Studies suggested that immobility, as measured by MAC, is mediated by the action of inhaled anaesthetics on the spinal cord 34 - Separating perfusion of the spinal cord and brain, and delivering anaesthetic agent preferentially to the brain, increased MAC for isoflurane from 1. An analogous study demonstrated a similar increase in MAC for halothane from 0. Complementary studies in rats introduced lesions in the central nervous system CNS to sever connections between the spinal cord and the brain, such as precollicular decerebration and spinal cord transection at the thoracic level; neither of these lesions altered MAC 36 , The lack of movement in these rats was produced by inhaled anaesthetic concentrations similar to controls, suggesting that the primary site of action is the spinal cord 36 , Studies of animals and humans have identified regions of the brain, such as the amygdala, hippocampus and cortex, that contribute to the formation of explicit episodic memory and may be targets for the amnesic effects of inhaled anaesthetics 42 , Inhaled anaesthetics produce hypnosis by suppressing both arousal and subjective experience.
The actions of inhaled anaesthetics on thalamocortical and corticocortical networks are thought to inhibit subjective experience. Many factors alter MAC, thereby increasing or decreasing the anaesthetic concentration required to prevent movement Fig. When a factor is present that increases MAC in an individual, volatile anaesthetics have decreased potency for that person. Thus, in the presence of these factors, patients will require a higher concentration of the volatile agent. Conversely, for factors that decrease MAC, the volatile anaesthetic agents have increased potency.
In the presence of these factors, patients require a lower concentration of the volatile agent. Many physiological factors have been associated with alterations in MAC. For example, the concentration of anaesthetic agent required to suppress movement varies inversely with the age of an individual. Using this equation, charts outlining equipotent concentrations of volatile anaesthetics adjusted for age and concurrent nitrous oxide use have been developed to guide clinical practice Fig.
Variations in body temperature have also been associated with differences in anaesthetic requirements. Studies in various animal models have demonstrated a positive linear relationship between temperature and anaesthetic requirement 48 - The effect of temperature on MAC is hypothesised to be attributable to effects of temperature changes on cerebral oxygen consumption Elevations or reductions in serum sodium coinciding with changes in sodium concentration and osmolality in cerebrospinal fluid may alter anaesthetic requirements.
Several other physiological derangements, such as anaemia, hypercarbia and hypoxia, have also been shown to decrease MAC 53 , The potency of inhaled anaesthetics is greater in women during pregnancy and in the early postpartum period, resulting in a decrease in MAC Preliminary studies have provided evidence for other factors that may affect MAC, notably genetic background 56 , Consistent with other investigations, this study also supported genetic background, but not biological sex, as an important determinant of MAC.
Numerous studies have demonstrated reductions in MAC with the use of opioid analgesics Again, it is important to emphasise that these drugs might all alter MAC, but not similarly affect other MAC derivatives. For example, through their nociceptive actions, potent opioid analgesics might help to prevent movement in response to surgical stimulation, but might not as profoundly decrease the concentration of volatile anaesthetic required to achieve amnesia. For example, drugs that increase catecholamine release in the CNS, such as cocaine, increase MAC during acute intoxication In contrast, chronic cocaine exposure is associated with a decrease in MAC for isoflurane Conversely, the results of several studies suggest that chronic ethanol use increases MAC, while acute ethanol administration decreases MAC 75 - In general, patients presenting with a depressed level of consciousness due to trauma or cerebrovascular insult have decreased anaesthetic requirements.
A murine model of Alzheimer's disease demonstrated that the neuropathology associated with Alzheimer's disease confers a resistance to the hypnotic actions of inhaled anaesthetics 78 , Several studies before the introduction of MAC suggested that alveolar partial pressures would mirror brain partial pressures after a sufficient period of time.
As a dosing metric of anaesthesia, MAC has several advantages.
Minimum Alveolar Concentration - StatPearls - NCBI Bookshelf
That is, for a given stimulus, the anaesthetic concentration required to prevent movement is similar from person to person. In contrast to previously proposed metrics of anaesthetic depth, MAC applies similarly to all inhaled anaesthetics and thus allows for ready comparison of potency. Furthermore, the concept of MAC has aided investigation of factors that alter anaesthetic requirements and helped guide investigation into the mechanisms of action of inhaled anaesthetics.
Minimum alveolar concentration is a useful construct that has advanced clinical care and scientific investigation. However, it does have limitations. For example, MAC is not a reliable indicator of hypnosis or unconsciousness. Of interest, spontaneous movement of the spared hand was rare if not absent in these patients, including those who reported pain. It is not known why a behavioural response was elicited by a verbal stimulus but not by the experience of noxious stimulus.
These and other data that dissociate responsiveness and consciousness 26 , 80 suggest that MAC — a measure of responsiveness — may not necessarily be informative regarding the state of consciousness. One of the most notable limitations of MAC is that it is not directly applicable to the administration of total intravenous anaesthesia.
The use of neuromuscular blockade to produce paralysis renders the concentration of inhaled anaesthetic required to suppress movement uninformative. Given that multimodal or balanced anaesthetic techniques e. The former is a rare but feared complication of inadequate general anaesthesia associated with devastating psychological sequelae 81 , Efforts to prevent AWR events have focused on attaining adequate depth of anaesthesia by adjustment of anaesthetic dosing in response to MAC or electroencephalographic measures.
Currently, there is no evidence to suggest that, within a clinically relevant range, volatile anaesthetic agents promote adverse outcomes For example, there are studies that suggest that exposure to higher anaesthetic concentrations might increase the likelihood of postoperative delirium and cognitive decline 89 , Despite its limitations, It remains the most commonly used measure of anaesthetic potency for inhaled drugs.
Measurement of volatile anaesthetic concentration in real time is reliable, inexpensive and available around the world. Future candidate measures of anaesthetic depth will have to prove their superiority to MAC. Volume 68 , Issue 5.
MAC indicates "ED50 (median effective dose) for im- mobility"
The full text of this article hosted at iucr. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account. If the address matches an existing account you will receive an email with instructions to retrieve your username. Anaesthesia Volume 68, Issue 5.
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Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Summary Since its introduction in , minimum alveolar concentration MAC has served as the standard measure of potency for volatile anaesthetic agents.
Therefore, MAC can be estimated from the following equation 13 :. Figure 1 Open in figure viewer PowerPoint. In general, decreased solubility is associated with decreased potency, increased MAC and faster onset of action. Figure 2 Open in figure viewer PowerPoint. Relationship between anaesthetic concentration and the percent of people not moving in response to a surgical stimulus 4.
Minimum alveolar concentration: A reconsideration
This figure illustrates the relatively narrow interperson variability in the anaesthetic concentration required to suppress movement. Factors that shift the curve to the left i. SD, standard deviation. E-mail this Article Clinical Research Article. Home E-submission E-submission old ver.
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Sitemap Contact us. Search Korean J Anesthesiol Search. Clinical Research Article. Korean Journal of Anesthesiology ;44 3 Although it has been well-known that BIS correlates highly with the concentration of inhalation anesthetics, it is not clear whether analgesic potency expressed as MAC is comparable to hypnotic potency described as BIS50 in inhaled anesthetics.