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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