Objectives: Although global respiratory mechanics are usually used to determine the settings of mechanical ventilation, this approach does not adequately take into account alveolar mechanics. However, it should be expected that the ventilatory condition (quasi-static vs. dynamic) and lung condition (noninjured vs. injured) affect alveolar mechanics in a clinically relevant way. Accordingly, the aim of this study was to investigate alveolar mechanics during quasi-static and dynamic ventilatory maneuvers in noninjured and injured lungs. We hypothesized that alveolar mechanics vary with ventilatory and lung conditions.
Design: Prospective animal study.
Setting: Animal research laboratory.
Subjects: Male Wistar rats.
Interventions: Alveolar mechanics (derived from alveolar size and airway pressure) were determined in noninjured (n = 9) and in lungs lavaged with saline (n = 8) at quasi-static (low flow at a peak pressure of 40 cm H₂O) and dynamic ventilatory maneuvers (increase and decrease in positive end-expiratory pressure from 0 to 15 and back to 0 cm H₂O in steps of 3 cm H₂O). Alveoli were recorded endoscopically and alveolar mechanics were extracted using automated tracking of alveolar contours.
Measurements and main results: The increase in alveolar size during quasi-static maneuvers was significantly greater than during dynamic maneuvers in noninjured (mean difference 18%, p < 0.001) but not in injured lungs (mean difference 3%, p = 0.293). During dynamic maneuvers, slope of the intratidal alveolar pressure/area curve (reflecting distensibility) decreased with increasing positive end-expiratory pressure (p = 0.001) independent of lung condition (noninjured and injured lungs). In contrast, independent of positive end-expiratory pressure but dependent on lung condition, the maximal tidal change in alveolar size was greater by an average of 40% in injured compared with noninjured lungs (p = 0.028).
Conclusions: Alveolar mechanics during mechanical ventilation differed between quasi-static and dynamic conditions and varied with lung condition. Our data thus confirm that analysis of respiratory system mechanics under dynamic conditions is preferable to analysis during static conditions.