MINIMUM ALVEOLAR ANESTHETIC CONCENTRATIONS FOR AIRWAY OCCLUSION IN CATS - A NEW CONCEPT OF MINIMUM ALVEOLAR ANESTHETIC CONCENTRATION AIRWAYOCCLUSION RESPONSE

Minimum alveolar anesthetic concentration (MAC) is defined as an end-t idal concentration of inhaled anesthetic required to prevent purposefu l movement (positive motor response) in 50% of subjects to somatic nox ious stimuli. Although MAC for visceral noxious stimuli has not been w ell investigated, airway occlusion can be a noxious respiratory stimul us that can induce a visceral sensation of choking. In this study, MAC for airway occlusion (MAC-AOR) was determined during halothane, isofl urane, and sevoflurane anesthesia and compared with the MAC values for somatic noxious stimuli such as toe pinch (MAC-pinch) or tetanic stim ulus (MAC-tetanus) in cats. Thirty-few adult cats were used. In 24 cat s, the motor responses to three different stimuli (toe pinch, tetanic stimulus, and airway occlusion for 6 min) were observed during inhaled anesthesia and rated as positive or negative. The concentration of an inhaled anesthetic was changed in steps of 0.1%-0.2 vol% until the br acketing procedure (i.e., the highest concentration of inhaled anesthe tic permitting a positive motor response and the lowest concentration preventing the response were determined) was completed. In 10 cats, th e effect of anesthetic duration on MAC-AOR was also investigated. Each mean MAC (MAC-pinch, MAC-AOR, and MAC-tetanus, respectively) was as f ollows: halothane 0.99, 1.13, and 1.46; isoflurane 1.50, 1.65, and 2.2 2; and sevoflurane 3.07, 3.38, and 3.95. The first and last MAC-AOR va lues determined during 6-h halothane anesthesia were 1.10 and 1.11, re spectively. Ln conclusion, airway occlusion can be a noxious stimulus and can induce an all-or-none type of motor response, depending on the depth of inhalational anesthesia in cats. This phenomenon can permit the introduction a new concept of MAC-AOR. Implications: Airway occlus ion can be a noxious visceral stimulus and induce all-or-none type of motor response in cats, depending on the depth of inhalational anesthe sia. This permits the introduction of a new concept of minimum alveola r anesthetic concentration airway occlusion response.