What minimum MAC is the patient receiving with 3 L/min N2O, 1 L/min O2, and isoflurane 0.85%?

What minimum MAC is the patient receiving with 3 L/min N2O, 1 L/min O2, and isoflurane 0.85%?

• A. 0.8
• B. 1.25
• C. 1.50
• D. 1.75

Answer: (C)

Think of MAC as a measure of anesthetic potency that adds across inhaled agents. Each agent contributes a fraction of one MAC toward the total depth of anesthesia, and you can sum these fractions. Isoflurane has a MAC of about 1.15%. The delivered concentration is 0.85%, which corresponds to 0.85/1.15 ≈ 0.74 MAC. Nitrous oxide has a MAC value of 105% (1.05 in decimal terms). With a fresh gas mixture of 3 L/min N2O and 1 L/min O2, the inspired fraction of N2O is 3/4 = 0.75, so its MAC contribution is 0.75/1.05 ≈ 0.714 MAC. Add the two contributions: 0.74 + 0.714 ≈ 1.45 MAC, which rounds to about 1.5 MAC. So the minimum MAC the patient is receiving is approximately 1.50. The oxygen fraction (25%) doesn’t add to MAC itself, but sets the overall gas mixture.

Think of MAC as a measure of anesthetic potency that adds across inhaled agents. Each agent contributes a fraction of one MAC toward the total depth of anesthesia, and you can sum these fractions.

Isoflurane has a MAC of about 1.15%. The delivered concentration is 0.85%, which corresponds to 0.85/1.15 ≈ 0.74 MAC.

Nitrous oxide has a MAC value of 105% (1.05 in decimal terms). With a fresh gas mixture of 3 L/min N2O and 1 L/min O2, the inspired fraction of N2O is 3/4 = 0.75, so its MAC contribution is 0.75/1.05 ≈ 0.714 MAC.

Add the two contributions: 0.74 + 0.714 ≈ 1.45 MAC, which rounds to about 1.5 MAC.

So the minimum MAC the patient is receiving is approximately 1.50. The oxygen fraction (25%) doesn’t add to MAC itself, but sets the overall gas mixture.