Amount Drained at Ultrasound-Guided Thoracentesis and Risk of Pneumothorax

Background: It has been suggested that no upper limit of the fluid amount drained is necessary when performing ultrasound-guided thoracentesis, but the risk of pneumothorax when large amounts of fluid are drained has not been studied in detail. Purpose: To study the amount of drained fluid at ultrasound-guided thoracentesis and the subsequent risk of pneumothorax. Material and Methods: Prospectively collected information on all ultrasound-guided thoracenteses performed at a county hospital between 2004 and 2006 was evaluated. In total, 735 thoracenteses in 471 patients were included. Chest radiographs performed within 14 days after thoracentesis were identified to obtain cases of pneumothorax and cases treated with tube thoracostomy. Data were analyzed by logistic regression. The study was approved by the regional research ethics committee. Results: There was a steep increase in risk for pneumothorax when large amounts of fluid were drained. Compared to a thoracentesis of 0.8-1.2 l, drainage of 1.8-2.2 l was associated with a more than threefold increase in risk for pneumothorax (odds ratio [OR] 3.8, 95% confidence interval [CI] 1.28-11.2), and after drainage of 2.3 l or more, the increase in risk was almost sixfold (OR 5.7, 95% CI 1.30-24.7). The association between the amount drained and the risk of pneumothorax was even more pronounced for pneumothoraces requiring tube thoracostomy (P for trend 0.0001). Nine of 11 tube thoracostomies occurred after thoracenteses of 1.8 l or more. Conclusion: Our study suggests that drainage of large amounts of fluid at ultrasound-guided thoracentesis is a risk factor for pneumothorax.