Determination of acute tolerance and hyperalgesia to remifentanil constant rate infusion in dogs undergoing sevoflurane anaesthesia.

ABSTRACT

OBJECTIVE: To determine if acute opioid tolerance (AOT) or opioid-induced hyperalgesia (OIH) could develop and limit the remifentanil-induced reduction in the sevoflurane minimum alveolar concentration (MAC). The response to mechanical nociceptive threshold (MNT) was evaluated and related to OIH. STUDY DESIGN: A crossover, randomized, experimental animal study. ANIMALS A total of nine Beagle dogs. METHODS: The dogs were anaesthetized with sevoflurane in 50% oxygen. Baseline sevoflurane MAC was measured (MACb1). Remifentanil (0.3 µg kg-1 minute-1) or 0.9% saline constant rate infusion (CRI) was administered intravenously (IV). Sevoflurane MAC was determined 20 minutes after CRI was initiated (MACpostdrug1), 30 minutes after MACpostdrug1 determination (MACpostdrug2) and after 1 week (MACb2). The MNT was determined at baseline (before anaesthesia), 3 and 7 days after anaesthesia. An increase of MACpostdrug2 ≥0.25% compared to MACpostdrug1 was considered evidence of AOT. A decrease in MNT at 3 and 7 days or an increase in MACb2 or both with respect to MACb1 were considered evidence of OIH. RESULTS: Remifentanil CRI reduced sevoflurane MACpostdrug1 by 43.7% with respect to MACb1. MACpostdrug2 was no different from MACpostdrug1 with the saline (p = 0.62) or remifentanil (p = 0.78) treatments. No significant differences were observed in the saline (p = 0.99) or remifentanil (p = 0.99) treatments between MACb1 and MACb2, or for MNT values between baseline, 3 and 7 days. CONCLUSION AND CLINICAL RELEVANCE In dogs, under the study conditions, remifentanil efficacy in reducing sevoflurane MAC did not diminish in the short term, suggesting remifentanil did not induce AOT. Hyperalgesia was not detected 3 or 7 days after the administration of remifentanil. Contrary to data from humans and rodents, development of AOT or OIH in dogs is not supported by the findings of this study.