A 19-year-old man with large malignant pleural and pericardial effusions with tamponade physiology and signs of congestive heart failure presented for emergent subxiphoid pericardial window. Surgical drainage of the pericardium was complicated by a paradoxical cardiovascular collapse that failed to respond to pressors and intravenous fluids. Suspecting a pericardial perforation, a median sternotomy was performed and revealed an intact heart. The arterial pressure was promptly restored after drainage of the pleural effusion. It is proposed that, in patients presenting with tamponading pericardial and pleural effusions, drainage of the pleural effusion be given priority. The pathophysiology of low cardiac output states resulting from pericardial and large pleural effusion is discussed and the literature reviewed.