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Alveolar recruitment/de-recruitment (R/D) seems to play an important role in the development of VILI . Many clinicians base their determination of PEEP settings during mechanical ventilation of ARDS/ALI patients on an estimate of alveolar recruitability . This project aims to establish an online tool that provides estimates of R/D in patients at the bedside.
In volume-controlled ventilated piglets as ARDS models, the airway pressure Paw (SI-Special Instruments, Nördlingen, Germany) and the flow rate Q (F + G GmbH, Hechingen Germany) were continuously recorded at 200 Hz. The pressure curve shows high nonlinearity being a suspect of recruitment effects during inspiration and a relaxation process during the end inspiratory pause. Based on the obtained data, the parameters of the linear viscoelastic model  are calculated by a LSE fitting process. As the parameter C1 represents a static constant compliance of the lung, this model is not capable of reproducing the nonlinear effects during inspiration. To improve on this, the constant compliance C1 is replaced by a nonlinear pressure-dependent compliance describing recruitment and dilatation as proposed by Hickling .
Since the nonlinear model has far more variable parameters to be optimized in the fitting process than the linear model, an approach via fitting the linear model first is helpful. Therefore, the estimated parameters of the linear model fit can be used as starting values for fitting the nonlinear model where the focus can be put on the recruitment phenomena. With the new nonlinear model, using the estimated values of R1, R2, C2 from the linear model (Figure (Figure11 left), it is now possible to reproduce the nonlinear characteristics (Figure (Figure11 right).
Using this new model it is possible to fit nonlinear behavior due to alveolar recruitment separately from viscoelastic effects with minimized error.