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Minimum alveolar concentration or MAC is the concentration 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.
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Formal definition[edit]
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.
Meyer-Overton hypothesis[edit]
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[edit]
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, height and weight 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.[4] 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[edit]
Values are known to decrease with age and the following are given based on a 40-year-old (MAC40):[8]
- Nitrous oxide - 104 [8]
- Xenon - 72 [8]
- Desflurane - 6.6 [8]
- Ethyl Ether - 3.2
- Sevoflurane - 1.8 [8]
- Enflurane - 1.63 [8]
- Isoflurane - 1.17 [8]
- Halothane - 0.75 [8]
- Chloroform - 0.5
- Methoxyflurane - 0.16
References[edit]
- ^'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. PMID5844267.
- ^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. ISSN0003-3022. PMID13935000.
- ^ abEger, 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. ISSN0003-3022. PMID5844267.
- ^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. PMID9447854.
- ^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. PMID9952144.
- ^ abcdefgh* 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. PMID12878613.CS1 maint: multiple names: authors list (link)
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Choosing a foundation can be really daunting, but it’s a key part to our makeup and can make or break a flawless face so it’s important to get it right. Many clients ask me about the labelling of MAC foundations as they don’t understand what NC and NW means and how to colour match themselves.
In colour theory, we’re taught that cool undertones are reds, pinks and blues and that warm undertones are yellow, peaches and golds. However, in MAC labelling, the undertones are the opposite.
NC: Neutral Cool – golden beige undertones.
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NW: Neutral Warm – red, pink or blue undertones.
N: Neutral – beige undertones.
C: Cool – Yellow, golden or olive undertones
So a person with yellow, peach or golden undertones would be suited well to an NC foundation whereas a person with red, pink or blue undertones would be better suited to an NW foundation. A person with a beige undertone would be perfectly suited to an N foundation.
You might have heard that some ladies use an NC foundation but have been given an NW concealer, this is because the pink undertone of an NW concealer helps to cancel out dark green under eye circles. For blue tinged under eye circles, you could cancel them out with an NC concealer.
My client Nina has warm undertones (reds in MAC terms) with cool under eye circles (yellow). I used MAC Studio Fix in NC20 over her face to even out and correct the redness and NW10 concealer to correct the dark circles
The number that follows each NC, NW, N or C is simply the colour of each foundation. Sharon, from Sharonthemakeupartist.com explains this perfectly:
“…there are some odd numbers peppered along the way in some of the foundations. For example there is an NC27 or an NW18 in Studio Fix Fluid . These colours don’t have quite as strong an undertone as those that end in 5 or 0. These odd numbers have more of a neutral undertone. So an NC27 is not as yellow as an NC25 but it is a little bit darker.
Conversely there are also plain C’s and plain N’s. These have a stronger yellow or pink undertone than their NC/NW counterparts. Very few of the foundations in the range now have these strong undertones, it’s mostly the old school formulas like Face and Body, or Studio Fix Powder.”
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The shade I wear myself is NC15 – this is much more flattering to my skin tone and helps to cancel out any unwanted redness over my cheek area. This is my personal preference too and I find that the NC blends seamlessly into my skin. I find that with the majority of my clients, an NC foundation always looks better and evens the skin tone beautifully.
The most important thing is to make sure that your foundation matches with your chest and looks natural. If you are still struggling, don’t be afraid to ask a counter makeup artist for advice and a colour match. Makeup is all about having fun so there is no wrong or right way to wear your foundation, as long as you have a good match and avoid the dreaded ‘tideline’ around the neck, you’re good to go!
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