On the separate determination of lung mechanics in in- and expiration

Decisions concerning mechanical ventilation setting at the ICU highly depend on knowledge about respiratory mechanics of the patients' lung. Airway resistance and compliance are different between inspiration and expiration - as e.g. shown by means of body plethysmography in healthy spontaneously breathing subjects. Some authors e.g. [1] propose to apply a least-squares fit (LSF) - based on the equation of motion - to the expiration phase alone. But in theory the passive unloading of a capacitor (compliance) via a resistor (resistance) leads to a linear relationship between volume and flow and thus to an underspecified problem. To decouple this linear volume-flow dependency and to enable - at the bedside and online - a separate analysis of respiratory mechanics in inspiration and expiration a new ventilation mode is introduced: Active Expiration Control (AEC). Six healthy sheep - ventilated by an Evita4Lab system (Draeger Medical, Lubeck, Germany) that was reprogrammed to enable AEC - were enrolled in this study. The LSF analysis of normal passive expirations without AEC delivered faulty results. If the analysis includes all expiratory data, i.e. the noisy phase of the expiration valve opening and the end-expiratory PEEP regulation, parameter estimation is stable but the fitting quality is bad. If the LSF is constrained to the undisturbed phase of the expiration, we found huge parameter variations as predicted by theory but a small sum of squared errors (SSE) indicating a perfect fit. If the parameter estimation is performed on expirations with AEC we get both: small confidence intervals and at the same time a small SSE. AEC decouples the linear dependency between volume and flow present in passive expiration and provides the basis for separate analysis of lung mechanics in inspiration and expiration. The method is ready for implementation in ICU ventilators.