CT data obtained in patients with ARDS lying supine show that a majority of them have substantial parts of their upper lobes that remain normally aerated at ZEEP. In contrast, aeration loss is always massive in caudal parts of the lungs and, because an external compression is exerted on lower lobes by the heart, the abdomen and the presence of pleural effusion, juxta-diaphragmatic lung regions are often entirely atelectatic. One of the most intriguing results from CT studies of the whole lung is that marked increases in lung tissue are not automatically associated with a massive loss of aeration. The hypothesis that the injured lung collapses under its own weight is not supported by recent experimental and human data. Very likely, the aeration loss in ARDS is mainly resulting from alveolar flooding by edema fluid and/or inflammatory infection. As a consequence, human VILI is basically characterized by alveolar overinflation of aerated lung areas. The uneven distribution of the aeration loss in a majority of patients with ARDS, creates a risk of overinflating normally aerated lung areaswhen high PEEP is implementedforrecruitingnonaer-ated lung regions. In other words, the injured lung of the majority of ARDS patients cannot be kept fully aerated at end-expiration without overinflating some parts of the lungs. These data form the rationale for selecting the right PEEP level, which should be a compromise between the benefits of lung recruitment and the risks of lung overdistension. Adequate body positioning by reversing the external compression on lower lobes appears an attractive complement to PEEP for re-establishing lung aeration in ARDS.

Acknowledgments. We would like to give special thanks to Priscyla Girardi who actively contributed to the present manuscript. Cassio Girardi was the recipient of a scholarship provided by the Ministère Français des Affaires Etrangères (ref 359530G/P330281D).

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