Medetomidine, an alpha 2-agonist, alleviates post-thoracotomy pain in dogs.

Twelve laboratory beagles underwent a routine left thoracotomy to insert permanent instrumentation. Every second dog was given 10 micrograms/kg of medetomidine, an alpha 2-agonist sedative. The rest of the animals were treated with 20 micrograms/kg of buprenorphine, an opioid agonist-antagonist, which is regularly used to treat postoperative pain in laboratory animals. The drugs were given at the end of operation (0) and 4, 8, 20, and 24 h postoperatively. Blood samples for catecholamines (adrenaline and noradrenaline) and blood gases (pCO2 and pO2) and pH were drawn immediately before any drug administration, and 30 min later. At the same time points, the pain level was subjectively evaluated using a pain score, and heart rate and rectal temperature were measured. Adrenaline and noradrenaline concentrations were lower in the medetomidine group than in the buprenorphine group. Accordingly, it was concluded that medetomidine had better analgesic effect than buprenorphine in the treated animals. This result was supported by subjective evaluation of the severity of pain, even though subjective evaluation is not considered very reliable in the present kind of open studies. pO2 was lower in the buprenorphine group than in the medetomidine group after the first injection of the analgesics. pCO2 and pH were similar in both of the groups. Medetomidine decreased heart rate after every injection, this fall and subsequent rise might be avoided by a lower dose regime. Buprenorphine did not effect heart rate. Rectal temperature did not differ in either group. It was concluded that medetomidine, and other alpha 2-agonists, possess some potential in postoperative pain alleviation.