Which circuit cannot be used to deliver nitrous oxide to a patient?

Which circuit cannot be used to deliver nitrous oxide to a patient?

• A. Circle
• B. Mapleson D
• C. Mapleson A
• D. Bains

Answer: (A)

The way a circuit handles rebreathing and gas concentrations determines whether nitrous oxide can be delivered safely and predictably. The Circle breathing system recycles exhaled gases after CO2 removal, so any nitrous oxide present in the loop tends to be re-breathed. Nitrous oxide is not absorbed by the CO2 absorber, so it stays in the circuit and can accumulate over time, making the inspired concentration hard to predict and control. This also complicates scavenging and can risk over- or under-dosing the patient. Because of these rebreathing and control issues, delivering nitrous oxide through a Circle system is not reliable. Mapleson circuits (such as A and D) and the Bains modification are designed to minimize rebreathing and allow more straightforward control of fresh gas flow, making nitrous oxide delivery safer and more predictable with those designs.

The way a circuit handles rebreathing and gas concentrations determines whether nitrous oxide can be delivered safely and predictably. The Circle breathing system recycles exhaled gases after CO2 removal, so any nitrous oxide present in the loop tends to be re-breathed. Nitrous oxide is not absorbed by the CO2 absorber, so it stays in the circuit and can accumulate over time, making the inspired concentration hard to predict and control. This also complicates scavenging and can risk over- or under-dosing the patient. Because of these rebreathing and control issues, delivering nitrous oxide through a Circle system is not reliable. Mapleson circuits (such as A and D) and the Bains modification are designed to minimize rebreathing and allow more straightforward control of fresh gas flow, making nitrous oxide delivery safer and more predictable with those designs.