Minimal alveolar anesthetic concentration

Current anesthesia protocols usually include several agents in combinations that vary according to the depth of anesthesia required for specific procedures. Inhalational anesthetics, which include nitrous oxide and six halogenated hydrocarbons, have varying potency in proportion to their lipid solubilities. MAC value, a measure of anesthetic potency, is defined as the minimal alveolar anesthetic concentration (% of inspired air) at which 50% of patients do not respond to a surgical stimulus. MAC values are additive, lower in elderly patients, and lower in the presence of opioid analgesics and sedative hypnotics. 

A MAC (minimal alveolar anesthetic concentration) is defined as the concentration of inhaled anesthetic, as a % of inspired air, at which 50% of patients do not respond to a surgical stimulus. 

Minimal alveolar anesthetic concentration (MAC) The alveolar concentration of an anesthetic that is required to prevent a response to a standardized painful stimulus in 50% of patients... 

Anesthetic Blood Gas Partition Coefficient Minimal Alveolar Anesthetic Concentration (%)... 

Eger II, E. I., Koblin, D. D., Laster, M. J., Schurig, V., Juza, M., Ionescu, P., and Gong. D. (1997) Minimum alveolar anesthetic concentration values for the enantiomers of isoflurane differ minimally. Anesth, Analg. 85, 188-192. 

Gong D, Fang Z, Ionescu P et al. (1998) Strain minimally influences anesthetic and convulsant requirements of inhaled compounds in rats. Anesth Analg 87 963-966 Hall RI, Murphy MR, Hug CC (1987) The enfluorane sparing effect of sufentanyl in dogs. Anesthesiol 67 518-525 Ide T, Sakurai Y, Aono M, Nishino T (1998) Minimum alveolar anesthetic concentrations for airway occlusion in cats A new concept of minimum alveolar anesthetic concentration-airway occlusion response. Anaesth Analg 86 191-197... 

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. 

Nitrous oxide is the only inhalation anesthetic that is a gas. It is chemically inert. Nitrous oxide has little effect on overall cardiovascular function. Disadvantages are that it has no muscle relaxing effect and that it cannot be used on its own because of high Minimal Alveolar Concentration values needed for adequate anesthesia. During recovery there is a risk for hypoxia and anesthesia should be slowly tapered off to prevent this event. 

Anesthetic Blood Gas Partition Coefficient1 Brain Blood Partition Coefficient1 Minimal Alveolar Concentration (MAC) (°/o)2 Metabolism Comments... 

The slope of the regression line implies that the MAC (minimal alveolar concentration effective in 50 percent of animals) is inversely proportional to partition coefficient or potency is directly proportional to partition coefficient. The Meyer-Overton correlation suggests that the site at which anesthetics bind is primarily a hydrophobic environment. Although a wide variety of compounds lie on the Meyer-Overton correlation line, there are many compounds that do not. This suggests that the chemical properties of the anesthetic site differ from those of olive oil. 

Salmempera et al. (1992) studied in dogs the potency of remifentanil, a short acting opioid analgesic, which is used as anesthetic adjunct by variable-rate infusion. Enfluorane minimal alveolar concentration was measured by the tail-clamp method in dogs before and after sequential infusion of various doses of remifentanil. The plasma concentration causing a 50% reduction of enfluorane minimal alveolar concentration was determined. 

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. 

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