THE EFFECT OF ABDOMINAL-SURGERY ON THRESHOLDS TO THERMAL AND MECHANICAL STIMULATION IN SHEEP

Thresholds to noxious mechanical and thermal stimulation were measured in 6 groups of sheep prior to induction of anaesthesia and subsequent ly for a period of 2 h in the post-anaesthetic period. Groups 1-4 were anaesthetised using thiopentone and underwent ventral midline laparot omy. Four animals (group 5) underwent anaesthesia but not surgery, and a further 6 sheep (group 6) undergoing surgery were anaesthetised usi ng ketamine. Groups 1-3 were intravenously administered the following drugs intra-operatively: flunixin meglumine, carprofen and buprenorphi ne, respectively. Groups 4-6 received no additional treatment. Thresho lds to the mechanical test were not changed in the post-anaesthetic pe riod for any group. There was a significant reduction in the responses to thermal stimulation after surgery for sheep in group 4 (45 and 60 min), while sheep in group 2 had thresholds to thermal stimulation gre ater than those recorded in the remaining groups at all time points po st-operatively. Responses to thermal stimulation in sheep undergoing a naesthesia but not surgery (group 5) were unaltered during the 2 h rec ording period after anaesthesia ended. These data indicate that abdomi nal surgery induces thermal but not mechanical hyperalgesia in sheep, which appears to be centrally mediated. Moreover, the absence of mecha nical hyperalgesia raises the possibility that central changes in noxi ous information processing may not be detected using mechanical stimul i in the same time course as thermal stimuli.