Alveolar mechanics studied by in vivo microscopy imaging through intact pleural space

•From in vivo microscopy absolute alveolar compliance (Cabs) was up to tenfold lower in smaller compared to larger alveoli.•Specific alveolar compliance (Csp) accounting for initial alveolar surface was independent on alveolar size.•The constancy of Csp might contribute to reduce differences in acting forces in a pre-stressed lung by assuring uniform stretching for mechanically inter-dependent alveoli.

In six male anesthetized, tracheotomized, and mechanically ventilated rabbits we derived indications on alveolar mechanics from in vivo imaging, using a “pleural window” technique (pleural space intact) that allows unrestrained movement of the same subpleural alveoli (N = 60) on increasing alveolar pressure from 4 to 8 cmH2O.Absolute compliance (Cabs, ratio of change in alveolar surface area to the change in alveolar pressure) was significantly lower in smaller compared to larger alveoli. Specific compliance, Csp, obtained by normalizing Cabs to alveolar surface area, was essentially independent of alveolar size. Both Cabs and Csp were affected by large variability likely reflecting the complex matching between elastic and surface forces.We hypothesize that the relative constancy of Csp might contribute to reduce interregional differences in parenchymal and surface forces in the lung tissue by contributing to assure a uniform stretching in a model of mechanically inter-dependent alveoli.