Extradural pethidine with and without adrenaline during labour: wide variation in effect.

The pain-relieving effect of a single extradural dose of pethidine 25 mg with and without adrenaline was studied in 20 healthy women during labour. The study was open regarding the effects of pethidine but double-blind regarding the addition of adrenaline. In 14 of 19 women good or excellent analgesia was achieved for a period of 50-160 min. Pethidine with adrenaline 25 micrograms was not more effective than pethidine alone. Eight of the 14 women showed signs of regional analgesia to pin-prick and temperature discrimination. The patients had small (45-188 ng ml-1) concentrations of pethidine in plasma. In eight patients the plasma concentrations of pethidine were maintained for at least 1.5 h. Extradural pethidine thus induces analgesia of short and variable duration. Repeated doses may be needed, resulting in accumulation of the drug in plasma with the risk of respiratory depression in mother or child.