Motor responses to stimulation during isoflurane anaesthesia in pigs.

Objective: To investigate motor responses to stimulation during the transition from 'deep' (burst suppression) to 'light' isoflurane anaesthesia in pigs.

Study Design: Prospective, randomized observational study.

Animals: Five castrated male and five female Norwegian landrace pigs, weighing 19-29 kg.

Materials And Methods: Anaesthesia was induced with isoflurane and the inspired concentration gradually increased until a burst suppression electroencephalogram (EEG) was recorded. End-tidal isoflurane concentration (F) was then allowed to equilibrate for 30 minutes after which the eyelashes, cornea, nasal septum, anus, interdigital skin fold, periople, tail and claw were stimulated. The motor response to stimulation at each location was graded from 0 to 5. End-tidal isoflurane concentration was decreased 0.3% and the areas re-stimulated; this was repeated three times in each pig. A linear regression analysis using response as dependent and anaesthetic level as independent variable was performed for each stimulus in each pig. Using Student's t-statistic a 95% confidence interval for the mean slope of each stimulus was constructed.

Results: No pig responded to eyelash brushing. The mean slopes for the other stimuli indicated increasing responses with decreasing F. Responses to periople pinching and tail and claw clamping showed significant increases. No stimuli consistently increased the magnitude of response in all pigs, and the appearance and absence of a response was inconsistent between pigs. Motor responses occurred in at least one pig during isoflurane burst suppression anaesthesia to all stimuli except eyelash brushing.

Conclusions: All the stimuli investigated may elicit movement responses during burst suppression anaesthesia with isoflurane except eyelash brushing. No consistent response pattern between pigs was observed with decreasing isoflurane concentration. Of the stimuli evaluated, clamping the tail or claw and pinching the periople appear the most reliable indicators of anaesthetic depth.

Clinical Relevance: The absence or presence of single reflexes does not accurately reflect the degree of isoflurane-induced cortical depression in individual pigs.