A mixture change leads to the expired isoflurane concentration rising to 2.3% after 1 minute; the best explanation is which of the following?

A mixture change leads to the expired isoflurane concentration rising to 2.3% after 1 minute; the best explanation is which of the following?

• A. Intermittent back pressure
• B. Diffusion hypoxia
• C. Concentration effect
• D. Effect of N2O solubility in isoflurane

Answer: (D)

When nitrous oxide is used with a volatile anesthetic, the delivered concentration of the volatile agent can rise because N2O diffuses into the liquid isoflurane in the vaporizer. Nitrous oxide dissolves in the isoflurane liquid, increasing its vapor pressure and pushing more isoflurane into the gas phase. That means the fresh gas—and thus the expired end-tidal concentration—can climb even if the dial setting hasn’t changed. Seeing the end-tidal isoflurane reach about 2.3% within a minute fits this diffusion into the liquid phenomenon, driven by the solubility of N2O in isoflurane. Intermittent back pressure would affect flow or ventilation, not specifically the vaporizer’s output. Diffusion hypoxia is a concern when N2O is stopped, causing dilution of alveolar O2, not a rise in isoflurane concentration. The concentration effect describes a rapid rise in alveolar concentration of a volatile agent due to high inspired concentration and low solubility for uptake dynamics, but it doesn’t specifically account for the interaction of N2O with the liquid anesthetic in the vaporizer.

When nitrous oxide is used with a volatile anesthetic, the delivered concentration of the volatile agent can rise because N2O diffuses into the liquid isoflurane in the vaporizer. Nitrous oxide dissolves in the isoflurane liquid, increasing its vapor pressure and pushing more isoflurane into the gas phase. That means the fresh gas—and thus the expired end-tidal concentration—can climb even if the dial setting hasn’t changed. Seeing the end-tidal isoflurane reach about 2.3% within a minute fits this diffusion into the liquid phenomenon, driven by the solubility of N2O in isoflurane.

Intermittent back pressure would affect flow or ventilation, not specifically the vaporizer’s output. Diffusion hypoxia is a concern when N2O is stopped, causing dilution of alveolar O2, not a rise in isoflurane concentration. The concentration effect describes a rapid rise in alveolar concentration of a volatile agent due to high inspired concentration and low solubility for uptake dynamics, but it doesn’t specifically account for the interaction of N2O with the liquid anesthetic in the vaporizer.