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1 Section of Environmental Physiology, Department of Physiology and Pharmacology, Karolinska Institutet, Stockholm, Sweden
2 Department of Anesthesiology and Intensive Care, Karolinska Hospital, Stockholm, Sweden
3 Departments of Medicine and of Physiology and Biophysics, University of Washington, Seattle, USA; Department of Anesthesiology and Intensive Care, Karolinska Hospital, Stockholm, Sweden
* To whom correspondence should be addressed. E-mail: malin.rohdin{at}fyfa.ki.se.
When normal subjects are exposed to hypergravity (5 G) there is an impaired arterial oxygenation that is less severe in the prone compared to supine posture. We hypothesized that under these conditions the heterogeneities of ventilation and/or perfusion distributions would be less prominent when prone compared to supine. Expirograms from a combined rebreathing-single breath washout maneuver (Rohdin et al. J Appl Physiol 96: 1470-1477, 2004) were analyzed for vital capacity (VC), phase III slope, and phase IV amplitude, in order to analyze heterogeneities in ventilation (Ar) and perfusion (CO2/Ar) distribution, respectively. During hypergravity VC decreased more when supine than when prone (ANOVA, P=0.02). There was a more positive phase III slope for Ar (P=0.003) and a more negative for CO2/Ar (P=0.007) in the supine compared to prone posture at 5 G in agreement with the notion of more severe hypergravity-induced heterogeneities of ventilation and perfusion in supine posture. Phase IV amplitude became lower in the supine than in the prone posture for both Ar (P=0.02) and CO2/Ar (P=0.004) during hypergravity as a result of the more reduced VC in the supine posture. We speculate that results of VC and phase IV amplitude are due to the differences in heart-lung interaction and diaphragm position between postures; a stable position of the heart and diaphragm in prone hypergravity in contrast to supine where the weight of the heart and a cephalad shift of the diaphragm compress lung tissue.
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