Urinary excretion and metabolism of pethidine and norpethidine in the newborn

In seven neonates, whose mothers were given pethidine during labour, urine was collected for the first 24-40 h of life. Urinary volume and pH, and concentrations of pethidine and norpethidine in the urine were measured. Urine flow rate was low for the first 7-22 h, and then high for about 12 h. The rate of excretion of pethidine and norepethidine was approximately parallel to the urine flow rate. However, the ratio of the rate of excretion of norpethidine to that of pethidine increased with time and the concentration of norpethidine in urine decreased first and then, after 18 h, increased significantly. These findings that the neonate can metabolize pethidine, although the rate of metabolism is probably less than in the adult. The total amounts of pethidine and norpethidine excreted in the first 24 h after birth were positively related to the dose-delivery interval in the mother for intervals up to at least 5 h. From the data it is estimated that 95% of the total pethidine transferred from the mother would be eliminated by the baby by the 2nd to 3rd day after birth.