We recently encountered a patient with a large anterior mediastinal tumor who developed severe hypoxemia during general anesthesia. This life-threatening hypoxemia was treated by extracorporeal membrane oxygenation using emergent percutaneous cardiopulmonary support followed by extirpation of the tumor. We found that total arteriovenous shunt resulting from compression by the mediastinal tumor caused this severe hypoxemia (total obstruction of left main bronchus and of the right pulmonary artery).