Alveolar Concentration MAC

In short, the recent development in the field of protein drug interactions has more or less challenged this theory squarely. 

MAC may be defined as — the concentration at 1 atmosphere of anaesthetic in the alveoli required to produce immobility in 50% of adult patients being subjected to a surgical procedure.  

It has been duly observed that a further inerease to 30% MAC i.e., 1.3 MAC) invariably affords apparent 

Mechanism Probably at equilibrium, the prevailing concentration of a volatile anaesthetic in the alveoli is equal to that in the brain and eonsequently this partieular eoneentration in the brain that very intimately exhibits the eoneentration at the site responsible for the anaesthetie aetivities. Therefore, the MAC of a volatile anaesthetic is most frequently employed as a reliable yardstick to ascertain the exact potency of an individual general anaesthetic agent. Table-1 depicts the MAC values of several gaseous and volatile anaesthetics commonly put into practice nowadays. 

Interestingly, when these general (volatile) anaesthetics are employed in combination, the MACs for the Inhaled Anaesthetics are Additive. 

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Potency is described by the minimum alveolar concentration (MAC) of an agent and lipid solubility by the oil gas solubility coefficient. 

The mechanism of action of inhalational anesthetics is unknown. The diversity of chemical structures (inert gas xenon hydrocarbons halogenated hydrocarbons) possessing anesthetic activity appears to rule out involvement of specific receptors. According to one hypothesis, uptake into the hydrophobic interior of the plasmalemma of neurons results in inhibition of electrical excitability and impulse propagation in the brain. This concept would explain the correlation between anesthetic potency and lipophilicity of anesthetic drugs (A). However, an interaction with lipophilic domains of membrane proteins is also conceivable. Anesthetic potency can be expressed in terms of the minimal alveolar concentration (MAC) at which 50% of patients remain immobile following a defined painful stimulus (skin incision). Whereas the poorly lipophilic N2O must be inhaled in high concentrations (>70% of inspired air has to be replaced), much smaller concentrations (<5%) are required in the case of the more lipophilic halothane. 

Anesthetic Blood Gas Partition Coefficient1 Brain Blood Partition Coefficient1 Minimal Alveolar Concentration (MAC) (°/o)2 Metabolism Comments... 

Depth of anesthesia is determined by the concentration of anesthetic agent that reaches the brain. Brain concentration, in turn, depends on the solubility and transport of the anesthetic agent in the bloodstream and on its partial pressure in inhaled air. Anesthetic potency is usually expressed as a minimum alveolar concentration (MAC), defined as the percent concentration of anesthetic in inhaled air that results in anesthesia in 50% of patients. As shown in Table 9.6, nitrous oxide, N2O, is the least potent of the common anesthetics. Fewer than 50% of patients are immobilized by breathing an 80 20 mix of nitrous oxide and oxygen. Methoxyflurane is the most potent agent a partial pressure of only 1.2 mm Hg is sufficient to anesthetize 50% of patients, and a partial pressure of 1.4 mm Hg will anesthetize 95%. 

The potency of inhaled anesthetics is defined quantitatively as the minimum alveolar concentration (MAC), which is the concentration of anesthetic gas needed to eliminate movement among 50% of patients challenged by a standardized skin incision. The MAC is... 

Davis NL, Nunnally RL, Malinin TI (1975) Determination of the minimal alveolar concentration (MAC) of halothane in the white New Zealand rabbit. Br J Anesthesiol 47 341-345... 

Eger El II, Xing Y, Laster M et al. (2003) Halothane and isofluroane have additive minimum alveolar concentration (MAC) effects in rats. Anesth Analg 96 1350-1353 Fang Z, Gong D, Ionescu P et al. (1997) Maturation decreases ethanol minimum alveolar anesthetic concentration (MAC) more than desflurane MAC in rats. Anaesth Analg 84 852-858... 

Anesthetic potency can be expressed in terms of the minimal alveolar concentration (MAC) at which 50% of patients remain immobile following a defined painful stimulus (skin incision). Whereas the poorly lipophilic nitrous oxide must be inhaled in high concentrations, much smaller concentrations are required in the case of the more lipophilic halothane. 

LeDez, K.M., and J. Lerman. 1987. The minimum alveolar concentration (MAC) of isoflurane in preterm neonates. Anesthesiology 67(3) 301-307. 

Stevens, W.D., W.M. Dolan, R.T. Gibbons, A. White, E.I. Eger, R.D. Miller, R.H. deJong, and R.M. Elashoff. 1975. Minimum alveolar concentration (MAC) of isoflurande with and without nitrous oxide in patients of various ages. Anesthesiology 42(2) 197-200. 

