RONE POSITIONING WAS ADVOcated 30 years ago 1 to improve oxygenation in patients with hypoxemic acute respiratory failure (ARF) receiving mechanical ventilation. Dramatic oxygenation improvement using prone positioning was reported in severely hypoxemic patients. 2 The mechanism of how the prone position improves oxygenation in this setting is still unclear. Postulated hypotheses in humans include alveolar recruitment, 3 redistribution of ventilation 4 toward dorsal areas that remain well Author Affiliations are listed at the end of this article.

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.

To assess incidence and magnitude of the "lower inflection point" of the chest wall, the sigmoidal equation was used in 36 consecutive patients intubated and mechanically ventilated with acute lung injury (ALI). They were 21 primary and 5 secondary ALI, 6 unilateral pneumonia, and 4 cardiogenic pulmonary edema. The lower inflection point was estimated as the point of maximal compliance increase. The low constant flow inflation method and esophageal pressure were used to partition the volume-pressure curves into their chest wall and lung components on zero end-expiratory pressure. The sigmoidal equation had an excellent fit with coefficients of determination >0.90 in all instances. The point of maximal compliance increase of the chest wall ranged from 0 to 8.3 cmH2O (median 1 cmH2O) with no difference between ALI groups. The chest wall significantly contributed to the lower inflection point of the respiratory system in eight patients only. The occurrence of a significant contribution of the chest wall to the lower inflection point of the respiratory system is lower than anticipated. The sigmoidal equation is able to determine precisely the point of the maximal compliance increase of lung and chest wall.