Talk:Minimum alveolar concentration

Defined
--koshime 23:10, September 9, 2005 (UTC) Defined and expanded on the MAC concept

Values greater than 100%
The article Nitrous oxide states 'It has a MAC of 105%'.

Either this statment is false, or the definition of MAC given on this page is incomplete/incorrect. Can somone please provide a complete and concise definition here, or else correct the other page.


 * Nitrous oxide is an anagesic. This does not mean that it readily immobilizes the patient.  MAC is the concentration at 1atm that produces immobility in 50% of patients.  In other words, nitrous oxide will kill the pain but must be used with other agents (such as halothane MAC 0.8 or isoflurane MAC 1.5)if immobility is desired.  I hope that helps. 194.176.105.39 13:07, 7 March 2007 (UTC)


 * MAC is conveniently expressed as a percentage of the atmospheric content at 1 atmosphere so isoflurane with a mac of 1.2% means that at 1 atmosphere isoflurane must make up 1.2% of the inhaled gas to have a mac effect. This is for convenience only as mac is related to partial pressure. So at 1 atmosphere 1.2% equates to about 1.2 KPa of pressure.
 * Now if you are in a situation where the ambient pressure is 2 atmospheres (underwater for example) then isoflurane need only make up .6% of the atmospheric content in order to exert 1 MAC effect because the partial pressure would be 1.2 KPa. At altitude this would be reversed so that the mac would 2.4%.
 * This then explains how the mac of nitrous oxide can be 105%. what is really meant that the mac is 105KPa so that you could achieve mac effect in hyperbaric situations but never at 1 atmosphere (earth atmosphere that is!) so that for clinical purposes nitrous is either described as a mac sparing agent (ie fractional contribution to the mac) or simply as an analgesic.


 * hope this helps 194.176.105.39 13:07, 7 March 2007 (UTC)
 * Both of the previous responses answered the question but left out two important points: one must remember that MAC is a median and is really similar to MAC50, sort of like LD50. One mustn't have the full MAC% to achieve surgical anesthesia in 100% of patients -- it may be (this is speculation for the purpose of explanation) that a MAC of 100% N2O would produce surgical anesthesia in 20% of patients.  That's fine until the second point -- surgical anesthesia won't be needed if the patient asphyxiates with 100% N2O and 0% O2.  DRosenbach  ( Talk 03:38, 3 September 2009 (UTC)