The patient was a 70-year-old female who had been diagnosed with Mixed Connective Tissue Disease (MCTD) and treated with oral steroids at another clinic for the previous 7 years. As she developed exercise-induced respiratory discomfort and paroxysmal atrial fibrillation, and received a diagnosis of aortic stenosis and angina pectoris, aortic valve replacement using a biological valve and coronary artery bypass to LAD were performed. An intra-aortic balloon pump (IABP) was placed due to LOS associated with acute thrombocytopenia and bleeding tendency during the operation. Although postoperative hemodynamics were stable, an infiltrative shadow was observed in the entire right lung. On Day 4, remarkable thrombocytopenia reappeared. On Day 5, the IABP was removed and the anticoagulant changed from heparin to nafamostat; however, thrombocytopenia continued. On Day 9, CAG was performed, and complete obstruction was detected at the proximal part of the circumflex branch, which was not observed before the operation. Hence, emergency PCI was performed to re-insert the IABP. Aggravation of thrombocytopenia was observed along with progressive necrosis of fingertips and oral cavity as well as mucous-bloody stool. On day 24, the patient's condition was complicated by subarachnoid hemorrhage of unknown cause and she died on Day 34 due to extensive colonic necrosis.