Background: Lung sonography (LS) is a valuable tool in the management of acute respiratory failure (ARF) in critical care units. LS can diagnosis several lung abnormalities, modify the initial diagnosis and determinate therapeutic changes. Besides, LS has demonstrated to be superior to chest radiography in several points with A and B grades of scientific evidence, but thoracic Tomography Scan (TC) is still considered to be the gold standard in pulmonary diagnosis. 
Case Presentation: In this report, we presented a case of postoperative ARF with admission to ICU criteria. Large left pleural effusion was evident in chest x ray studies. Non-invasive mechanical ventilation (NIMV) was applied with response, but the patient kept needing this therapy to maintain good parameters of oxygenation and ventilation. A chest tube drainage was punctured with the aim of improving the NIMV weaning. In the first two hours, 700ml of amber liquid with transudate characteristics were obtained. In an x ray control study, an image of whole white left lung similar to the previous one was obtained. The chest tube was well placed but it did not drain anymore. A thoracic CT scan was solicited; a septated or loculated pleural effusion was ruled out. 
Nevertheless, fibrotic tracts were evident in pleural effusion during a systematic lung sonography exploration. According to these results, we proceeded to apply intra-pleural fibrinolytic therapy. More than 2000ml were drained in the first four hours, with improvement in radiological studies and allowing NIVM removal. 
Discussion: As reflected in the International evidence-based recommendations for point-of-care lung ultrasound, chest sonography could be a more sensitive test than TC; for example regarding to pleural effusion characteristics. We think this point is a novel approach in clinical management of ARF because we would not have used intra-pleural urokinase only with CT scan results.

Intra-pleural fibrinolytic therapy in a patient with fibrotic tracts in pleural effusion observed in lung sonography but not in tomography scan