Initial size of unilateral pleural effusion determines impact of thoracocentesis on oxygenation

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.

We studied 122 patients, aged (mean±SD) 61.2±16.8 years, with unilateral pleural effusion and no remarkable parenchymal lesion, by determining PaO₂, PaCO₂ and [A-a] PaO₂ 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.

In group A, at T3, PaO₂(mm Hg) showed a statistically significant increase versus T1 (p<0.001) and T2 (p=0.002), while [A-a] PaO₂ displayed a statistically significant decrease compared with T1 (p<0.001) and T2 (p=0.001). In group B, at T2, PaO₂ presented significant decrease versus T1 (p<0.001) and T3 (p<0.001), while [A-a] PO₂ was found to be significantly increased compared with both T1 and T3 (p<0.001).

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).