Mechanism by which the prone position improves oxygenation in acute lung injury.

Abstract

The mechanism by which oxygenation improves when patients with ARDS are turned from supine to prone position is not known. From results of our previous studies we reasoned that (1) when supine, in the setting of lung injury, transpulmonary pressure will be less than airway opening pressure and (2) atelectasis will develop preferentially in dorsal lung areas, and (3) both ventilation and ventilation/perfusion ratios would improve in these regions on turning prone. To study this directly, we measured regional ventilation and perfusion using 81mKr and 99mTc-MAA, respectively, and single photon emission computed tomography, both prone and supine, in four control animals and four given oleic acid. After oleic acid, the prone position improved (1) oxygenation (mean +/- SD PaO2 = 140 +/- 112 versus 453 +/- 54 mm Hg), (2) median ventilation/perfusion ratios (0.77 versus 0.95), (3) ventilation/perfusion heterogeneity (coefficient of variation 86 +/- 15 versus 61 +/- 6), and (4) the gravitational ventilation/perfusion gradient (dependent to non-dependent slopes of 0.22 versus -0.02, all p < 0.05). The prone position generates a transpulmonary pressure sufficient to exceed airway opening pressure in dorsal lung regions, i.e., in regions where atelectasis, shunt, and ventilation/perfusion heterogeneity are most severe, without adversely affecting ventral lung regions.

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

“…It is notable that humans, when recumbent, most frequently assume a lateral position to initiate sleep and orient themselves prone almost as often as supine. 23 Experimental data 24,25 and clinical observations [26][27][28] demonstrate uncontestable physiologic benefit from prone positioning during acute lung injury. It is clear that prone positioning redistributes stretching (transpulmonary) forces within the thoracic cavity, helping to recruit lung tissue, Fig.…”

“…Inclusion of only subjects with severe ARDS, use of appropriate lung-protective ventilation, liberal use of NMBAs, substantial institutional experience proning patients, and exclusion of patients likely to experience complications from prone positioning are important factors to 15,20,25 Aside from increased duration of prone positioning from 8 to 16 h, the major differences in the studies from 2004 to 2013 were exactly the factors discussed above. These factors included a baseline inclusion P aO 2 Marini: It's for someone who's refractory and you're entering dangerous pressure and F IO 2 .…”

Effects of prone and supine position on oxygenation and inflammatory mediator in a hydrochloric acid-induced lung dysfunction in rats

“…Animal studies have demonstrated improved oxygenation and ventilation perfusion ratios, along with restoring aeration, decreasing shunt, and preserving perfusion 1,9,15 . In addition, prone positioning has been shown to eliminate compression of the lungs by the heart 16 .…”

Efeito da posição prona sem PEEP na oxigenação e complacência em modelo experimental com lesão pulmonar

“…Lamm et al 19 demonstrated that the prone position was associated with a stricter distribution of the ventilation /perfusion ratio (more homogeneous) and with a relative increase in the ventilation/perfusion ration in the dorsal region of the lungs.…”

“…17 Many clinical and experimental studies have since been carried out, but the mechanism responsible for the improvement remains highly controversial. 18,19 The relationship between PEEP, prone position and improved oxygenation is also controversial.…”