Sustained vs. Intratidal Recruitment in the Injured Lung During Airway Pressure Release Ventilation: A Computational Modeling Perspective

Introduction: During mechanical ventilation, cyclic recruitment and derecruitment (R/D) of alveoli result in focal points of heterogeneous stress throughout the lung. In the acutely injured lung, the rates at which alveoli can be recruited or derecruited may also be altered, requiring longer times at higher pressure levels to be recruited during inspiration, but shorter times at lower pressure levels to minimize collapse during exhalation. In this study, we used a computational model to simulate the effects of airway pressure release ventilation (APRV) on acinar recruitment, with varying inspiratory pressure levels and durations of exhalation. Materials and Methods: The computational model consisted of a ventilator pressure source, a distensible breathing circuit, an endotracheal tube, and a porcine lung consisting of recruited and derecruited zones, as well as a transitional zone capable of intratidal R/D. Lung injury was simulated by modifying each acinus with an inflation-dependent surface tension. APRV was simulated for an inhalation duration (T-high) of 4.0seconds, inspiratory pressures (P-high) of 28 and 40cmH(2)O, and exhalation durations (T-low) ranging from 0.2 to 1.5seconds. Results: Both sustained acinar recruitment and intratidal R/D within the subtree were consistently higher for P-high of 40cmH(2)O vs. 28cmH(2)O, regardless of T-low. Increasing T-low was associated with decreasing sustained acinar recruitment, but increasing intratidal R/D, within the subtree. Increasing T-low was associated with decreasing elastance of both the total respiratory system and transitional subtree of the model. Conclusions: Our computational model demonstrates the confounding effects of cyclic R/D, sustained recruitment, and parenchymal strain stiffening on estimates of total lung elastance during APRV. Increasing inspiratory pressures leads to not only more sustained recruitment of unstable acini but also more intratidal R/D. Our model indicates that higher inspiratory pressures should be used in conjunction with shorter exhalation times, to avoid increasing intratidal R/D.