Minimum alveolar concentration
Encyclopedia
Minimum alveolar concentration or MAC is a concept used to compare the strengths, or potency
, of anaesthetic vapours; in simple terms, it is defined as the concentration of the vapour in the lung
s that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus. Thus, it is actually a median value; the use of minimum would appear to be descended from the original paper in which the concept appeared, although the term there was minimal alveolar concentration. A lower MAC value represents a more potent volatile anesthetic.
The concept was introduced in 1965.
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.
) 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
. 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.
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. 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:
and thyrotoxicosis. Likewise, hypothermia, hypotension, hypothyroidism, and pregnancy seem to decrease MAC. Gender, height and weight seem to have little effect on MAC. Opioid analgesics and sedative-hypnotics, often used as adjuvants to anesthesia, decrease MAC. It should also be noted that 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.
Potency
Potency may refer to:* Potency , a measure of the activity of a drug in a biological system* Virility* Potency is a measure of the differentiation potential of stem cells...
, of anaesthetic vapours; in simple terms, it is defined as the concentration of the vapour in the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus. Thus, it is actually a median value; the use of minimum would appear to be descended from the original paper in which the concept appeared, although the term there was minimal alveolar concentration. A lower MAC value represents a more potent volatile anesthetic.
The concept was introduced in 1965.
Other uses of MAC include MAC-BAR (1.7-2.0 MAC), which is the concentration required to block autonomic
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
reflexes to nociceptive stimuli
Nociception
Nociception is defined as "the neural processes of encoding and processing noxious stimuli." It is the afferent activity produced in the peripheral and central nervous system by stimuli that have the potential to damage tissue...
, 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 pressurePartial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
) 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
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
and the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. 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 lipidLipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
solubility (oil:gas coefficient) , in most cases. This is the Meyer-Overton hypothesis put forward in 1899–1901 by Hans Horst Meyer
Hans Horst Meyer
Hans Horst Meyer was a German pharmacologist. He studied medicine and did research in pharmacology. The Meyer-Overton hypothesis on the mode of action on general anaesthetics is partially named after him...
and Charles Ernest Overton. 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 Alpha2 agonists (dexmedetomidine) and/or opioid receptor agonists (morphine/fentanyl) can decrease the MAC
- Mullin's critical volume hypothesis
- Positive modulation of GABAGabâGabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...
at GABAA receptors by barbiturates or benzodiazepines
Altered MAC
Certain physiological and pathological states may alter MAC. MAC is higher in infants and lower in the elderly. Also, MAC increases with anxietyAnxiety
Anxiety is a psychological and physiological state characterized by somatic, emotional, cognitive, and behavioral components. The root meaning of the word anxiety is 'to vex or trouble'; in either presence or absence of psychological stress, anxiety can create feelings of fear, worry, uneasiness,...
and thyrotoxicosis. Likewise, hypothermia, hypotension, hypothyroidism, and pregnancy seem to decrease MAC. Gender, height and weight seem to have little effect on MAC. Opioid analgesics and sedative-hypnotics, often used as adjuvants to anesthesia, decrease MAC. It should also be noted that 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
- Nitrous oxideNitrous oxideNitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
- 104 - XenonXenonXenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
- 72 - DesfluraneDesfluraneDesflurane is a highly fluorinated methyl ethyl ether used for maintenance of general anesthesia. Like halothane, enflurane and isoflurane, it is a racemic mixture of and optical isomers...
- 6 - Ethyl Ether - 3.2
- SevofluraneSevofluraneSevoflurane , also called fluoromethyl hexafluoroisopropyl ether, is a sweet-smelling, nonflammable, highly fluorinated methyl isopropyl ether used for induction and maintenance of general anesthesia. Together with desflurane, it is replacing isoflurane and halothane in modern anesthesiology...
- 2 - EnfluraneEnfluraneEnflurane is a halogenated ether that was commonly used for inhalational anesthesia during the 1970s and 1980s. Developed by Ross Terrell in 1963, it was first used clinically in 1966....
- 1.7 - IsofluraneIsofluraneIsoflurane is a halogenated ether used for inhalational anesthesia. Together with enflurane and halothane, it replaced the flammable ethers used in the pioneer days of surgery. Its name comes from being a structural isomer of enflurane, hence they have the same empirical formula...
- 1.2 - HalothaneHalothaneHalothane is an inhalational general anesthetic. Its IUPAC name is 2-bromo-2-chloro-1,1,1-trifluoroethane. It is the only inhalational anesthetic agent containing a bromine atom; there are several other halogenated anesthesia agents which lack the bromine atom and do contain the fluorine and...
- 0.75 - ChloroformChloroformChloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
- 0.5 - MethoxyfluraneMethoxyfluraneMethoxyflurane is a halogenated ether that was in clinical use as an volatile inhalational anesthetic from its introduction by Joseph F. Artusio et al in 1960 until around 1974. It was first synthesized in the late 1940s by William T...
- 0.16