Dynamic behavior of alveolar surface-to-volume ratio in live dogs by light-scattering stereology

To investigate the behavior of the surface-to-volume ratio (S/V) in lung lobes of live dogs, we measured the backscattered light pattern produced by applying a laser beam to the pleural surface. This pattern was used to estimate the optical mean free path, which is approximately proportional to the inverse square of the geometric S/V. Measurements were performed in mechanically ventilated animals with the chest widely opened. We found that during quasi-static large volume excursion, S/V changed approximately as the one-third power of lung volume but with hysteresis such that S/V during deflation was higher than during inflation. This finding is consistent with a relatively large stress hysteresis of surface tension compared with that of the tissue. During tidal breathing, however, the geometric hysteresis was inconsistent among animals at low frequency but was consistently changed at higher frequencies in a direction suggesting a lesser role of surface tension relative to tissue stress hysteresis.