The following case report details an 88-year-old woman with severe mitral stenosis and moderate mitral regurgitation who presented with worsening dyspnea on exertion. The patient had undergone 4-vessel coronary artery bypass graft and mitral valve replacement 14 years before and was deemed high risk for redo sternotomy. A transseptal mitral valve-in-valve replacement was performed which resulted in intraoperative hypoxia and hypotension after atrial septal defect creation for valve deployment. A right-to-left shunt had developed due to the patient's underlying pulmonary hypertension. Successful atrial septal defect closure resolved the hypoxia and hypotension. The patient had a brief and uncomplicated postoperative course.