Effects of prone position on alveolar recruitment and oxygenation in acute lung injury.

OBJECTIVE: To investigate the effects of prone position (PP) on alveolar recruitment and oxygenation in acute respiratory failure.

DESIGN: Prospective physiologic study.

SETTING: Medical ICU two in a university hospital.

PATIENTS: Twelve adult patients intubated and mechanically ventilated with medical primary acute lung injury/adult respiratory distress syndrome (ALI/ARDS) in whom PP was indicated.

MEASUREMENTS AND RESULTS: We constructed the static inflation volume-pressure curves (V-P) of the respiratory system in the 12 patients and differentiated between lung and chest wall in ten of them. We determined the difference between end-expiratory lung volume on positive end-expiratory pressure (PEEP) and relaxation volume of the respiratory system on zero PEEP (delta FRC). The recruited alveolar volume was computed as the delta FRC times the ratio of static elastance of the respiratory system to the lung. These measurements together with arterial blood gases determination were made in supine position (SP1), after 1 h of PP and after 1 h of supine repositioning (SP2) at the same level of PEEP. The PaO2/FIO2 ratio improved from SP1 to PP (136 +/- 17 vs 204 +/- 24 mm Hg; p < 0.01). An PP-induced alveolar recruitment was found in five patients. The change in oxygenation correlated to the recruited volume. The static elastance of the chest wall decreased from 4.62 +/- 0.99 cmH2O/l in SP1 to 6.26 +/- 0.54 cmH2O/l in PP (p < 0.05) without any correlation to the change in oxygenation.

CONCLUSIONS: Alveolar recruitment may be a mechanism of oxygenation improvement in some patients with acute hypoxemic respiratory failure. No correlation was found between change in oxygenation and chest wall elastic properties.