Which Mapleson circuit is considered the least effective for controlled ventilation?

The Mapleson circuit considered the least effective for controlled ventilation is Mapleson A. This classification is based on the circuit's performance in delivering anesthetic gases during controlled ventilation, particularly regarding how well it facilitates the exchange of gases and allows for effective ventilation.

Mapleson A has a high freshness gas flow requirement, which means that it does not effectively prevent rebreathing of exhaled gases when used for controlled ventilation. This makes it less efficient compared to other Mapleson systems, such as Mapleson D, which has better characteristics for controlled ventilation due to its lower resistance and higher efficiency in gas delivery during the phases of ventilation.

The inefficiency of Mapleson A in controlled ventilation scenarios stems from its design, where the limb lengths and configuration contribute to a longer time constant for gas exchange. Therefore, in a setting requiring controlled or mechanical ventilation, other Mapleson configurations are preferred for their more favorable characteristics, contributing to a more effective delivery of anesthetic gases to the patient.