Ventilatory depression of the newborn of women receiving pethidine or pentazocine. A double-blind comparative trial.

Forty-two women at term received pethidine 100 mg and 43 received pentazocine 45 mg, both given in a double-blind randomized manner. There was no difference in analgesic effect between the groups, but twelve patients were judged to need more than one injection of pentazocine, compared with seven in the pethidine group. The Apgar scores at 1 and 5 min were significantly less in the pethidine group. Four neonates in the pethidine group and two in the pentazocine group were severely depressed and received naloxone between 5 and 15 min after birth. The remainder of the infants received naloxone 200 microgram i.m. 15 min after birth. From 5 min after birth, end-expiratory carbon dioxide concentrations and from 15 min transcutaneous PO2 were recorded. A significantly smaller end-tidal carbon dioxide concentration was measured when more than one injection of pentazocine had been given. Repeated doses of pethidine, on the other hand, resulted in a greater end-tidal carbon dioxide concentration. Vigorous ventilation was even more pronounced when naloxone was given indicating an analeptic effect when two drugs with antagonistic activity are combined. At no time was transcutaneous PO2 less than 6.1 kPa. We conclude that both pethidine and pentazocine produce adequate pain relief during labour, but more than one injection of pethidine is associated with greater neonatal depression.