Respiratory mechanics in anaesthesia.

The present knowledge of respiratory mechanics in anaesthetized humans is astonishingly scanty. This probably reflects the notion that measurement of the mechanics of ventilation during anaesthesia is difficult to perform. In fact, in mechanically ventilated anaesthetized-paralysed humans, a detailed analysis of respiratory mechanics can be performed readily with simple and commonly available equipment, namely a pneumotachograph to measure flow (V), an integrator to obtain volume changes (A V) from the flow signal, and a pressure transducer to measure the pressure at the airway opening (Pao) or, preferably, in the trachea (Ptr) some distance beyond the distal end of the tracheal tube [12, 33]. Several commercial ventilators allow direct measurement of these variables (e.g. Siemens 900C, Puritan-Bennett 7200). With this equipment it is possible to determine, non-invasively, the static and dynamic elastance of the total respiratory system, the flow-resistances of the total respiratory system, airways and thoracic tissues and intrinsic PEEP [11, 34]. By adding an oesophageal balloon catheter system, overall respiratory system mechanics data can be partitioned into lung and chest wall components. In fact, contrary to previous belief, the oesophageal balloon technique is valid in the supine position in both awake [6] and anaesthetized subjects [21].