The influence of surgery and anaesthesia on the pharmacokinetics of pethidine in the horse.

The plasma concentration of pethidine was measured in 16 horses, after its administration intravenously (i.v.) at a dose rate of 1 mg/kg bodyweight (bwt). In eight animals studied before surgery, the plasma levels of the drug decreased in a bi-exponential manner with a distributive half life of 3 mins and an elimination half life of 57.7 mins. Total body clearance was 17.7 ml/kg bwt/min. The remaining horses were investigated immediately after a period of anaesthesia and surgery and in these animals the drug exhibited smaller volumes of distribution (V1 cand Vdarea) and a significantly lower clearance rate of 12.8 ml/kg bwt/min in these animals. Correlation between the plasma concentration of pethidine and information published previously on the duration of action of the agent in the horses suggests that plasma levels of approximately 0.4 microgram/ml are required to produce analgesia in this species.