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.
Circle systems for pediatrics have the same advantages as for adults including low fresh gas flow requirements, conservation of heat, humidity, and anesthetic agents, and minimal environmental pollution. In addition, shorter, narrow-caliber tubing and Y-pieces help to minimize the compliance and dead space of pediatric circuits.
Dead space in a breathing system only exists where fresh gas mixes with exhaled gas (i.e. at the Y-piece). Dead space is minimized with a septum in the Y-piece of a circle system or a fresh gas inflow port in the elbow of the Mapelson D system (Normal elbow).