Effects of remifentanil on measures of anesthetic immobility and analgesia in cats

Objective - To evaluate effects of various doses of remifentanil on measures of analgesia in anesthetized cats. Animals - 6 healthy adult cats. Procedures - Minimum alveolar concentration (MAC) for isoflurane and thermal threshold responses were evaluated in anesthetized cats. Remifentanil infusions of 0 (baseline), 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8, and 16 μg/kg/min were administered; after a 45-minute equilibration period, isoflurane MAC and responses were determined. Isoflurane MAC was determined in anesthetized cats once for each remifentanil infusion rate by use of a standard tail clamp technique. Thermal threshold was measured in awake cats by use of a commercially available analgesiometric probe placed on the lateral portion of the thorax; remifentanil infusions were administered in randomized order to anesthetized cats, and thermal threshold determinations were made by an investigator who was unaware of the infusion rate. Results - Mean ± SEM median effective concentration (EC₅₀) for remifentanil and its active metabolite, GR90291, for the thermal threshold test was 1.00 ± 0.35 ng/mL and 307 ± 28 ng/mL of blood, respectively. Dysphoria was detected in all awake cats at the 2 highest remifentanil infusion rates. However, isoflurane MAC during remifentanil infusions was unchanged from baseline values, even at blood opioid concentrations approximately 75 times the analgesic EC₅₀. Conclusions and Clinical Relevance - Immobility and analgesia as reflected by thermal threshold testing were independent anesthetic end points in the cats. Results of MAC-sparing evaluations should not be used to infer analgesic potency without prior validation of an MAC-analgesia relationship for specific drugs and species.