Monitoring the breathing system

All breathing systems need to be monitored in clinical use, but little is amenable to the unaided observation of the anaesthetist. Reliance is therefore placed on the components of the breathing system itself and on the monitoring equipment. The purpose of monitoring is mainly to confirm our expectations of its results. When an unexpected finding appears the anaesthetist´s response may range from immediate action to ignoring the result. Several factors influence this decision: the anaesthetist´s confidence in his ability to predict the correct result, the clinical importance of the finding, support from different monitoring equipment, etc. It is, of course, possible to overwhelm an observer with information, and monitoring must be selective. It is also impossible to monitor everything, and although we may yearn for a central nervous system PO₂-meter, we can´t have one. Even when it is possible to monitor something (e.g. intrapulmonary shunt) we may not consider it worth the expense and effort. It is with these points in mind that I have approached the subject of monitoring the breathing system. Some monitoring applies equally to open and to low flow breathing systems, but some is of much greater importance as the flow is reduced because the anaesthetist´s confidence in his own predictions is weakened. I will consider airway pressure, gas flow, gas temperature briefly, and concentrate on the analysis of gas composition