Figure 1.

Column I shows an isolated lung (cone) and alveolar units (circles) removed from the chest wall. This illustrates how the unhindered lung contains more alveolar units in the dorsal regions than in the ventral regions and how a gravitational pleural pressure gradient leads to compression of dependent segments. When the patient is in a prone position, this results in a smaller fraction of compressed alveolar units than when the patient is supine. Column II illustrates the effects of compressing the native conical shape of the lungs into the rigid chest wall. While the patient is supine, the compressive effects of gravity are magnified by the chest wall, further compressing the dorsal segments while expanding the ventral segments. Conversely, when the patient is prone, the chest wall effects oppose gravimetric effects, leading to more homogeneous aeration. Column III displays experimental data supporting this model. The curves describe how pulmonary aeration (gas to tissue ratio on CT) varies as one moves along the lung's vertical axis in human patients with ARDS. Note the marked asymmetry in aeration (and thus ventilation) along the ventral/dorsal axis when supine and a much more uniform gas to tissue ratio when prone. The white arrows signify recruitment of dependent regions, and the black arrows signify reduced regional hyperinflation in well-aerated lung.

(Adapted with permission from Gattinoni et al.²⁵)