Dynamic alveolar mechanics as studied by videomicroscopy.

The in vivo morphology of rat alveoli has been recorded and studied on videotape. The dynamic internal structure has been compared to that observed by histologic section from lung rapidly frozen with liquid nitrogen. The irreversible nature of local alveolar wall movement has been demonstrated in animals ventilated both with and without positive end-expiratory pressure (10 cm water) by parametrically comparing the distances between two sets of paired points measured sequentially across internal alveolar surfaces at intervals of 0.017 sec. Geometric hysteresis was significantly less in the animals ventilated with positive end-expiratory pressure. This hysteresis can be explained by irreversibility in alveolar surface area apart from any irreversible dependence on surface tension. Sharp reverses and rapid changes in geometric hysteresis suggest that the length-tension properties of elastic tissue within the alveolar wall and interface forces from adjacent alveoli are important determinants of local alveolar wall motion and pulmonary hysteresis.