Background: During general anesthesia in patients with mediastinal mass compression effect on the heart, great intra thoracic vessels, or tracheal tree can lead to decrease venous return, cardiovascular collapse or tracheal obstruction. These complications may be worsened after induction of general anesthesia or prescribing muscle relaxants. Case report: A twenty one years old female with huge anterior mediastinal mass was a candidate for right sided thoracotomy. Compression effect on the adjacent structures has been reported in chest CT scan and in echocardiography. Awake oro-tracheal intubation was performed in sitting position. Then patient positioned to left lateral decubitus for thoracotomy During surgery asystole occured. We changed the position to remove compression effect on the heart and great vessels. Fortunately these maneuvers changed asystole to sinus rhythm. Conclusion: Decreased thoracic muscular tone and removed spontaneous ventilation after muscle relaxant may threaten patient’s life. Change of patient’s position can remove the compression effect.