Severe air leak (more than one chest tubes per hemithorax with persistent air leak for more than 72 hours) Systolic blood pressure lower than 90 mmHg, despite maximum support with fluids and vasopressor drugs Significant heart disease Severe chronic obstructive pulmonary disease or asthma Intracranial hypertension Chronic interstitial lung disease with bilateral lung infiltrates Lung biopsy or incision during the current admission Previous lung transplantation or bone marrow transplantation Pregnancy