The definition of MAC is the concentration of the vapor (measured as a percentage at 1 atmosphere, i.e the partial pressure) that prevents the reaction to a standard surgical stimulus in 50% of subjects. Since most of us work at about 1 atmosphere, we can still think in terms of % concentration, but what is physiologically important is the partial pressure (mm Hg), not the concentration.
Modern conventional vaporizers (for halothane, isoflurane, sevoflurane) are agent specific, temperature compensated, variable bypass vaporizers. They automatically compensate for changes in altitude because they put out a partial pressure that is determined by the position of the dial. Even though the units on the dial are percentages, it’s actually partial pressure that is determined. The partial pressure of the anesthetic agent is what determines whether a patient is anesthetized, and it does not change at different altitudes. So if you are doing anesthesia with isoflurane at high altitude, setting the dial to 1% will have the same effect as it would at sea level.
However, if the question relates to volume concentration then use equation:
Where VO=vapor output (ml), CG= carrier gas flow(mL.min), SVP=saturated vapor pressure (mm Hg) at room temp, and Pb is barometric pressure (mm Hg)
At a higher altitude where the barometric pressure is ½ that at sea level, the amount of isoflurane vapor output increases due to the lower barometric pressure. Therefore, the settings that delivered 2% isoflurane now deliver 4% isoflurane. However, according to Dalton’s law, the partial pressure of isoflurane delivered would be approximately the same at both altitudes since 2% isoflurane at 760mm Hg (15.2 mm Hg) is the same as 4% isoflurane at 380mm Hg (15.2 mm Hg).
According to Shafer, who presents an example on Stanford University’s website, “we find that to deliver 8.4 mm Hg (1 MAC) of isoflurane at 8000 feet, we need an anesthetic concentration of 1.5%. However, our vaporizer, set for 1.1%, is actually producing 1.7%, or a partial pressure of 9.9 mm Hg. Thus, the vaporizer slightly overcompensates for the reduced atmospheric pressure… Note that this does NOT apply to the desflurane vaporizer“(Source 1).
Alternatively, the desflurane vaporizer is electrically heated to 39 degrees centigrade, which creates a vapor pressure of 2 atmospheres inside the vaporizer, regardless of ambient pressure. The number on the dial reflects the percentage that will be delivered. So at any altitude, when you dial 5%, it will give you 5%. But when that 5% desflurane leaves the vaporizer at high altitude, what is delivered to the patient is 5% of a decreased ambient pressure, so the partial pressure of desflurane in the alveoli will be much less that it would be at sea level. Thus, you will need to dial a higher concentration at high elevation to attain the same clinical effect as at sea level with desflurane (Tec-9) vaporizer.