Abstract
In six male anesthetized, tracheotomized, and mechanically ventilated rabbits, we imaged subpleural alveoli under microscopic view (60×) through a “pleural window” obtained by stripping the endothoracic fascia and leaving the parietal pleura intact. Three different imaging scale levels were identified for the analysis on increasing stepwise local distending pressure (Pld) up to 16.5 cmH2O: alveoli, alveolar cluster, and whole image field. Alveolar profiles were manually traced, clusters of alveoli of similar size were identified through a contiguity‐constrained hierarchical agglomerative clustering analysis and alveolar surface density (ASD) was estimated as the percentage of air on the whole image field. Alveolar area distributions were remarkably right‐skewed and showed an increase in median value with a large topology‐independent heterogeneity on increasing Pld. Modeling of alveolar area distributions on increasing Pld led to hypothesize that absolute alveolar compliance (change in surface area over change in Pld) increases fairly linearly with increasing initial alveolar size, the corollary of this assumption being a constant specific compliance. Clusters were reciprocally interweaved due to their highly variable complex shapes. ASD was found to increase with a small coefficient of variation (CV <25%) with increasing Pld. The CV of lung volume at each transpulmonary pressure was further decreased (about 6%). The results of the study suggest that the considerable heterogeneity of alveolar size and of the corresponding alveolar mechanical behavior are homogenously distributed, resulting in a substantially homogenous mechanical behavior of lung units and whole organ.
Highlights
Lung parenchyma is considered as a system composed by polyhedral alveoli opening on the lumen of alveolar ducts (Wilson and Bachofen 1982)
The coefficient of variation (CV) of each distribution was computed by dividing the interquartile range (75th percentile value minus 25th percentile value) by the median value and did not show any specific trend to increase with increasing distending pressure: for local distending pressure of 3, 4.5, 8.5, 10.5, 13, and 16.5 cmH2O on the inflation limb, the CV values were 88%, 79%, 87%, 92%, 79%, 80%, respectively
The method had the advantage to allow unrestrained movement of subpleural units, exposed to physiological distending pressure, by preserving the integrity of the respiratory system. This allowed to relate the morphology of the alveoli, assessed through a two-dimensional analysis, to the expected local distending pressure (Agostoni and Miserocchi 1970)
Summary
Lung parenchyma is considered as a system composed by polyhedral alveoli opening on the lumen of alveolar ducts (Wilson and Bachofen 1982). The tissue structure surrounding alveoli is a complex interconnected network of macromolecules with viscoelastic and elastic properties. Whole mechanical lung behavior reflects the mechanical properties of single alveoli whose distending pressure is given by the sum of an elastic component, developed by the parenchymal structures, and a pressure component reflecting surface forces. Several studies have considered how considerable perturbation of physiological condition at lung periphery affect the intrinsic viscoelastic properties and/or surface forces creating regional heterogeneities in the mechanical behavior. A recent article by Wilson (2013) proposed that, in physiological conditions, regional differences in volume oscillations may result from the complex interaction between the stiffness of the elastic elements of the alveoli and surface tension relaxation. We wished to provide a contribution to this hypothesis, addressing two related questions: (1)
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