Effect of positional change on ventilation-perfusion distribution in unilateral pleural effusion.

The effect of pleural effusion on lung function and gas exchange has not been adequately defined. We used the multiple "inert" gas technique to study gas exchange and ventilation-perfusion relationships in both the left and the right lateral decubitus positions in a patient with a moderate unilateral (left) pleural effusion. With position change, only a small change in oxygenation (3 mm Hg) and no change in carbon dioxide tension occurred. The ventilation-perfusion relationships were also nearly identical for both body positions. We conclude that a pleural effusion of moderate size has little effect on overall gas exchange and ventilation-perfusion relationships and that complex mechanisms of the respiratory system compensate for the effusion.