EFFECT OF BODY POSITION ON GAS-EXCHANGE IN PATIENTS WITH UNILATERAL PLEURAL EFFUSION - INFLUENCE OF EFFUSION VOLUME

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 c onsecutive patients with unilateral pleural effusion, without evidence of parenchymal pulmonary involvement, were entered into the study. Ar terial 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, FEV(1), T LC and RV were determined. The influence of the presence or absence of pleuritic pain on gas exchange was also assessed. There was no signif icant difference in PaCO2 between right and left lateral decubitus bod y positions (31 . 1 +/- 4 . 2 vs. 31 . 0 +/- 4 . 5 mmHg). The differen ces 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 signific ant 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 2 2 of 30 patients (conventional), and lower in eight patients (paradoxi cal). 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 plethysmogr aphic values. The mean difference between PaO2 values in the two posit ions (Delta PaO2 N-E) (6 . 3 +/- 9 . 8 mmHg) in patients with chest pa in was significantly greater (P < 0 . 05) than the Delta PaO2 N-E in p atients without pain (-3 . 3 +/- 10 . 7 mmHg). Moreover, only pleuriti c chest pain showed a significant statistical influence on Delta PaO2 N-E, when all factors were analysed simultaneously. Although gas excha nge is improved in most patients in the lateral decubitus position wit h normal-side down, some patients behave paradoxically. This differenc e does not appear to be related to the effusion volume, but seems to b e influenced by the presence of pleural pain.