Atelectasis is a common problem in the critical care setting and when it causes acute life-threatening hypoxemia it is necessary its re-expansion. Ultrasonography is capable to diagnose atelectasis at the bedside and accurately assesses lung aeration changes after reexpansion maneuver. We describe a case in which a male patient receiving mechanical ventilation developed acute hypoxemia due atelectasis, being applied recruitment maneuver guided by ultrasonography combined with lateral decubitus positioning to reexpansion of collapsed region. CASe RePoRTCheck for updates disease and receiving mechanical ventilation. Bedside lung ultrasonography was very useful to obtain the diagnosis, to evaluate the patient response to therapy and to guide fine-tune the ventilator. CaseA 56-year-old male patient with history of epilepsy was admitted to the ICU due to severe pneumonia resulting from infection with the SARS-CoV-2 coronavirus. He evolved with acute hypoxemic respiratory failure and was intubated 2 days after his admission.After 12 days of mechanical ventilation, he had a sudden drop in peripheral arterial oxygen saturation. The patient was sedated, well adapted to the ventilator and hemodynamically stable. Blood gas analysis revealed PaO 2 /FiO 2 ratio of 140. A chest X-ray was performed which showed opacification of the left hemithorax with erasure of the contours of the cardiac area and the left phrenic dome.Thoracic ultrasound was performed at the bedside and visualized volumetric reduction of the pulmonary segment above left diaphragm and the presence of tissue pattern with hyperechoic punctiform images and no change in its characteristics during respiratory incursions, besides abolished lung sliding and cardiac vibrations visible at the pleural line, corresponding to a left lower lobe atelectasis. Arterial vessels within consolidations were visualized by means Color Doppler ultrasound indicanting intrapulmonary shunt.