Surfactant-Depleted Rat Lungs: A Characterization of Organ- and Tissue-Level Mechanics Under Negative and Positive Pressure Ventilation

Multiple types of lung diseases damage epithelial cell layers producing pulmonary surfactant, a substance vital for decreasing lung surface tension, and therefore increase the risk of ventilator induced lung injury (VILI). VILI may be exacerbated by positive pressure ventilation (PPV; standard clinical method) compared to negative pressure ventilation (NPV; diaphragm physiological loading), warranting investigations of the comparative effect of PPV and NPV on surfactant-depleted lungs. Surfactant depletion was induced using ex vivo rat lungs via saline wash. An electromechanical ventilator imposed PPV or NPV, where digital image correlation technology continuously measured the associated inflation deformations simultaneously; both bulk-level pressure-volume measures and local tissue-level strain values were used to compare the two ventilation modes. For PPV, compared to control counterparts, the surfactant-depleted lungs showed decreased starting compliance, increased hysteresis, and decreased skewness of major strain distribution at maximum inflation, known to be associated with increased VILI risk; these same measures were not significantly different between NPV controls and surfactant-depleted specimens. For surfactant-depleted lungs, NPV showed a pressure-time curve fit closer to one (associated with the clinical stress index, less pressure inhomogeneities, and safer ventilation), and lower regional and 75th percentile compliance values at maximum inflation compared to PPV. The negative impact of surfactant depletion is suggested to be ameliorated under NPV via a more gradual initial pressure increase and resulting lower local compliance values compared to PPV. For lungs with surfactant loss, findings indicate NPV supports safer mechanical ventilation in contrast to PPV.