MAC
The minimum alveolar concentration at steady state that prevents the reaction to a standard surgical stimulus (traditionally measured as a response to a set depth and width of skin incisions) in 50% of subjects and measured as a percentage at 1 atmosphere (i.e the partial pressure effects of a gas are proportional to the concentration in the brain, assuming a constant alveolar concentration after 15 minutes has occured, allowing for a equlibration between the gasses in the alveoli = blood = brain, and thus the partial pressure in the alveolar (lungs) are said to be equivalent to the partial pressure in the brain).
When measured in comparison to the lipid solubility of a volatile agent (oil:gas coefficient) using logarithmic scales, the relationship generated is linear. This relationship is described as the Meyer overton hypothesis (unfortunately, there is still no specific and satisfactory method that explains how volatile agents cause anaesthesia or how anaesthesia works) which explains some of the general physio-chemical properties of volatile agents.
The Meyer-Overton hypothesis correlates lipid solubility of an anaesthetic agent with the MAC and suggests that onset of anaesthesia occurs when sufficient molecules of the anaesthetic agent have dissolved in the lipd membrane resulting in anaesthesia. Exceptions to the Meyer-Overton hypothesis vary depending on:
- convulsant property of an agent
- specific receptor (variosu agents may exhibit an additional effect through specific receptors)
- Alpha 2 agonist effects decreases the MAC
- Mullin's critical volume hypothesis
- activation of GABA