Minimum Alveolar Concentration (MAC) is a measure of potency of volatile and gaseous anaesthetic agents: the lower the MAC, the more potent the agent.

Definition: The MAC is that concentration required to prevent 50% of patients moving when subjected to a standard midline incision.

MAC values are quoted in terms of % concentration at atmospheric pressure. However, potency is determined by partial pressure rather than concentration. Unlike concentration, partial pressure is independent of ambient pressure. Vaporizers are calibrated in % and because one atmospheric pressure is approximately 100 kPa, there is a simple relationship between % and partial pressure of the agent: 1% is equivalent to a partial pressure of 1 kPa.

One MAC of sevoflurane is 2 kPa, which is approximately equal to a concentration of 2%.

Question: How is it possible for nitrous oxide to have a MAC of 103%?

Answer

Minimum Alveolar Concentration (MAC) is a measure of potency of volatile and gaseous anaesthetic agents: the lower the MAC, the more potent the agent.

Definition: The MAC is that concentration required to prevent 50% of patients moving when subjected to a standard midline incision.

MAC values are quoted in terms of % concentration at atmospheric pressure. However, potency is determined by partial pressure rather than concentration. Unlike concentration, partial pressure is independent of ambient pressure. Vaporizers are calibrated in % and because one atmospheric pressure is approximately 100 kPa, there is a simple relationship between % and partial pressure of the agent: 1% is equivalent to a partial pressure of 1 kPa.

One MAC of sevoflurane is 2 kPa, which is approximately equal to a concentration of 2%.

Question: How is it possible for nitrous oxide to have a MAC of 103%?

Answer: Although 103% of 1 atmosphere makes little sense, in a hyperbaric chamber at 1.5 atmospheres (150 kPa) a partial pressure of 103 kPa can readily be achieved in a non-hypoxic mixture. At this pressure the concentration of nitrous oxide is 103/150 or 69%.