Minimum alveolar concentration or MAC is the concentration, often expressed as a percentage by volume, of a vapour in the alveoli of the
lungs that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus. MAC is used to compare the strengths, or
potency, of anaesthetic
vapours.
The concept of MAC was first introduced in 1965.
MAC actually is a ''median'' value, not a minimum as term implies. The original paper proposed MAC as the ''minimal'' alveolar concentration, which was shortly thereafter revised to ''minimum'' alveolar concentration.
A lower MAC value represents a more potent volatile anesthetic.
Other uses of MAC include MAC-BAR (1.7–2.0 MAC), which is the concentration required to block
autonomic reflexes to
nociceptive stimuli, and MAC-awake (0.3–0.5 MAC), the concentration required to block voluntary reflexes and control perceptive awareness.
Formal definition
The MAC is the concentration of the vapour (measured as a percentage at 1 atmosphere, i.e. the
partial pressure) that prevents patient ''movement'' in response to a supramaximal 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
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
and the
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...
. MAC is accepted as a valid measure of potency of inhalational general anaesthetics because it remains fairly constant for a given species even under varying conditions.
Meyer-Overton hypothesis
The MAC of a volatile substance is inversely proportional to its
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids includ ...
solubility (oil:gas coefficient), in most cases. This is the Meyer-Overton hypothesis put forward in 1899–1901 by
Hans Horst Meyer and
Charles Ernest Overton
Charles Ernest Overton (1865–1933) was a British physiologist and biologist, now regarded as a pioneer of the theory of the cell membrane.
In the last years of the 19th century Overton did experimental work, allowing the distinction to be drawn ...
. 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)
* co-administration of Alpha
2 agonists (dexmedetomidine) and/or opioid receptor agonists (morphine/fentanyl) can decrease the MAC
* Mullin's critical volume hypothesis
* Positive modulation of
GABA at GABA
A receptors by barbiturates or benzodiazepines
Factors affecting MAC
Certain physiological and pathological states may alter MAC. For example, MAC increases with hyperthermia and hypernatremia. Conversely, anemia, hypercarbia, hypoxia, hypothermia, hypotension (MAP < 40 mmHg), and pregnancy seem to decrease MAC. Duration of anesthesia, gender seem to have little effect on MAC.
Age has been shown to affect MAC. MAC begins to rise at one month of age with a peak at approximately 6 months of age. There is a subsequent steady decline in MAC with increasing age, with the exception of another peak during puberty.
There is a linear model that describes the change in MAC of approximately 6% per decade of age.
[
Medications, illicit drugs, and prior substance use history have also been found to affect MAC. For example, acute use of amphetamines, cocaine, ephedrine, and chronic use of alcohol increase MAC. Whereas, administration of propofol, etomidate, barbiturates, benzodiazepines, ketamine, opiates, local anesthetics, lithium, verapamil, and alpha 2-agonists (dexmedetomidine, clonidine) decrease MAC. Acute alcohol intoxication and chronic amphetamine use have also been found to decrease MAC.][
MAC values are additive. For instance, when applying 0.3 MAC of drug X and 1 MAC of drug Y the total MAC achieved is 1.3 MAC. In this way nitrous oxide is often used as a "carrier" gas to decrease the anesthetic requirement of other drugs.]
Common MAC values
Values are known to decrease with age and the following are given based on a 40-year-old (MAC40):[* ]
* Nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has ...
– 104Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
– 72
References
{{Anesthesia
Anesthesia