Lung function in micro- and in hypergravity

Abstract: The lung is extremely susceptible to gravity, even during short-term changes of the gravity vector, because it is a mixture of air and elastic tissue components. Yet, there is limited knowledge of the effects of gravity on the respiratory system. This is an important issue, for two reasons: 1) for understanding the physiology of space flight; and 2) because a better knowledge of these effects could help to understand the pulmonary changes that occur in a patient confined to bed, which is a partial simulation of a weightlessness (microgravity, 0 G) state. The present thesis aims to understand the influence of moderately increased gravity (1.7-2 G), and of 0 G or simulated 0 G, on some pulmonary features, on a large (the whole lung) or small scale. In a first study, six subjects were studied before, during, and after 120 days of bed rest. Forced expiratory flows and volumes, and diffusing capacity for carbon monoxide (DLCO) were recorded. Peak flow did not change, whereas mid-maximal expiratory flow decreased in the supine posture by ~22%. DLCO also decreased during bed rest, by 14%. These decreases had not recovered two weeks after bed rest. Such results speak against major muscle deconditioning caused by bed rest but are in favour of a decrease in lung elastic recoil and an alteration in gas exchange. These alterations are unlikely to influence daily life after bed rest but may limit maximal work capacity. In two further experimental studies, an anti-G suit was used to manipulate stroke volume (SV) and central blood volume (CBV). Nine and twelve subjects were studied at 1 G and at 2 G in a human centrifuge. It was found that SV and CBV, which change with gravity, play major roles in cardiopulmonary interactions: they influence mechanically the emptying of lung alveoli. Moreover, when SV and CBV increase, the indices of perfusion heterogeneity decrease. In a last study, twelve and six subjects were studied in different combinations of 1.7 G and 0 G during two series of parabolic flights. Indices of small and large-scale pulmonary perfusion heterogeneity were measured. All decreased but still existed at 0 G, compared to 1 G and to 1.7 G. This confirms previous reports of a large degree of gravity-independent heterogeneity of pulmonary perfusion. In conclusion, gravity in the head-to-foot direction is useful for maintaining a normal lung capacity. In contrast, the heterogeneity of pulmonary perfusion distribution is only partially influenced by gravity. Cardiopulmonary interactions on a small scale are not mainly influenced by gravity but rather by changes in intra-thoracic blood content during the cardiac cycle.