A comparison of methods for evaluation of pain and distress after orthopaedic surgery in horses

Post operative pain was evaluated in 13 horses subjectively and by measuring pre-defined behavioural and physiological variables. Twelve of the horses had undergone painful orthopaedic surgery and one had developed post anaesthetic myopathy and nerve damage after radiological examination under general anaesthesia. Venous blood samples were collected for catecholamine, ß-endorphin and cortisol assays before premedication and up to 72 h after surgery. No differences were seen in head position, ground pawing or use of the operated leg between the pre-and post anaesthetic periods. The subjective pain score was higher at 4 h after anaesthesia than the pre-operative score. Mean plasma ß-endorphin concentration increased above pre-operative values at 6 and 12 h after anaesthesia. Anaesthesia and surgery did not change mean plasma cortisol concentration, but it was lower in samples collected at 48 and 72 h. There was considerable individual variation in plasma catecholamine concentrations. At some time points the plasma concentration of catecholamines was higher than before premedication, but the maximal concentrations for adrenaline, noradrenaline and dopamine did not occur simultaneously. Heart rate was increased at 2 and 4 h, and abdominal sounds were decreased at 2 h. Packed cell volume was decreased in samples taken 2 to 12 h after anaesthesia. There was poor correlation between all the parameters measured in the study The subjective pain score and the plasma ß-endorphin concentration correlated with each other at several time points, but not during the immediate post operative period.