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Prone Position in ARDS: Are We Looking at a Half-Empty or Half-Full Glass?

Alain F. Broccard
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Affiliations: St. Paul, MN
 ,  Dr. Broccard is Associate Professor of Medicine, University of Minnesota, and Medical Director of the Medical Intensive Care Unit, Regions Hospital.

Correspondence to: Alain F. Broccard, MD, FCCP, University of Minnesota, Medical Director of the Medical Intensive Care Unit, Pulmonary and Critical Care Division, Regions Hospital, 640 Jackson St, St Paul, MN 55101-2595; e-mail: brocc001@umn.edu



Chest. 2003;123(5):1334-1336. doi:10.1378/chest.123.5.1334
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In 1976, Piehl and Brown1 reported for the first time that ventilation in the prone position improves “oxygenation” in patients with acute hypoxemic respiratory failure. Since then, numerous clinical series have been published regarding the use of prone positioning in patients with ARDS, but its impact on outcome remains uncertain.

Overall, prone positioning helps to improve gas exchange in approximately two thirds of the patients with ARDS.2 The mechanisms that account for the rise in the arterial blood oxygen content have essentially been investigated in animal models of lung injury.3 In these models,4 as well as in many patients with ARDS,2 the poorly and/or nonaerated lung units appear to be mainly localized in the dependent regions both in the supine and prone position. The time constant of the dependent collapsed/flooded lung units is such that tidal ventilation distributes preferentially to the “open” nondependent lung units.4 Since the distribution of perfusion is largely gravity-independent, at least under West zonal 3 conditions, the largest proportion of the perfusion goes through the dorsal lung regions, with patients in both the supine and prone positions.5 As a result, perfusion is largest in the dependent regions with the patient in the supine position and is largest in the nondependent region when the patient is in the prone position, and this remains true in the setting of lung injury.6 Regardless of position, positive-pressure ventilation (ie, the creation of West zonal conditions 2 or 1) alters the vertical distribution of perfusion, and blood flow tends to be redistributed from the nondependent region to the dependent regions. Positive airway pressure thus tends to reduce the vertical perfusion gradient with the patient in the prone position and tends to amplify the gradient with the patient in the supine position.7 It follows that with the patient in the supine position the vertical ventilation and perfusion gradients of mechanically ventilated injured lungs vary in opposite direction, promoting ventilation-perfusion mismatch and shunting. In contrast, a larger proportion of perfusion distributes to the well-ventilated nondependent regions (dorsal) and, everything else being equal, a smaller amount of desaturated blood perfuses the poorly and/or nonaerated lung regions with the patient in the prone position. This helps to explain the fact that the ventilation-perfusion relationship is more favorable with patients in the prone position than in the supine position.3,8

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