Decision making in the ICU concerning mechanical ventilation depends on knowledge about physiological characteristics of the patients' lung. The analysis of respiratory resistance and compliance usually assumes both parameters to be identical in inspiration and expiration, which is not true in general -as e.g. shown by means of body plethysmography in healthy spontaneously breathing subjects [1]. Therefore some authors e.g. [2] propose to apply a least-squares fit (LSF) -based on the equation of motion- to the respiratory data of the expiration phase alone. But as theory about the passive unloading of a capacitor (compliance) via a resistor (resistance) predicts, passive expiration leads to a quasi linear relationship between volume and flow and thus to an underspecified problem. To decouple this linear volume-flow dependency and to enable a separate analysis of respiratory mechanics in inspiration and expiration online at the bedside a new ventilation mode was introduced: Active Expiration Control (AEC) [3]. The AEC allows even separate analysis of intratidal nonlinearity of compliance and resistance as will be demonstrated. Six healthy sheep -ventilated by an Evita4Lab system (Draeger Medical, Germany) that was reprogrammed to enable AEC- were enrolled in this study. The adaptive slice method (ASM) [4, 5] was used to obtain intratidal changes in respiratory parameters. Considerable differences could be observed in the resistance of inspiration and expiration. This may lead to a different interpretation of dynamic compliance obtained with current methods. 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.
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