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 | Contribs)} 03:38, 3 September 2009 (UTC)