The objective of this study was to evaluate the effect of lateral body position on gas exchange in patients with unilateral pleural effusion, with special reference to the influence of effusion volume. Thirty consecutive patients with unilateral pleural effusion, without evidence of parenchymal pulmonary involvement, were entered into the study. Arterial blood gas tensions (PaO2, PaCO2) were randomly measured in both right and left lateral decubitus body positions, while breathing room air. To assess the influence of the effusion volume, roentgenographic and functional parameters were used. Among the latter, FVC, FEV1, TLC and RV were determined. The influence of the presence or absence of pleuritic pain on gas exchange was also assessed. There was no significant difference in PaCO2 between right and left lateral decubitus body positions (31.1 +/- 4.2 versus 31.0 +/- 4.5 mmHg). The differences in PaO2 between the two body positions ranged from 0.5-25 mmHg (mean 9.3 +/- 6.6 mmHg). Mean PaO2 with the normal-side (control) down (PaO2-N) (81.4 +/- 8.5 mmHg) was higher, but without significant statistical difference, than mean PaO2 with the effusion-side down (PaO2-E) (78.0 +/- 12.5 mmHg). PaO2-N was higher than PaO2-E in 22 of 30 patients (conventional), and lower in eight patients (paradoxical). No consistent relationship was found for alterations in PaO2 in different positions with the volume of effusion, either when estimated by a roentgenographic method or when using spirometric or plethysmographic values.

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