that observed in the newborn, 15.94 h.