Abstract
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.
