In most patients, the balance favors clearing of dorsal atelectasis, thus increasing the net amount of well-aerated tissue. Therefore, what we understood regarding “typical” ARDS might be summarized as follows: prone positioning in ARDS, a condition characterized by extensive inflammatory edema, leads to decreased frontal chest wall compliance, to partial clearing of dorsal atelectasis, and to the development of new ventral atelectasis. Significant survival benefits were demonstrated when prone position was applied for 12–16 consecutive hours in the more severe forms of respiratory failure (Pa o 2/F io 2 < 150 mm Hg) ( 5), whereas its application does not provide convincing survival advantage in patients with milder disease ( 6). Over the ensuing years, diverse studies of prone positioning led on one hand to better understanding of the mechanisms that improve oxygenation on the other hand, observational and clinical trials that progressively refined the indications for its use in ARDS. The improvement of oxygenation, consistently found both clinically and experimentally, prompted the parallel implementation of clinical trials.
This discovery led to the formulation of the “sponge” model of the lung in which the rapid resolution and formation of atelectasis in different regions of the heavily edematous ARDS lung are primarily due to changes of superimposed hydrostatic pressure which follow the change in gravitational axis ( 4). On the contrary, it became apparent that the main effect following prone position was the redistribution of lung densities from the dorsal to the ventral lung regions. However, further studies of CT scans taken with patients in the prone position disproved the mechanistic hypothesis that change in perfusion determined its effects on oxygenation ( 3). The assumption was that the normally ventilated lung located in the “ventral” regions (i.e., the anatomical concept of “baby lung”) would be better perfused if placed in a gravitationally dependent position, improving the overall ventilation/perfusion (V/Q) ratio, and hence gas exchange. This finding provided the anatomical hypothesis for the improvement in gas exchange and led to an increase in the clinical use of prone position ( 2). This observation remained essentially a curiosity until the first CT scan of acute respiratory distress syndrome (ARDS) patients showed that parenchymal densities were disproportionately distributed in the dorsal lung regions. Prone positioning was first applied in critically ill patients by Piehl and Brown ( 1) in 1976 who reported a marked oxygenation improvement in five patients with acute respiratory failure.
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