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1 Department of Medicine, University of Washington, Seattle, WA, USA
2 Department of Medicine, University of Washington, Seattle, WA, USA; Department of Physiology and Biophysics, University of Washington, Seattle, WA, USA
* To whom correspondence should be addressed. E-mail: billa{at}u.washington.edu.
Although recent high-resolution studies demonstrate the importance of non-gravitational determinants for both pulmonary blood flow and ventilation distributions, posture has a clear impact on whole lung gas exchange. Deterioration in arterial oxygenation with repositioning from prone to supine posture is caused by increased heterogeneity in the distribution of ventilation-perfusion ratios. This can result from increased heterogeneity in regional blood flow distribution, increased heterogeneity in regional ventilation distribution, decreased correlation between regional blood flow and ventilation, or some combination of the above (33). We hypothesize that, although repositioning from prone to supine has relatively small effects on overall blood flow and ventilation distributions, regional changes are poorly correlated resulting in regional ventilation:perfusion mismatch and reduction in alveolar oxygen tension. We report ventilation and perfusion distributions in seven anesthetized, mechanically ventilated pigs measured with aerosolized and injected microspheres. Total contributions of pulmonary structure and posture on ventilation and perfusion heterogeneities were quantified using analysis of variance. Regional gradients of posture-mediated change in ventilation, perfusion, and calculated alveolar oxygen tension were examined in the caudo-cranial and ventro-dorsal directions. We found that pulmonary structure was responsible for 74.0±4.7% of total ventilation heterogeneity and 63.3±4.2% of total blood flow heterogeneity. Posture-mediated redistribution was primarily oriented along the caudo-cranial axis for ventilation and along the ventro-dorsal axis for blood flow. These mismatched changes reduced alveolar oxygen tension primarily in the dorso-caudal lung region.
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