Figure EQ.4 Mapleson classification of breathing systems

The expiratory or adjustable pressure-limiting (APL) valve is designed to vent or spill excess gas from the circuit. A relatively weak spring acts on a disc to close the expiratory orifice. With the valve control fully open the pressure on the valve is minimised to about 2 cm^O (0.2 kPa). This is sufficient to bias the valve closed and allow the reservoir bag to fill, yet allows the disc to lift off easily as the patient exhales. Screwing the valve towards the closed position allows a controlled leak when the bag is squeezed during hand ventilation. While earlier APL valves could be completely closed, this is now prevented by the action of a second, stronger spring that limits the pressure to about 50 cm^O (5 kPa) to prevent barotrauma to the lungs. Modern valves are shrouded within an assembly to permit the collection of gas into a scavenging system.

The most efficient arrangement for a spontaneously breathing adult is the Mapleson A or Magill circuit. Early in the expiratory phase the breath consists of anatomical dead space gas that contains little CO2. This flows down the tubing towards the reservoir bag that distends until the spring loaded expiratory valve opens. Alveolar gas containing CO2 is then vented out through the valve. During inspiration, the valve closes and the patient draws in the anatomical dead space volume from the bag as well as fresh gas. The minimum fresh gas flow rate during spontaneous ventilation need only equal the alveolar ventilation. If the anaesthetist takes over respiration by manually squeezing the bag, the valve opens during inspiration and a mixture of fresh gas and expired gas is forced into the lungs and out through the valve. This is inefficient and thus a much higher flow of fresh gas is required to flush out carbon dioxide (typically two to three times the alveolar ventilation).

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