Minimum alveolar concentration
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.[1] The concept of MAC was first introduced in 1965.[2]
MAC actually is a median value, not a minimum as term implies. The original paper proposed MAC as the minimal alveolar concentration,[3] which was shortly thereafter revised to minimum alveolar concentration.[4] 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[5] 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.[citation needed]
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 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:
- 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[6][7]
- Mullin's critical volume hypothesis
- Positive modulation of GABA at GABAA 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.[8]
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.[4] There is a linear model that describes the change in MAC of approximately 6% per decade of age.[8]
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.[8]
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.[9]
Common MAC values
Values are known to decrease with age and the following are given based on a 40-year-old (MAC40):[10]
- Nitrous oxide – 104[10]
- Xenon – 72[10]
- Desflurane – 6.6[10]
- Ethyl ether – 3.2
- Sevoflurane – 1.8[10]
- Enflurane – 1.63[10]
- Isoflurane – 1.17[10]
- Halothane – 0.75[10]
- Chloroform – 0.5
- Methoxyflurane – 0.16
References
- ^ "Policy: Ban on Use of Ether". Laboratory Animal Science Center. Archived from the original on 2008-06-09. Retrieved 2008-11-10.
- ^ Eger EI, Saidman LJ, Brandstater B (1965). "Minimum alveolar anesthetic concentration: a standard of anesthetic potency". Anesthesiology. 26 (6): 756–63. doi:10.1097/00000542-196511000-00010. PMID 5844267.
- ^ Merkel, Giles; Eger, Edmond I. (1963-05-01). "A Comparative Study of Halothane and Halopropane AnesthesiaIncluding Method for Determining Equipotency". The Journal of the American Society of Anesthesiologists. 24 (3): 346–357. doi:10.1097/00000542-196305000-00016. ISSN 0003-3022. PMID 13935000. S2CID 35750587.
- ^ a b Eger, Edmond I.; Saidman, Lawrence J.; Brandstater, Bernard (1965-11-01). "Minimum Alveolar Anesthetic ConcentrationA Standard of Anesthetic Potency". The Journal of the American Society of Anesthesiologists. 26 (6): 756–763. doi:10.1097/00000542-196511000-00010. ISSN 0003-3022. PMID 5844267.
- ^ Miller ANESTHESIOLOGY
- ^ * Daniel M, Weiskopf RB, Noorani M, Eger EI (January 1998). "Fentanyl augments the blockade of the sympathetic response to incision (MAC-BAR) produced by desflurane and isoflurane: desflurane and isoflurane MAC-BAR without and with fentanyl". Anesthesiology. 88 (1): 43–9. doi:10.1097/00000542-199801000-00009. PMID 9447854. S2CID 19866907.
- ^ Katoh T, Kobayashi S, Suzuki A, Iwamoto T, Bito H, Ikeda K (February 1999). "The effect of fentanyl on sevoflurane requirements for somatic and sympathetic responses to surgical incision". Anesthesiology. 90 (2): 398–405. doi:10.1097/00000542-199902000-00012. PMID 9952144.
- ^ a b c Lobo, S. A.; Ojeda, J.; Dua, A.; Singh, K.; Lopez, J. (2022). "Minimum Alveolar Concentration". StatPearls. PMID 30422569.
- ^ Eger Ei, 2nd (2001). "Age, minimum alveolar anesthetic concentration, and minimum alveolar anesthetic concentration-awake". Anesthesia and Analgesia. 93 (4): 947–953. doi:10.1097/00000539-200110000-00029. PMID 11574362. S2CID 31218667.
{{cite journal}}
: CS1 maint: numeric names: authors list (link) - ^ a b c d e f g h * Nickalls, R. W. D., & Mapleson, W. W. (August 2003). "Age-related iso-MAC charts for isoflurane, sevoflurane, and desflurane in man". British Journal of Anaesthesia. 91 (2): 170–4. doi:10.1093/bja/aeg132. PMID 12878613.
{{cite journal}}
: CS1 maint: multiple names: authors list (link)
- v
- t
- e
- Anticholinergics
- Antiemetics
- Butyrophenones
- Benzodiazepines
- General anesthetics
- Inhalational anesthetics
- Local anesthetics
- Neuromuscular-blocking drugs
- Opioids
- Sedatives
- Blood–gas partition coefficient
- Concentration effect
- Fink effect
- Minimum alveolar concentration
- Second gas effect
- Allergic reactions
- Anesthesia awareness
- Drug-induced amnesia
- Effects of early-life exposures to anesthesia on the brain
- Emergence delirium
- Local anesthetic toxicity
- Malignant hyperthermia
- Perioperative mortality
- Postanesthetic shivering
- Postoperative nausea and vomiting
- Postoperative residual curarization
- ACE mixture
- Helsinki Declaration for Patient Safety in Anaesthesiology
- History of general anesthesia
- History of neuraxial anesthesia
- History of tracheal intubation
- American Association of Nurse Anesthetists
- American Society of Anesthesia Technologists & Technicians
- American Society of Anesthesiologists
- Anaesthesia Trauma and Critical Care
- Association of Anaesthetists of Great Britain and Ireland
- Royal College of Anaesthetists
- Association of Veterinary Anaesthetists
- Australian and New Zealand College of Anaesthetists
- Australian Society of Anaesthetists
- International Anesthesia Research Society
- European Society of Anaesthesiology and Intensive Care
- Category
- Outline