Critical Care Medicine
Despite the involvement of all lung fields on a frontal chest radiograph, computed tomography in patients with acute respiratory distress syndrome has demonstrated preferential distribution of alveolar consolidation in the dependent lung areas. It might be assumed that the improvement in oxygenation when the patient was turned to the prone position is a consequence of increasing perfusion to better-ventilated lung regions. However, Gattinoni and colleagues1 have demonstrated that the lung densities redistribute rapidly from the dorsal to ventral regions when a patient is turned prone, suggesting that ventilation redistribution is the main factor influencing oxygenation in the prone position, rather than perfusion redistribution. In the supine position, dorsal lung regions are below closing volume at end-inhalation. However, in the prone position, the transpulmonary pressure may exceed the airway opening pressure in the dorsal lung regions, thereby recruiting previously unventilated lung units.
- Increased ventilation to the nondependent ventral lung regions.
1. Gattinoni L, Pelosi P, Vitale G, et al. Body position changes redistribute lung computed-tomographic density in patients with acute respiratory failure. Anesthesiology 1991;74:15-23.
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3. Lamm WJ, Graham MM, Albert RK. Mechanism by which the prone position improves oxygenation in acute lung injury. Am J Respir Crit Care Med 1994;150:184-93.
4. Pappert D, Rossaint R, Slama K, et al. Influence of positioning on ventilation-perfusion relationships in severe adult respiratory distress syndrome. Chest 1994;106:1511-6.
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