When the inspiratory valve malfunctions by staying open during exhalation, which gas concentration will increase in the inspired gas mixture?

When the inspiratory valve malfunctions by staying open during exhalation, which gas concentration will increase in the inspired gas mixture?

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

Answer: (B)

This question centers on what happens in a circle anesthesia circuit when the inspiratory valve stays open during expiration. With the valve not closing as it should, exhaled, CO2-rich gas can flow back into the inspiratory limb and be re-inhaled. Since the freshly delivered gas in the circuit is relatively low in CO2, mixing in this exhaled gas causes the concentration of carbon dioxide in the inspired gas to rise. The consequence is rebreathing of CO2, leading to higher inspired and end-tidal CO2 levels. Oxygen concentration does not increase from this setup; it can become diluted as exhaled gas mixes with fresh gas, and the patient may retain less oxygen if ventilation isn’t adequate. Nitrous oxide and sevoflurane may also be carried back into the inspired mix, but the gas whose concentration most characteristically increases due to this specific valve malfunction is carbon dioxide, which is the key clue clinicians would monitor for rebreathing.

This question centers on what happens in a circle anesthesia circuit when the inspiratory valve stays open during expiration. With the valve not closing as it should, exhaled, CO2-rich gas can flow back into the inspiratory limb and be re-inhaled. Since the freshly delivered gas in the circuit is relatively low in CO2, mixing in this exhaled gas causes the concentration of carbon dioxide in the inspired gas to rise. The consequence is rebreathing of CO2, leading to higher inspired and end-tidal CO2 levels.

Oxygen concentration does not increase from this setup; it can become diluted as exhaled gas mixes with fresh gas, and the patient may retain less oxygen if ventilation isn’t adequate. Nitrous oxide and sevoflurane may also be carried back into the inspired mix, but the gas whose concentration most characteristically increases due to this specific valve malfunction is carbon dioxide, which is the key clue clinicians would monitor for rebreathing.