Halothane minimal alveolar concentration

Muir W W III, Sams R 1992 Effects of ketamine infusion on halothane minimal alveolar concentration in horses. American Journal of Veterinary Research 53 1802-1806... 

The mechanism of action of inhalational anesthetics is unknown. The diversity of chemical structures (inert gas xenon hydrocarbons halogenated hydrocarbons) possessing anesthetic activity appears to rule out involvement of specific receptors. According to one hypothesis, uptake into the hydrophobic interior of the plasmalemma of neurons results in inhibition of electrical excitability and impulse propagation in the brain. This concept would explain the correlation between anesthetic potency and lipophilicity of anesthetic drugs (A). However, an interaction with lipophilic domains of membrane proteins is also conceivable. Anesthetic potency can be expressed in terms of the minimal alveolar concentration (MAC) at which 50% of patients remain immobile following a defined painful stimulus (skin incision). Whereas the poorly lipophilic N2O must be inhaled in high concentrations (>70% of inspired air has to be replaced), much smaller concentrations (<5%) are required in the case of the more lipophilic halothane. 

Davis et al. (1975) Determination of the minimal alveolar concentration of halothane in the white New Zealand rabbit was published. 

Anesthetic potency can be expressed in terms of the minimal alveolar concentration (MAC) at which 50% of patients remain immobile following a defined painful stimulus (skin incision). Whereas the poorly lipophilic nitrous oxide must be inhaled in high concentrations, much smaller concentrations are required in the case of the more lipophilic halothane. 

Flalothane, desflurane, enflurane, sevoflurane, and isoflurane all decrease mean arterial pressure in direct proportion to their alveolar concentration. With halothane and enflurane, the reduced arterial pressure appears to be caused by a reduction in cardiac output because there is little change in systemic vascular resistance despite marked changes in individual vascular beds (eg, an increase in cerebral blood flow). In contrast, isoflurane, desflurane, and sevoflurane have a depressant effect on arterial pressure as a result of a decrease in systemic vascular resistance with minimal effect on cardiac output. 

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