A simple bedside approach to measurement of respiratory mechanics in critically ill patients.
OBJECTIVE
To describe and evaluate clinically applicable approaches to measurement of respiratory mechanics in critically ill patients.


METHODS
Methodological and evaluation studies of respiratory mechanics in critically ill patients from relevant MEDLINE searches.


RESULTS
In ventilated subjects clinically important respiratory system mechanics can be measured using airway pressure and flow data. However, since the respiratory system consists of the lung and chest wall, and chest wall mechanics can markedly alter respiratory system mechanics, it is preferable to compartmentalise these parameters with concurrent measurement of oesophageal, and preferably gastric pressure. Additional care must be taken with interpretation of these data since elastance and resistance may be influenced by frequency, volume, volume history and flow. Tissue viscoelasticity and non-homogeneity of regional time constants are responsible for stress adaptation, which can be measured simply, and accounts for some of these effects on elastance and resistance, and for a systematic difference between static and dynamic intrinsic PEEP. Elastance can be measured using the end-inspiratory occlusion technique, or from either static or dynamic volume-pressure curves. PEEP-mediated recruitment can be measured following referencing of these curves to FRC. Similarly, resistance can be measured from either end-inspiratory occlusion or dynamic pressure and flow data.


CONCLUSIONS
Some of this information is available on modern ventilators, but greater insight requires measurement and manipulation of flow and pressure data using a pneumotachograph and pressure transducers. Given the importance of respiratory mechanics in the management of many critically ill patients, and given how poorly the respiratory system is monitored compared with the cardiovascular system, it is worth considering making this simple but additional effort.
Bersten
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Department of Critical Care Medicine, Flinders Medical Centre, Adelaide, South Australia. andrew.bersten@flinders.edu.au