Prone Position in Acute Respiratory Distress Syndrome. Rationale, Indications, and Limits

Abstract

In the prone position, computed tomography scan densities redistribute from dorsal to ventral as the dorsal region tends to reexpand while the ventral zone tends to collapse. Although gravitational influence is similar in both positions, dorsal recruitment usually prevails over ventral derecruitment, because of the need for the lung and its confining chest wall to conform to the same volume. The final result of proning is that the overall lung inflation is more homogeneous from dorsal to ventral than in the supine position, with more homogeneously distributed stress and strain. As the distribution of perfusion remains nearly constant in both postures, proning usually improves oxygenation. Animal experiments clearly show that prone positioning delays or prevents ventilation-induced lung injury, likely due in large part to more homogeneously distributed stress and strain. Over the last 15 years, five major trials have been conducted to compare the prone and supine positions in acute respiratory distress syndrome, regarding survival advantage. The sequence of trials enrolled patients who were progressively more hypoxemic; exposure to the prone position was extended from 8 to 17 hours/day, and lung-protective ventilation was more rigorously applied. Single-patient and meta-analyses drawing from the four major trials showed significant survival benefit in patients with PaO2/FiO2 lower than 100. The latest PROSEVA (Proning Severe ARDS Patients) trial confirmed these benefits in a formal randomized study. The bulk of data indicates that in severe acute respiratory distress syndrome, carefully performed prone positioning offers an absolute survival advantage of 10-17%, making this intervention highly recommended in this specific population subset.

Recruitment Maneuvers and PEEP Titration

“…[43][44][45] Prone position may reduce lung stress and strain in severe ARDS. Thus, prone positioning might be considered a recruitment maneuver.…”

“…46 One randomized controlled trial 47 and several meta-analyses report a survival benefit for prone position, 48,49 particularly in severe ARDS. 43 …”

Should Early Prone Positioning Be a Standard of Care in ARDS With Refractory Hypoxemia?

“…12,29 For the same tidal volume and PEEP combination, ventilating large animals in the prone position reduces the severity of ventilator-inflicted lung damage. 24,30,31 Using the very high but irrefutable outcome threshold of improved overall mortality, most large clinical trials conducted over the last 20 y were unable to confirm a survival benefit in diverse populations of subjects labeled as having acute lung injury/ARDS.…”

“…The truncated cone-shaped lung must fit into the more cylindrically contoured chest cavity, and this shaping mismatch is partially offset by the reconfiguration of the latter when prone. 12,35 Physiologic benefit follows as a direct consequence of assuming what for all other mammals is the preferred orientation (Table 1). There is little question that prone positioning can be expected to redistribute translung forces, reduce the supine gradient of trans-lung pressure, 11,36 recruit and stabilize dorsal lung units, relieve cardiac compression of lung tissue, 28 and favor mouthward migration of retained airway secretions.…”

“…[7][8][9] Prone positioning for ARDS has been extensively studied in the laboratory and at bedside, and in both settings, it improves oxygenation and lung recruitment during acute lung injury. [10][11][12][13] Although multiple large randomized controlled trials conducted in diverse populations failed to demonstrate a consistent mortality benefit to prone positioning in ARDS, [14][15][16][17][18] one published trial reported a very substantial improvement in mortality (Fig. 1).…”