| Minimum Alveolar Concentration |
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FORMAL DEFINITION The MAC is the concentration of the vapour (measured as a percentage at 1 atmosphere, i.e the Partial Pressure ) that prevents the reaction to a standard surgical stimulus (traditionally a set depth and width of skin incisions) in 50% of subjects. This measurement is done at steady state (assuming a constant alveolar concentration for 15 minutes), under the assumption that this allows for an equilibration between the gasses in the Alveoli , the Blood and the Brain . MEYER-OVERTON HYPOTHESIS The MAC of a volatile substance is inversely proportinal to its Lipid solubility (oil:gas coefficient) , in most cases. This is the Meyer-Overton hypothesis. MAC is inversely related to potency i.e. high mac equals low potency. The hypothesis correlates lipid solubility of an anaesthetic agent with potency (1/MAC) and suggests that onset of anaesthesia occurs when sufficient molecules of the anaesthetic agent have dissolved in the cell's lipid membranes, resulting in anaesthesia. Exceptions to the Meyer-Overton hypothesis can result from:
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