See table 1 for the list of MAC values for the most commonly used inhaled anesthetics in a 40-year-old patient. Where MACage is the MAC at a given age and MAC40 is the MAC value at age 40.
A comprehensive meta-analysis determined the relationship between MAC and age could be described with reasonable accuracy by the following equation : MAC peaks at 6 months of age, and then decreases by approximately 6% per decade, regardless of volatile anesthetic. įactors such as species, sex, hypothyroidism, hyperthyroidism, or duration of anesthesia do not affect MAC however, the age of the patient does influence the minimum alveolar concentration. It is important to note that the conventional values of MAC are determined at a reference pressure of 1 atm and that changes in ambient pressure will change MAC. The inverse is true for factors that decrease MAC.
When a factor is present that increases MAC, there is a decreased potency of volatile anesthetic for that person, and they require a larger amount of anesthetic to achieve immobility. Many variables alter minimum alveolar concentration (MAC), changing the amount of anesthetic required to prevent movement. Following the Meyer-Overton theory, MAC could be estimated by using the following equation : Although lipid solubility is not the sole determinant of potency, as proteins are likely the target site for volatile anesthetics, this concept holds true for volatile anesthetics. īefore the concept of MAC, the Meyer-Overton relationship was well established it stated that all fat-soluble agents would function as anesthetics due to their ability to cross the lipid bilayer of neurons. Studies in rats demonstrated this concept by introducing lesions in the central nervous system to sever connections between the spinal cord and the brain, and it was found that these lesions did not alter MAC. Immobilization is mediated by other aspects of anesthesia such as amnesia and hypnosis in the subcortical and cortical regions of the brain. Both animal and human studies have found that volatile anesthetics depress spinal motor-neuron excitability, which is why immobilization is mediated largely at the level of the spinal cord by inhaled anesthetics. It can be applied to all inhalational anesthetics and is used to compare anesthetic potency. Minimum alveolar concentration (MAC) provides a correlation between anesthetic dose and immobility. It is a useful and reproducible metric in both animals and humans across all volatile anesthetics, making it the standard for comparison of volatile anesthetic potency. MAC uses the measurement of end-tidal anesthetic as a measure of the level of anesthetic within the alveoli and, in turn, at the level of the central nervous system. It is defined as the concentration of inhaled anesthetic within the alveoli at which 50% of people do not move in response to a surgical stimulus. The concept of MAC negated the variability by measuring a single quantitative endpoint: immobility. Previous attempts to create a measure to determine the adequacy of anesthetic dosing, such as the Guedel stages of anesthesia or Woodbridge concept of “nothria” were all qualitative measures that varied depending on the inhaled anesthetics. introduced the concept of minimum alveolar concentration (MAC), and it has since been used as the standard measure of potency for volatile anesthetic agents.
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