Vaporisers and anaesthetic agent delivery

An important point when considering the behaviour of circle systems with low fresh gas flows is that the measured fresh gas composition no longer represents the gas mixture being delivered to the patient. As the total flow is reduced to the point where no gas is vented from the system, so called Closed System Anaesthesia, the fresh gas flow represents the replacement of the gases taken up by the patient. Typically as the gas flow is reduced there is a need for a more oxygen-rich supply since oxygen uptake exceeds nitrous oxide uptake after around 30 to 40 minutes of anaesthesia. Thus the monitoring of oxygen concentrations within the system is vital as is the anaesthetic vapour concentration since the uptake of this will vary with time according to its own characteristics and these vary from agent to agent. The four means by which a volatile agent may be introduced into the breathing system are discussed: vaporiser outside the circle; vaporiser in the circle; volumetric addition to the fresh gas flow; volumetric addition into the circle