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 pulmo nary perfusion are difficult to apply in the critically ill. Electron-beam computed tomography was used to study the effects of prone positioning on r egional pulmonary perfusion in six healthy subjects. Contrast-enhanced sect ions were obtained sequentially in the supine, prone, and (original) supine positions at full inspiration. Regions of interest were placed along the n ondependent to dependent axis and relative perfusion calculated. When corre cted for the redistribution of lung parenchyma, a gravitational gradient of pulmonary perfusion existed in both supine and prone positions. The distri bution of perfusion between the supine or prone positions did not differ, b ut data analysis using smaller regions of interest demonstrated marked hete rogeneity of perfusion between anatomically adjacent regions of lung. The d istribution of lung parenchyma was more uniform in the prone position. Grav ity was estimated to be responsible for 22-34% of perfusion heterogeneity i n the supine and 27-41% in the prone positions. These data support the hypo thesis that factors other than gravity may be at least as important in dete rmining the distribution of pulmonary perfusion in humans. The influence of nongravitational factors may not be detectable if techniques that sample l arge tissue volumes are employed.