Mapelson System: Mapelson systems are classified based on the location of the fresh gas flow inflow and overflow valves relative to the patient connection. They all share the benefit of decreased resistance due to the absence of unidirectional valves and canisters. Unfortunately rebreathing and the resulting hypercarbia and respiratory acidosis become a problem (although rebreathing conserves humidity, heat, and anesthetic gas). The degree of rebreathing depends on the fresh-gas flow.

Mapelson A systems result in no rebreathing during spontaneous ventilation if fresh gas inflow is > 75% of minute ventilation. It requires very high flows during controlled ventilation to eliminate rebreathing.

Mapelson D systems requires slightly higher flows during spontaneous ventilation than the Mapelson A system, but requires very little flow to prevent rebreathing during controlled ventilation. It is the most commonly used system for pediatric patients. The Bain system is a modification of the Mapelson D system where the fresh gas inflow is placed coaxially within the expiratory limb.

Mapelson F The most commonly used T-piece system is the Jackson-Rees’ modification of the Ayre’s T-piece (sometimes known as the Mapleson F system). This system connects a two-ended bag to the expiratory limb of the circuit; gas escapes via the `tail’ of the bag.