Comparison of the efficacy of inhalational agents is made by measuring the minimum alveolar concentration (MAC) in oxygen required to prevent movement in response to a standard surgical skin incision in 50% of subjects. The MAC of the volatile agent is reduced by the co-administration of nitrous oxide. 

Cyclopropane is an inhalational anesthetic gas. Its minimum alveolar concentration (MAC) is 9.2%. Because of the risk of explosion, it is usually administered by closed circuit (1). 

The effects of desflurane and sevoflurane on bronchial smooth muscle reactivity have been compared in a randomized study of 40 patients (36). Anesthesia was induced with thiopental, followed by muscle relaxation and ventilation. Airway pressures were recorded during administration of desflurane or sevoflurane at one minimal alveolar concentration (MAC). Airway resistance increased by 5% in the desflurane group and fell by 15% in the sevoflurane group. The increase in airways resistance was greater in smokers and with desflurane, but did not differ with sevoflurane. The result was a surprise, given that desflurane stimulates the sympathetic nervous system. Thiopental also increased airways resistance by 10%. The result is important, because induction of anesthesia can cause bronchospasm and desflurane can exacerbate this. 

The oil solubility of an anaesthetic is of interest, not only because it governs the passage of the anaesthetic into and out of the fat depots of the body, but also because there is a well-established correlation between anaesthetic potency and oil solubility. Figure 2.10 shows a linear inverse relationship between log narcotic concentration and log solubility in oleyl alcohol for a series of common anaesthetic gases. The ordinate of the graph represents the minimum alveolar concentration (MAC), which is that concentration of anaesthetic at which 50% of the patients cease to move in response to a stimulus. The abscissa shows the solubility expressed in terms of the oil/gas partition coefficient. Partition coefficients are widely used to express solubility and are the ratios of the concentration of the gas in the two phases in equilibrium at a given temperature. When, as in this case, one of the phases is the gas itself, the partition coefficient expressed as the liquid/gas (note the order of the phases) concentration ratio is equal to the... 

Rifabutin has better activity against the minimum alveolar concentration (MAC) organisms than does rifampin. Rifabutin is active in vitro against MAC bacteria isolated from both HIV-infected (where the majority of MAC infections are M. avium) and non-HIV-infected individuals (in whom approximately 40% of MAC infections are M. intracellulare). Rifabutin inhibits the growth of most MAC isolates at concentrations ranging from 0.25 to 1 pg/mL. Rifabutin also inhibits the growth of many strains of M. tuberculosis at concentrations of 0.125 pg/mL. 

Define minimum alveolar concentration (MAC) and explain its significance. 

Minimum alveolar concentration (MAC) is the anesthetic concentration that eliminates the response in 50% of patients exposed to a standardized painful stimulus. In this table, MAC is expressed as a percentage of the inspired gas mixture. 

General anaesthetics have been in use for more than a centuiy, but unfortunately so far no exact mechanism of action has been put forward. Of coiuse, a few theories, namely lipid, physical, biochemical, miscellaneous, Meyer-Overton, minimum alveolar concentration (MAC), stereochemical effects and ion-channel and protein receptor theories have been advocated from time to time in support of the mode of action of the general anaesthetics. These will be discussed briefly in this context. 

The measure of potency is the muu num alveolar concentration (MAC). MAC is the concentration of anesthetic that causes immobility in 50% of subjects exposed to the agent at one atmosphere. The MACs of commonly used anesthetics are ... 

The minimum alveolar concentration (MAC) is defined as the conoentration at 1 atmosphere of anesthetic in the alveoli that is required to produce immobility in 50% of adult patients subjected to a surgical incision. A further inorease to 1.3 MAC frequently will oause immobility in 99% of patients. At equilibrium, the conoentration (or partial pressure) of an anesthetic in the alveoli is equal to that in the brain, and it is this concentration in the brain that probably most olosely reflects the concentration at the site responsible for the anesthetio aotions. Thus, the MAC often is used as a measure of the potenoy of individual anesthetic agents. The MAC of many of the volatile and gaseous anesthetics in use today is shown in Table 18.2. 

The physical properties of inhalational anesthetics are provided in Table 19.2. Note that desflurane, compared to other agents, has a boiling point that is close to normal room temperature and a vapor pressure that is close to atmospheric pressure at sea level. These properties dictated a new type of vaporizer that heats the agent to 39°C in order to meter the flow of desflurane vapor into the gas flow stream. The minimum alveolar concentration (MAC) of halothane, from Table 19.2 indicates that the agent is the most potent of the agents available today with desflurane as the least potent. 

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