Recruitment/derecruitment models fitted to respiratory data of acute respiratory distress syndrome/acute lung injury patients

Introduction

Recruitment/derecruitment (R/D) seems to play an important role in the development of VILI [1]. Many clinicians base their determination of PEEP settings during mechanical ventilation of ARDS/ALI patients on an estimate of alveolar recruitability [2]. This project aims to establish an online tool that provides estimates of R/D in patients at the bedside.

Methods

We developed a computer simulation of R/D based on Matlab (Mathworks, Natick, MA, USA), which incorporates different approaches [1, 3, 4]. Our model is fitted (currently offline) to patient data acquired during controlled mechanical ventilation. For data acquisition the internal respiratory data of a ventilator (Evita 4; Dräger Medical, Lübeck, Germany) is read in real time. The simulation assumes a quantitative partition into pressure-dependent and time-dependent recruitment. Pure pressure-related approaches (for example [1]) are not able to describe transients (for example, a volume shift after a change in PEEP).

Results

A multistep optimization process is performed to reduce the difference between measured data and model prediction. At any moment during a tidal breath or during some respiratory maneuver the current state of the model can be visualized. The inflated volume splits up into extension of open alveoli and into temporal or pressure-dependent recruitment. Distribution of these compartments over time during a tidal inflation is depicted in Figure 1a. The pressure vs time and flow vs time curves are shown in Figure 1b.

Figure 1

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Conclusion

The fitting of recruitment models provides interesting insight into not directly observable R/D. It may be used for monitoring trends and drifts in recruitment. Currently results rely on certain assumptions; for example, distribution and quantity of superimposed pressure. With modern imaging techniques (for example, CT, EIT) a validation of the fitted models will come into reach and will be performed as a next step.