Ventilation distribution in the lung periphery measured by inert gas washout : influence of increased gravity, anti-G suit pressure, body posture, and breathing pattern
Abstract: The lung is highly sensitive to forces of gravity and acceleration, and to surrounding pressures. This thesis aimed to assess the effects on ventilation distribution in peripheral lung units when changing the direction or magnitude of the gravitational vector and when compressing the lower body half with an anti-G suit (AGS). The studies were performed on healthy adult male volunteers. Vital capacity single-breath washout (VC SBW) and tidal multiple-breath washout (MBW) tests were performed using two inert gases with widely differing diffusivities (SF6 and He). Lung volumes (VC and FRC; functional residual capacity) were measured and gas trapping was estimated from three VC breaths taken after the MBW. New analytical methods were used allowing separation of ventilation inhomogeneity into aspects resulting from differences in specific ventilation between parallel units joined at branch points proximal to the gas exchange zone (convective dependent inhomogeneity; cdi), and those caused by interaction between diffusion and convection due to geometrical asymmetries at the entrance to or within the acinus (diffusion-convection interaction dependent inhomogeneity; dedi).Study I and II assessed the changes occurring when assuming the supine posture from standing and the effects of changing tidal volume (VT). VC SBW showed a marked increase of the SF6 and He phase III slopes, which are indices reflecting overall ventilation inhomogeneity, by approximately 50% when supine, whilst the (SF6-He) phase III slope difference, a marker of intraacinar inhomogeneity, did not change with posture. A 10s breathhold (BH) reduced both SF6 and He slopes significantly by one-third in both postures. Marginal, but statistically significant, reductions in the (SF6-He) phase III slope differences were seen after supine BH. When tidal breathing, supine position was followed by a 24% reduction in FRC. Analysis of the progression of concentration normalized phase III slopes during MBW (SnIII analysis) in supine disclosed greater cdi, while dedi or the (SF6-He) slope difference did not change. Larger VT reduced dcdi in both postures, while cdi increased when supine.Study I and II show that transition to the supine posture leads to greater inhomogeneity between regions joined at branch points proximal to the gas exchange zone (cdi), some of it arising close to the zone (intraregional cdi). Ventilation distribution within the gas exchange zone is not changed, however. In the supine posture larger VT produce increased interregional inhomogeneity.Study III and IV assessed how increased gravity and pressurization of an AGS affect ventilation distribution in peripheral lung units, lung volumes, and gas trapping. Increased gravity in the head-to-foot (+Gz) direction up to +3Gz reduced VC modestly (5%) and increased the VC SBW phase III slopes by 42-63%, while the (SF6-He) slope difference did not change. Inflating an AGS to 12 kPa did not further increase the SF6 and He phase III slopes, but increased the (SF6-He) slope difference. FRC increased significantly by 14% in +3Gz. MBW disclosed moderate increases in gas trapping, cdi, dcdi and (SF6-He) slope difference in hypergravity. Pressurizing the AGS up to 12 kPa provoked large FRC reductions and marked gas trapping, particularly in +3Gz. While cdi decreased after inflating the AGS in + 3Gz, dcdi increased.Study III and IV show that hypergravity produces increased inhomogeneity of ventilation distribution between parallel regions joined at branch points located well proximal to the gas exchange zone, both at large and small breaths, and also within small lung units during tidal breathing. Inflating an AGS produces gas trapping which becomes extensive in hypergravity, and as a result interregional differences appear to decrease, while inhomogeneity increases in the gas exchange zone of ventilated units, possibly due to mechanical deformation.The studies confirm and extend previous knowledge about the effects of gravity on the lung and demonstrate that the AGS produces marked gas trapping, even in modest hypergravity. The methods used appear to be more sensitive than previous methods for detecting changes in ventilation distribution in the lung periphery.
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