|Anaesthetic gas exits the anaesthesia machine (via the common gas outlet) and then enters a breathing circuit. The function of the circuit is to deliver oygen and anaesthetic gases to the patient and to eliminate CO2. The CO2 may be eliminated by gas inflow or by soda lime absorption.
Various classification systems have been developed to aid understanding of how breathing systems operate.
Open, semi-open, semi-closed and closed
This most basic classification of breathing systems divides them into open, semi-open, semi-closed or closed.
Non-rebreathing and rebreathing systems
The classification of non-rebreathing versus rebreathing systems is more widely used. The term rebreathing implies that expired alveolar gas containing 5% CO2 is inspired as part of the next tidal volume. Anaesthetic circuits are designed to minimise this, as it may lead to serious elevations in blood CO2 levels. The amount of rebreathing that occurs with any particular anaesthetic breathing system depends on four factors: the design of the individual breathing circuit, the mode of ventilation (spontaneous or controlled), the fresh gas flow rate and the patient's respiratory pattern. Circuits may eliminate rebreathing either by ensuring an adequate flow of fresh gas which flushes the circuit clear of alveolar gas, or, in the case of a circle system, by the use of soda lime, which absorbs the CO2 so that low fresh gas flows may be used.
Mapleson described five different arrangements of breathing circuits. He classifed these circuits and they are now known as the Mapleson systems, termed A-E. This classification does not include systems with CO2 absorption.
Adjustable pressure-limiting valve (APL valve)
In the open position, the valve is actuated by pressures of less than 0.1 kPa (1 cm H2O).
In the closed position, the breathing system pressure, and therefore the intrapulmonary pressure, is protected by a pressure relief mechanism, actuated at 6 kPa (60 cmH2O).
Classification of anaesthetic breathing circuits