A 24-year-old undergoing anesthesia with sevoflurane, N2O and O2 via LMA experiences spontaneous breathing as the vaporizer dial is increased, but the inspiratory valve does not fully close. The likely result is an increase in the inspired concentration of which gas?

A 24-year-old undergoing anesthesia with sevoflurane, N2O and O2 via LMA experiences spontaneous breathing as the vaporizer dial is increased, but the inspiratory valve does not fully close. The likely result is an increase in the inspired concentration of which gas?

• A. Nitrous oxide
• B. Carbon dioxide
• C. Oxygen
• D. Sevoflurane

Answer: (B)

This question hinges on rebreathing of carbon dioxide when the inspiratory valve in a circle system doesn’t close properly. If the inspiratory valve can’t seal, a portion of the gas the patient inhales comes from the patient’s own exhaled breath or from the reservoir rather than solely from fresh gas. Exhaled gas contains higher levels of carbon dioxide, so this backflow raises the concentration of CO2 in the inspired mixture. Even though increasing the vaporizer dial raises the sevoflurane concentration in the fresh gas, the valve leak causes more CO2 to be drawn into the next breath, leading to an increase in inspired CO2.

This question hinges on rebreathing of carbon dioxide when the inspiratory valve in a circle system doesn’t close properly. If the inspiratory valve can’t seal, a portion of the gas the patient inhales comes from the patient’s own exhaled breath or from the reservoir rather than solely from fresh gas. Exhaled gas contains higher levels of carbon dioxide, so this backflow raises the concentration of CO2 in the inspired mixture. Even though increasing the vaporizer dial raises the sevoflurane concentration in the fresh gas, the valve leak causes more CO2 to be drawn into the next breath, leading to an increase in inspired CO2.