Effects of gravity and posture on the human lung

Abstract: The presence of the gravitational force at the surface of Earth affects all of the organ systems in landliving creatures. The function of the lung is particularly susceptible to changes in the direction and magnitude of gravity because of the elastic structure of this organ. Gravity-dependent deformation of lung tissue in turn is an important determinant of gas transfer between the gas and the blood in the lungs. For example, the impaired arterial oxygenation characteristic of patients with acute respiratory distress syndrome (ARDS) become less severe when turned from supine (face-up) to prone (face-down) posture. In the 1st part of this thesis, we explored if this influence of the direction of gravity also existed in healthy subjects in whom acute lung insufficiency was induced by hypergravity. When exposing healthy subjects to 5 times normal gravity (5 G) in the human centrifuge, the arterial oxygen saturation was 84.6 + 1.2% (mean ± SEM) in the supine and 89.7 ± 1.4% in the prone posture. Thus, there was a protective effect of prone positioning during hypergravity, due to more effective preservation of alveolar-to-arterial oxygen transport. In the 2nd part, our goal was to develop and assess a procedure for rapid and non-invasive determination of the lung diffusing capacity and tissue volume, as well as of the distributions of ventilation and perfusion, in order to further characterize this effect of posture on pulmonary function. Our novel approach was first applied to seated subjects exposed to hypergravity, since there are a relatively large number of earlier reports on this situation that could be used for comparison. We employed a combined rebreathing-single breath washout maneuver using soluble and insoluble inert gases. Lung diffusing capacity was reduced by 33% at 3 G, compared to 1 G, most likely as a consequence of a more heterogeneous distribution of alveolar volume with respect to pulmonary-capillary blood volume. The lung tissue volume was increased by 38% at 3 G, probably caused by a sequestration of blood in the dependent parts of the pulmonary circulation, just as occurs in the systemic circulation. We also found that in seated subjects, not only large-scale (apex-to-base), but even smallerscale (acinar level) heterogeneities in ventilation and perfusion are enhanced by hypergravity. In the 3rd section of this thesis, I describe application of this novel methodology in studies on recumbent humans exposed to hypergravity (5 G). Lung diffusing capacity was decreased by 46% in the supine posture during hypergravity, but only by 25% with prone posture. These data were in agreement with our previous findings of more extensively impaired arterial oxygenation in supine hypergravity. In addition, the ventilation and perfusion heterogeneities induced by hypergravity were more severe in the supine than in the prone posture. The striking similarities observed between sitting and prone postures probably reflected heart-lung and diaphragm-lung interactions that are more similar than those that occur with supine posture. We conclude that pulmonary function is more effectively preserved in the prone than in the supine posture upon exposure to hypergravity. Apparently, the differences in cardiopulmonary function associated with these two postures is of little consequence in healthy subjects at normal gravity, but becomes significant under conditions where pulmonary gas exchange is impaired, as in patients with ARDS or upon exposure to hypergravity. We speculate that mammals have developed cardiopulmonary structures and functions that are favourable to a life on four legs.