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Development of a system for in vivo optical alveolar elastometry

Introduction

Micromechanical properties of alveolar walls are needed for finite element modeling of the lung that in turn may be used to guide mechanical ventilation therapy. The aim of this project is to develop an endomicroscopic device [1] to measure the local mechanics of alveolar walls in vitro and in vivo under varying conditions. Here we report on the development and the evaluation of the endoscopic system, which is – on the first run – performed in an artificial environment (bioreactor) with known mechanical properties.

Methods

A system of two concentric trocars was built to adjust and monitor the local pressure by means of a flushing fluid in the endoscopic field of view. The fluid is pumped through the double trocar system that is surrounding the endoscope. By adjusting the flow rate of the fluid, the mean pressure P2 in the endoscopic field of view, can be kept constant. The alveolar pressure is changed by adjusting the airway pressure P1. The transpulmonary pressure (Pt) for the observed subpleural alveoli is thus Pt = P1 - P2 (Figure 1b). Pt is varied by applying different continuous positive airway pressure values to animal-model airways. The mechanical reaction of the observed alveoli is, thereby, recorded by video endoscopy. Assuming that the recorded outlines reflect changes in the diameter of an observed alveolus allows calculation of mechanical properties such as the stress–strain relationship of the alveolar wall. For evaluation, the endoscopic system is applied to an artificial membrane with known mechanical properties in a bioreactor. A pattern of particles on the membrane allows quantifying the three-dimensional deformation under pressure changes. Mechanical membrane properties can be determined by the relation between membrane-deformation and transmembrane pressure Pm = Pa - Pb (Figure 1a).

Figure 1
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abstract P52

Results

Preliminary results were obtained in the bioreactor with a polymer membrane (polydimethylsiloxan) of 100 μm thickness. Graphite particles were used to produce a particle pattern. Deformation due to different Pt' was observed and recorded successfully.

Conclusion

The in vivo estimation of micromechanical properties such as the stress–strain relationship of elastic walls is feasible. For that, the developed system has to be technically optimized.

References

  1. 1.

    Stahl CA, et al.: J Biomech. 2006, 39: 598. 10.1016/S0021-9290(06)85480-1

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Schwenninger, D., Möller, K., Schumann, S. et al. Development of a system for in vivo optical alveolar elastometry. Crit Care 13, P52 (2009). https://doi.org/10.1186/cc7216

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Keywords

  • Continuous Positive Airway Pressure
  • Airway Pressure
  • Strain Relationship
  • Graphite Particle
  • Alveolar Wall