Critical Care Medicine

Answer 4 Increased ventilation to the nondependent ventral lung regions. 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. REFERENCE 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. SUGGESTED READING 1. Bonten MJ, Gaillard CA, Wouters EF, et al. Problems in diagnosing nosocomial pneumonia in mechanically ventilated patients: a review. Crit Care Med 1994;22:1683-91. 2. Chiles C, Ravin CE. Radiographic recognition of pneumothorax in the intensive care unit. Crit Care Med 1986;14:677-80. 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. 5. Tuxen DV. Detrimental effects of positive end-expiratory pressure during controlled mechanical ventilation of patients with severe airflow obstruction. Am Rev Respir Dis 1989;140:5-9. 6. Tuxen DV, Williams TJ, Scheinkestel CD, et al. Use of a measurement of pulmonary hyperinflation to control the level of mechanical ventilation in patients with acute severe asthma. Am Rev Respir Dis 1992;146(5 Pt 1):1136-42. Click here to return to the questions

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