Determination of minimum alveolar concentration of isoflurane in dogs and cats using the up-and-down method. A preliminary study

Minimum alveolar concentration (MAC) is a reliable measurement of the potency of inhaled anesthetic agents. The determination of MAC in different species has followed a fairly consistent methodology. In people, MAC is determined with the up-and-down method, whereas in animal the bracketing technique is commonly used. The objectives of this study were to determine the MAC value of isoflurane in dogs and cats using the up-and-down method and to determine the MAC value of isoflurane at extubation (MACex). General anesthesia was induced in 13 dogs and 5 cats with 5% isoflurane in oxygen. An initial end-tidal isoflurane concentration of 1.3% was used for the first dog and of 1.6% for the first cat and maintained constant for ≥ 20 min. A noxious stimulus in the form of Carmalt forceps was applied to the base of the tail for no more than one minute or until movement was noticed. After stimulation, the response was recorded as positive (movement) or negative (no movement) and the animal was recovered. The end-tidal isoflurane concentration of the following animal was increased or decreased by 0.1% if the response of the previous animal to the stimulus was negative or positive, respectively. Isoflurane MAC values in dogs were 1.27% and 1.23%. Isoflurane MAC value in cats was 1.58%. MACex value was 0.45% in dogs and in cats. The up-and-down method for MAC determination achieved similar results when compared to MAC values of isoflurane in dogs and cats reported in the literature using the bracketing technique.