The potency of inhalation anesthetics is evaluated by use of the concept of MAC, which is the alveolar concentration of an anesthetic at which 50% of the population does not respond with purposeful movement to a noxious stimulus.1 Individual MAC values are usually determined in animal studies because the noxious stimulus may be applied repeatedly. Although the concept of MAC is widely accepted, its limitations are the subjectivity in the determination of purposeful movement and the fact that 50% of patients will move purposefully in response to surgical stimulation. Therefore, derivatives of MAC, such as MACNM2 and MACBAR,3–5 have been proposed.
Nitrous oxide is an anesthetic gas with many desirable properties, but it lacks potency in animals. The MAC for N2O in dogs is > 200%;6,7 thus, it cannot be used as the sole anesthetic agent in this species. Nevertheless, N2O is used as an adjunctive anesthetic, and it significantly decreases the MAC for halothane6 and desflurane8,9 in dogs.
The objective of the study reported here was to investigate the effects of 70% N2O on the MAC, MACNM, and MACBAR for sevoflurane in dogs. We hypothesized that N2O would significantly reduce MAC and its derivatives.
Minimum alveolar concentration
Minimum alveolar concentration derivative that blocks autonomic responses
Minimum alveolar concentration derivative that blocks motor movement
End-tidal partial pressure of carbon dioxide
North American Drager, Telford, Pa.
Criticare Systems, Waukesha, Wis.
Nellcor N-20V pulse oximeter, Nellcor, Pleasanton, Calif.
Milacath, Mila International, Erlanger, Ky.
Normosol-R, Abbott Laboratories, North Chicago, Ill.
Baxter Healthcare Corp, Irvine, Calif.
Bair Hugger, Arizant, Minn.
Grass Instrument Co, Quincy, Mass.
PROC GLIMMIX, SAS, version 9.2, SAS Institute Inc, Cary, NC.
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