A 31-year-old moderately obese female is undergoing anesthesia with isoflurane. Inspired isoflurane is 1.7%, expired 0.6%, nitrous oxide flow is 500 mL/min and oxygen flow is 250 mL/min. Approximately how many MAC of anesthesia is represented by the alveolar concentration?

A 31-year-old moderately obese female is undergoing anesthesia with isoflurane. Inspired isoflurane is 1.7%, expired 0.6%, nitrous oxide flow is 500 mL/min and oxygen flow is 250 mL/min. Approximately how many MAC of anesthesia is represented by the alveolar concentration?

• A. 0.5 MAC
• B. 0.85 MAC
• C. 1.1 MAC
• D. 1.8 MAC

Answer: (C)

MAC is the alveolar concentration at which an inhaled anesthetic prevents movement in 50% of patients; it’s the potency benchmark for that agent. For isoflurane, the MAC in air is about 1.15%. When nitrous oxide is used, it lowers the required concentration of the volatile agent—a MAC-sparing effect. A practical approximation is that the isoflurane MAC in the presence of nitrous oxide is around 0.55% (roughly half of the MAC in air). The end-tidal (alveolar) concentration of isoflurane given is 0.6%. Expressed as MAC under these gas conditions, that’s 0.6% ÷ 0.55% ≈ 1.1 MAC. So the alveolar concentration corresponds to about 1.1 MAC. End-tidal isoflurane reflects alveolar partial pressure and is the relevant value for comparing to MAC, while inspired concentration is higher due to uptake but doesn’t directly set the depth of anesthesia.

MAC is the alveolar concentration at which an inhaled anesthetic prevents movement in 50% of patients; it’s the potency benchmark for that agent. For isoflurane, the MAC in air is about 1.15%. When nitrous oxide is used, it lowers the required concentration of the volatile agent—a MAC-sparing effect. A practical approximation is that the isoflurane MAC in the presence of nitrous oxide is around 0.55% (roughly half of the MAC in air).

The end-tidal (alveolar) concentration of isoflurane given is 0.6%. Expressed as MAC under these gas conditions, that’s 0.6% ÷ 0.55% ≈ 1.1 MAC. So the alveolar concentration corresponds to about 1.1 MAC.

End-tidal isoflurane reflects alveolar partial pressure and is the relevant value for comparing to MAC, while inspired concentration is higher due to uptake but doesn’t directly set the depth of anesthesia.