Alterations in feline tracheal permeability after mechanical ventilation

Objectives: Previous investigations of ventilator-induced airway injury focused on histopathologic changes associated with various ventilators and strategies for their use. We hypothesized that mechanical ventilation is associated with alterations in tracheal epithelial permeability, and designed a study using an animal model to evaluate changes in tracheal epithelial permeability after administering different types of mechanical ventilation to test this hypothesis. Design: Prospective, multiple-group, controlled trial. Five groups of animals were studied and compared. Eight animals were studied without intubation or mechanical ventilation. A total of 28 animals (seven in each group) were studied after conventional mechanical ventilation, high- frequency positive-pressure ventilation, high-frequency jet ventilation, or high-frequency flow interruption at respiratory rates of 20, 150, 400, and 900 breaths/min, respectively. Comparison of data for each group was done using the Kruskall-Wallis analysis of variance. Between-group comparisons were made using standard error of the mean comparisons. For airway pressures and other physiologic data, one-way analysis of variance was performed. Between-group comparisons were made using the Student-Newman-Keuls' test. Setting: Small animal physiology laboratory. Subjects: Thirty-six adult cats. Interventions: Mechanically ventilated animals were treated for 8 hrs and then killed. Inspired oxygen concentration, BP, and mean airway pressures were comparable in mechanically ventilated animals. Spontaneously breathing control animals were killed without endotracheal intubation or exposure to mechanical ventilation. Measurements and Main Results: Permeability values in isolated tracheal segments were calculated for ¹⁴C-sucrose, ³H-inulin, and fluorescein isothiocyanate-dextran-20. Tracheal epithelial permeability to all studied molecules increased after exposure to mechanical ventilators. These different mechanical ventilators increased epithelial permeability in a progressive manner that paralleled ventilatory frequency. The changes were greatest after ventilation at the highest frequency. These observed changes in tracheal permeability are consistent with previously observed alterations in tracheal histopathology after exposure to mechanical ventilation. Conclusions: Mechanical ventilation was associated with increases in tracheal permeability to large and small nonionic molecules. These changes occurred with all studied ventilators, used as they are clinically. Permeability changes paralleled ventilatory rate changes.