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 subsequently for a period of 2 h in the post-anaesthetic period. Groups 1-4 were anaesthetised using thiopentone and underwent ventral midline laparotomy. Four animals (group 5) underwent anaesthesia but not surgery, and a further 6 sheep (group 6) undergoing surgery were anaesthetised using ketamine. Groups 1-3 were intravenously administered the following drugs intra-operatively: flunixin meglumine, carprofen and buprenorphine, respectively. Groups 4-6 received no additional treatment. Thresholds to the mechanical test were not changed in the post-anaesthetic period 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 greater than those recorded in the remaining groups at all time points post-operatively. Responses to thermal stimulation in sheep undergoing anaesthesia but not surgery (group 5) were unaltered during the 2 h recording period after anaesthesia ended. These data indicate that abdominal surgery induces thermal but not mechanical hyperalgesia in sheep, which appears to be centrally mediated. Moreover, the absence of mechanical hyperalgesia raises the possibility that central changes in noxious information processing may not be detected using mechanical stimuli in the same time course as thermal stimuli.