ABSTRACT
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 (FE'ISO) 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 FE'ISO. 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.
ABSTRACT
Objective To evaluate electroencephalographic indices, invasive arterial blood pressure and pulse rate as indicators of nociception during isoflurane anaesthesia in pigs. Animals Ten Norwegian Landrace pigs, five castrated males and five females, weighing between 19 and 29 kg. Materials and methods The pigs were anaesthetized with isoflurane and prepared for the measurement of heart rate (HR), mean arterial pressure (MAP) and the electroencephalogram (EEG). The inspired isoflurane concentration (Fi'ISO) was then adjusted until the suppression ratio (SR) exceeded 20%, after which the corresponding end-expired concentration (Fe'ISO) was maintained for 30 minutes. The effects of noxious stimuli, consisting of pinching (nasal septum, interdigital web, anus, periople) and clamping (tail, claw) on spectral edge frequency 95%, median frequency, alpha/delta ratio, beta/delta ratio, theta/delta ratio, total power, suppression ratio, MAP and HR were recorded. Fe'ISO was then decreased by 0.3% and maintained for 30 minutes after which noxious stimulation was re-applied. This was repeated twice with Fe'ISO reductions of 0.3% being applied each time. Changes in measured variables before and after noxious stimulation were then compared at each of four levels of anaesthesia. Results Noxious stimulation caused significant MAP increases at the lowest two levels for Fe'ISO. There were no significant changes in the other variables examined. Conclusion Of the variables examined, mean arterial blood pressure is the most sensitive indicator of nociception in isoflurane-anaesthetized pigs. Clinical relevance When evaluating nociception under isoflurane anaesthesia in pigs mean arterial blood pressure should be monitored when surgery is performed.
