If the isoflurane vaporizer dial of an older non–pressure-compensating machine is set to deliver 1.15% in Denver, Colorado (barometric pressure 630 mm Hg), how many MAC will the patient receive?

If the isoflurane vaporizer dial of an older non–pressure-compensating machine is set to deliver 1.15% in Denver, Colorado (barometric pressure 630 mm Hg), how many MAC will the patient receive?

• A. About 20% more than 1 MAC
• B. About 10% more than 1 MAC
• C. One MAC
• D. About 10% less than 1 MAC

Answer: (B)

The key idea is how barometric pressure interacts with older, non–pressure-compensating vaporizers. Isoflurane’s MAC at sea level is about 1.15% in the inspired gas. When barometric pressure falls, these older vaporizers don’t automatically adjust the output to keep the same alveolar concentration. The result is that, for a given dial setting, the actual amount of isoflurane the patient ends up inhaling can be a bit higher than expected. In Denver, where the barometric pressure is about 630 mm Hg (lower than 760 mm Hg), this mismatch tends to push the delivered depth of anesthesia slightly deeper than 1 MAC. The magnitude is modest rather than dramatic, commonly around 10% more than 1 MAC for this altitude with an older non–pressure-compensating machine. So, the patient would be about 1.1 MAC rather than exactly 1 MAC. Modern, pressure-compensating vaporizers are designed to counteract this effect and keep the delivered MAC consistent across changes in ambient pressure.

The key idea is how barometric pressure interacts with older, non–pressure-compensating vaporizers. Isoflurane’s MAC at sea level is about 1.15% in the inspired gas. When barometric pressure falls, these older vaporizers don’t automatically adjust the output to keep the same alveolar concentration. The result is that, for a given dial setting, the actual amount of isoflurane the patient ends up inhaling can be a bit higher than expected.

In Denver, where the barometric pressure is about 630 mm Hg (lower than 760 mm Hg), this mismatch tends to push the delivered depth of anesthesia slightly deeper than 1 MAC. The magnitude is modest rather than dramatic, commonly around 10% more than 1 MAC for this altitude with an older non–pressure-compensating machine. So, the patient would be about 1.1 MAC rather than exactly 1 MAC.

Modern, pressure-compensating vaporizers are designed to counteract this effect and keep the delivered MAC consistent across changes in ambient pressure.