Journal of applied physiology
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Clinical Trial
Pulmonary perfusion in supine and prone positions: an electron-beam computed tomography study.
Acute respiratory distress syndrome is characterized by alterations in the ventilation-perfusion ratio. Present techniques for studying regional pulmonary perfusion are difficult to apply in the critically ill. Electron-beam computed tomography was used to study the effects of prone positioning on regional pulmonary perfusion in six healthy subjects. ⋯ Gravity was estimated to be responsible for 22-34% of perfusion heterogeneity in the supine and 27-41% in the prone positions. These data support the hypothesis that factors other than gravity may be at least as important in determining the distribution of pulmonary perfusion in humans. The influence of nongravitational factors may not be detectable if techniques that sample large tissue volumes are employed.
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Changes in the spatial distribution of perfusion during acute lung injury and their impact on gas exchange are poorly understood. We tested whether endotoxemia caused topographical differences in perfusion and whether these differences caused meaningful changes in regional ventilation-to-perfusion ratios and gas exchange. Regional ventilation and perfusion were measured in anesthetized, mechanically ventilated pigs in the prone position before and during endotoxemia with the use of aerosolized and intravenous fluorescent microspheres. ⋯ In contrast, there were no topographical differences in perfusion before endotoxemia and no topographical differences in ventilation at any time point. Consequently, endotoxemia increased regional ventilation-to-perfusion ratios in the caudal-to-cranial and dorsal-to-ventral directions, resulting in end-capillary PO2 values that were significantly lower in dorsal-caudal than ventral-cranial regions. We conclude that there are topographical differences in the pulmonary vascular response to endotoxin that may have important consequences for gas exchange in acute lung injury.