Hyperbaric chamber: MAC effect

Hyperbaric oxygen (HBO) is when oxygen is delivered in an environment where the barometric pressure is more than 1 atmosphere. This is common therapy for arterial air embolism, decompression sickness, severe profound anemia or hypoperfusion, severe CO poisoning, and gas gangrene from C. perfringens. There are some fundamental gas laws to be aware of when discussing HBO.

PV = K

where P = pressure, V = volume, K = constant

• Boyle’s: volume of gas is inversely proportional to pressure at constant temp.

• Gay-Lussac’s: at constant volume, pressure is proportional to temp.
• Charles’: at constant pressure, volume will vary directly with temp.
• Dalton’s: total pressure of mixture of gases is equal to sum of partial pressures
• Henry’s: at constant temp, amount of gas dissolved in liquid is proportional to partial pressure of the gas

MAC effect

Volatile agents produce anesthesia based on the partial pressure of the anesthetic in the body, not on the volume of agent in the lungs. HBO causes a decrease in MAC because there is an increase in partial pressure of the volatile agents at higher barometric pressures. There is an increase in gas density in HBO, which also causes the rotameter flowmeters to read falsely high, e.g., 2.0% sevoflurane 1 atm. would produce the same amount of anesthesia as 0.66% sevo at 3 atm.