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Background: Inconsistent and contradictory findings have appeared in the literature concerning the impact of body position on oxygenation in pleural effusion., Methods: We attempted to elucidate whether the size of the pleural effusion in patients with no parenchymal disease is the main determinant of posture-induced alterations in oxygenation parameters. We studied 62 spontaneously breathing patients aged 65.3±7.8 years (mean±SD), of whom 36 had large and massive-sized effusions (Group A) and 26 had small and moderate-sized effusions (Group B). Arterial blood gases were determined in four different body positions: sitting (SIT), supine (SUP), ipsilateral (IPS) and contralateral (CON) to the effusion side, after remaining relaxed for at least 20 min in each position. Separation into groups A and B was deliberately set from the position of the fluid meniscus line on a posteroanterior chest film just above the upper costal margin of the sixth anterior rib. A two-way ANOVA model with outcome variables PaO 2 , PaCO 2 and [A-a] DO 2 was used., Results: In both groups the best oxygenation was found in SIT. The worst oxygenation (highest [A-a] DO 2 value) occurred in group A in CON compared with IPS (59.4±7.6 vs 49.0±7.5 mm Hg, p<0.001) and in group B in IPS compared with CON (51.0±8.7 vs 39.5±9.2 mm Hg, p<0.001). Also, PaCO 2 showed significant differences in both groups in IPS compared with CON (p=0.002)., Conclusions: Patients with large-sized effusions exhibit the worst oxygenation when lying on the side contralateral to the effusion, while those with small-sized effusions exhibit the worst oxygenation when lying on the side ipsilateral to the effusion., Competing Interests: Competing interests: None declared., (© Article author(s) (or their employer(s) unless otherwise stated in the text of the article) 2018. All rights reserved. No commercial use is permitted unless otherwise expressly granted.)

Background: There have been contradicting reports in the literature regarding the impact of pleural fluid aspiration on patients' oxygenation. The aim of this study was to assess the role of the initial size of effusion on post-drainage oxygenation., Methods: We studied 122 patients, aged (mean±SD) 61.2±16.8 years, with unilateral pleural effusion and no remarkable parenchymal lesion, by determining PaO 2 , PaCO 2 and [A-a] PaO 2 just before thoracocentesis (T1), 30 min after its completion (T2) and 48 hours after the procedure (T3). Patients were divided into group A (75 patients) with small and moderate sized effusions and group B (47 patients) with large and massive effusions. The position of the meniscus line on the posteroanterior film, being arbitrarily set at just above the upper costal margin of the sixth anterior rib, was used to divide the two groups. Patients were studied at rest, breathing room air in the sitting position. Repeated measures ANOVA (related samples) and the Friedman test when the normality assumption was violated were used., Results: In group A, at T3, PaO 2 (mm Hg) showed a statistically significant increase versus T1 (p<0.001) and T2 (p=0.002), while [A-a] PaO 2 displayed a statistically significant decrease compared with T1 (p<0.001) and T2 (p=0.001). In group B, at T2, PaO 2 presented significant decrease versus T1 (p<0.001) and T3 (p<0.001), while [A-a] PO 2 was found to be significantly increased compared with both T1 and T3 (p<0.001)., Conclusion: Patients with smaller effusions showed a small improvement in their oxygenation 48 hours post-thoracocentesis (T3). Patients with larger effusions exhibited a transient reduction in their oxygenation immediately after fluid removal (T2)., Competing Interests: Competing interests: None declared., (© Article author(s) (or their employer(s) unless otherwise stated in the text of the article) 2017. All rights reserved. No commercial use is permitted unless otherwise expressly granted.)