1, we represent the gas tissue ratio in prone and in supine position, both in normal and ARDS patients. As shown, the inflation of the pulmonary units is far more homogeneous in prone compared to supine, meaning that the forces applied to distend the lungs (the trans-pulmonary pressure, i.e., the lung stress) are more homogeneously distributed. The primary reason is improved shape matching between the chest wall and the lung. The gravitational gradient of pleural pressure, regional end-expiratory and end-inspiratory lung volumes, regional ventilation and ventilation-perfusion ratios are all more uniform in the prone compared with the supine position. The gas/tissue ratio (it may be thought as a volume of the pulmonary unit) as a function of the distance between the sternum and the vertebrae. In prone position, the gas/tissue ratio is far more homogeneous, indicating a more even distribution of forces throughout the lung parenchyma As shown, in supine position, the gas/tissue ratio sharply decreases from the sternum to the vertebrae suggesting that both in normal and in ARDS patients the distending forces is about three times higher closer to the sternum than to the vertebrae. Somewhat unexpectedly, perfusion distribution is similar in prone and supine positions. Importantly, counter to the zonal explanation for regional perfusion heterogeneity, the gravitational distribution of pulmonary blood flow is only minimally altered by turning prone resulting in the bulk of perfusion continuing to go to dorsal regions when these are turned to the non-dependent position. It follows that the observed changes in gas exchange (a direct function of the ventilation/perfusion ratio) are primary due to changes in regional ventilation. The most striking change observed on CT scan when shifting from supine to prone position is the density redistribution from dorsal to ventral. To interpret this finding, subsequent CT scan analyses culminated in the sponge model due to superimposed pressure.
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