We report a case of cardiac arrest in a 71 year old male during the Valsalva maneuver that had been performed immediately after completing a pneumonectomy. The patient had a subclinical atrial septal defect (1.08 cm sized) and he previously undergone a left upper lobectomy of the lung without complications 11 months earlier. He underwent a left completion pneumonectomy due to recurrent lung cancer. After surgery a Valsalva maneuver was performed with 35 cmH₂O to relocate the mediastinum. During this procedure, the patient experienced a cardiac arrest and was resuscitated with difficulty. It is believed that the cardiac arrest was due to pulmonary hypertension, right ventricular failure and right to left shunt caused by several factors, such as the pneumonectomy, subclinical atrial septal defect (1.08 cm sized), and Valsalva maneuver. In addition, the hypovolemia caused by fluid restriction and the epidural injection of local anesthetics might have contributed to this incident. Although it is a rare complication, it is suggested that subclinical ASD can cause severe hypoxemia, dyspnea even cardiac arrest after pneumonectomy. In order to avoid these complications, ASD should be treated with percutaneous closure or surgical intervention. If these are not possible, care must be taken when anesthetizing the patient, and anything that can increase the pulmonary vascular resistance and right to left shunt should be avoided.