The history and development of low flow breathing systems

A breathing system is the assemblage of tubes, reservoirs and valves from and into which a subject breaths. The function of the breathing system is to supply oxygen and to eliminate expired carbon dioxide. A “low flow” system is one in which the carbon dioxide is chemically removed so that the fresh gas supplied to the system needs only to be sufficient to supply the subject´s oxygen requirements. By contrast, in a “high flow” system there is no chemical removal of carbon dioxide, instead it is diluted and dispersed by a fresh gas flow rate which must at least equal the alveolar ventilation. This high efficiency is only achieved in systems in which the dead space gas (one third of the tidal volume and free of carbon dioxide) is rebreathed. These considerations indicate that the minimum fresh gas flow rate into a completely closed system is 200 mls/min and in a high flow system of the highest efficiency the minimum flow rate is 3.2 L/min. These figures relate to the traditional 70 Kg anaesthetised subject. In practice high flow systems usually require at least 5 L/min and fresh gas flow rates below 4 L/min are loosely termed “low flow”. In all modes of operation, except the completely closed, excess gas in the system is vented via an adjustable pressure limiting valve. The evolution of the theory and practice of such closed breathing systems is discussed