Modeling airflow-related shear stress during heterogeneous constriction and mechanical ventilation

Abstract

Nucci, Gianluca, Bé la Suki, and Kenneth Lutchen. Modeling airflow-related shear stress during heterogeneous constriction and mechanical ventilation. J Appl Physiol 95: 348-356, 2003. First published March 21, 2003 10.1152/ japplphysiol.01179.2001.-Ventilator-induced lung injury has been proposed as being caused by overdistention and closure and reopening of small airways and alveoli. Here we investigate the possibility that heterogeneous constriction increases airflow-related shear stress to a dangerously high level that may be sufficient to cause injury to the epithelial cells during mechanical ventilation. We employed an anatomically consistent model of the respiratory system, based on Horsfield morphometric data, and solved for the time evolution of pressure and flow along the airway tree during mechanical ventilation. We simulated constant-flow ventilation with passive expiration in two different conditions: baseline and highly heterogeneous constriction. The constriction was applied with two strategies: establishing a simple diameter reduction or adding also a length shortening. The shear stress distribution on airway walls was analyzed for airways ranging from the trachea to the acini. Our results indicate that 1) heterogeneous constriction can amplify the maximal values of shear stress up to 50-fold, with peak values higher than 0.6 cmH 2O; 2) the highest shear stress is found in pathways constricted by 60-80%; 3) simultaneous diameter reduction and shortening amplifies the shear stresses by three-to fourfold, with shear stresses reaching 2 cmH 2O; and 4) there is a range of airways (diameters from 0.6 to 0.3 mm at baseline) that appear to be at risk of very high stresses. We conclude that elevated airflow-related shear stress on the epithelial cell layer can occur during heterogeneous constriction and conjecture that this may constitute a mechanism contributing to ventilator-induced lung injury. lung mechanics; ventilator-induced lung injury; simulation model MECHANICAL VENTILATION (MV) is often required to manage patients with acute respiratory failure (ARF) of different origins, including those associated with abnormalities in respiratory mechanics (31). However, there is considerable experimental evidence that MV itself can also cause or exacerbate lung injury, resulting in so-called ventilator-induced lung injury (VILI) (9,23,28). The mechanisms associated with VILI have not yet been fully elucidated. Two mechanisms have received the most attention. One is related to high airway pressures during MV, termed barotrauma, causing excessive overdistension of alveolar structures (26). More recently, it has become evident that injury results more from overstretching the alveolar tissues than from high pressures, and hence it is now termed volutrauma (10). Another potential mechanism is the repeated recruitment and derecruitment of atelectatic lung units (28). In principle, opening a collapsed airway would require very high-pressure drops across the fluid plug blocking the airway (13). During the process o...

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