In this case report, we describe isolated left ventricular cardiac tamponade, a rare complication of cardiac surgery, diagnosed on point-of-care ultrasound (POCUS) in the emergency department (ED). To our knowledge, this is the first report of such a diagnosis made on ED bedside ultrasound. Our patient was a young adult female with a history of recent mitral valve replacement who presented to the ED with dyspnea and was found to have a large loculated pericardial effusion causing left ventricular diastolic collapse. Rapid diagnosis via POCUS in the ED allowed for expedited definitive treatment by cardiothoracic surgery in the operative room and emphasizes the importance of a standard 5-view cardiac POCUS examination when post-cardiac surgery patients present to the ED.