Minimum alveolar concentration
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Minimum alveolar concentration or MAC is a concept used to compare the strengths of anaesthetic vapours; in simple terms, it is defined as the minimal concentration of the vapour in the lungs that is needed to prevent movement in 50% of subjects in respose to surgical stimulus.
[edit] 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.
[edit] Meyer-Overton hypothesis
The MAC of a volatile substance is inversely proportional 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:
- convulsant property of an agent
- specific receptor (various agents may exhibit an additional effect through specific receptors)
- Alpha 2 agonist effects decrease the MAC
- Mullin's critical volume hypothesis
- activation of GABA
