Pharmacokinetics of intramuscularly administered pethidine in dogs and the influence of anaesthesia and surgery

Department of Veterinary Surgery, University of Bristol, Langford.

The plasma concentration of pethidine was measured after it had been administered intramuscularly to fully conscious dogs, and to dogs in the postoperative period and during general anaesthesia. The absorption of the drug was erratic except in the anaesthetised animals and the plasma concentrations of the drug were also higher in this group. Correlation of plasma concentrations of the drug with its analgesic activity revealed a 'critical' concentration of pethidine of 0.4 micrograms/ml for complete analgesia; useful though not complete analgesia was achieved with concentrations above 0.2 micrograms/ml. These concentrations were maintained for 90 minutes after the administration of the drug at a dose of 2.0 mg/kg and for 120 minutes after a dose of 3.5 mg/kg.

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