The correlation of the thermal and mechanical antinociceptive activity of pethidine hydrochloride with plasma concentrations of the drug in sheep.

The analgesic activity of pethidine was measured in eight sheep using both thermal and mechanical test systems. Pethidine, at a dose rate of 5 mg/kg body weight given intravenously, produced a significant degree of antinociception to thermal pain for 30 min (on average) but gave only a few minutes of significant analgesia when tested in the mechanical pressure system. Analgesia in both systems was abolished by pretreatment with naloxone. Pharmacokinetic analyses of plasma levels of pethidine after intravenous (i.v.) injection were carried out. Plasma concentrations of the drug exhibited a rapid biexponential pattern of decline with an average distribution half-life of 0.99 min and an elimination half-life of 12.8 min. Correlation of plasma levels of the drug with the presence of a significant degree of antinociception in the thermal test system enabled 'critical' analgesic levels of pethidine to be defined for sheep (0.93 microgram/ml).