Mechanisms Of Differences In Ventilation Distribution In The Upright, Supine, And Prone Postures

Abstract

Rationale: Measurements of ventilation distribution using various imaging modalities have suggested that the distribution of ventilation in the supine and prone postures is less evidently gravitational than when the lung is upright, and with some studies showing little difference between ventilation distributions in the prone and supine lung. This is despite the concurrent observations of a significant gradient in tissue density when supine, and a typically smaller - or absent - tissue density gradient prone. In this study we use a computational model of lung tissue elasticity coupled to air-flow to study the relationship between posture, density distribution, and ventilation. Methods: An imaging-based geometric model of the lung and airway tree that was developed in a separate study was used here. Flow in the airways was simulated using a one-dimensional fluid dynamics model that includes flow-dependent airway resistance and coupling to tissue elasticity at the airway walls and at the acini. A finite deformation elasticity model was used to predict the effect of gravity on tissue deformation, and the non-linear elasticity of each acinar tissue unit during simulated breathing. The upright lung volume was defined from pulmonary function testing; the supine and prone volumes were assumed to be the same, and equal to the supine air volume as calculated from the subject's computed tomography imaging acquired at FRC. Tissue density and ventilation at each of the ~32,000 distributed acini in the model were averaged within iso-gravitational slices of 10 mm thickness. Results: The gradient of tissue density predicted by the model was markedly larger in supine than in upright or prone. Ratios of the maximum to minimum slice density were 1.95, 1.51, and 1.39 for supine, prone, and upright, respectively. Ventilation in the upright model increased on average towards the dependent tissue, whereas ventilation in supine and prone was decreased in the most dependent and non-dependent regions. The ratios of maximum to minimum slice ventilation were 1.09, 1.03, and 1.31 for supine, prone, and upright, respectively. Conclusions: A lack of gravitational distribution of ventilation in the supine and prone postures compared with upright is predicted on the basis of the smaller size of the horizontal lung and a shift of the dependent tissue to a less-compliant region of the sigmoidal pressure-volume curve at its lower asymptote. This is the same mechanism that results in early filling of the non-dependent tissue when inhaling from residual volume.