Abstract
A 25-year-old male with no prior medical history presented with a one-month history of nausea, weight loss, and dyspnea that progressed to syncope. The initial echocardiogram showed a dilated right ventricle with signs of systolic failure. The patient was admitted for suspected pulmonary embolism, but chest computed tomography (CT) revealed interstitial pneumonia. A transthoracic echocardiogram on day 6 of admission diagnosed cor triatriatum sinister (CTS), severe pulmonary hypertension, chronic cor pulmonale, and reduced right ventricular function. The patient was managed conservatively in the intensive care unit (ICU) without the need for mechanical ventilation and discharged after clinical improvement. This case highlights the importance of the early diagnosis of rare congenital heart defects such as cor triatriatum sinister, which can present with nonspecific symptoms and rapidly progress to right heart failure.