A 51-year-old woman who had previously undergone left mastectomy for left breast cancer accompanied by multiple metastasis experienced worsening dyspnea. Physical and imaging assessments of the hemodynamics suggested cardiac tamponade, and emergency pericardiocentesis was successfully performed. However, immediately after the procedure, the patient's condition deteriorated rapidly and showed pulseless electrical activity. Contrast-enhanced computed tomography with continuous mechanical support demonstrated massive thrombi in both pulmonary arteries. An abrupt decrease in the central venous pressure and an increase in the venous return following pericardiocentesis might result in the migration of a deep venous thrombus and fatal acute pulmonary thromboembolism.