Prone ventilation in acute respiratory distress syndrome

Abstract

Prone positioning has been used for many years in patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), with no clear benefit for patient outcome. Meta-analyses have suggested better survival in patients with an arterial oxygen tension (PaO2 )/inspiratory oxygen fraction (FIO2 ) ratio <100 mmHg. A recent randomised controlled trial was performed in ARDS patients after a 12-24 h stabilisation period and severity criteria (PaO2 /FIO2 <150 mmHg at a positive end-expiratory pressure ≥5 cmH2O). This trial has demonstrated a significant reduction in mortality from 32.8% in the supine group to 16% in the prone group (p<0.001). The reasons for this dramatic effect are not clear but probably involves a reduction in ventilator-induced lung injury due to prone positioning, for which there is ample evidence in experimental and clinical studies. The aims of this article are to discuss: the rationale of prone positioning in patients with ALI/ARDS; the evidence of its use based on trial analysis; and the limitations of its use as well as the current place of prone positioning in the management of patients with ALI/ARDS. From the currently available data, prone positioning should be used as a first-line therapy in patients with severe ALI/ARDS.

Figure 1.

A patient with acute respiratory distress syndrome receiving mechanical ventilation in the intensive care unit while in the prone position.

Figure 2.

a) The distribution of normally aerated (white circles), poorly aerated (grey circles), non-aerated (black rectangle) and consolidated (green rectangle) lung areas during acute respiratory distress syndrome while in the supine position during end-inspiration and end-expiration. b) Barotrauma (alveolar rupture with air leaks) and volutrauma (overdistension in the normally aerated lung areas). c) Atelectrauma, i.e. shear stress in the poorly aerated lung areas close to the consolidated non-recruitable lung areas. Biotrauma (biochemical and biological response) results from volutrauma and/or atelectrauma with activation of pro-inflammatory mediators within the lungs and distant end organs. The red circle represents the heart.

Figure 3.

Homogenisation of the distribution of lung aeration as a result of moving from a) the supine to b) the prone position during acute respiratory distress syndrome. The red circle represents the heart. White circles: normally aerated lung areas; grey circles: poorly aerated lung areas; black rectangle: non-aerated lung areas; green rectangle: consolidated lung areas.