Cardiac tamponade complicating a case of fever of unknown origin.

A 16-year-old male was transferred to Children's Hospital of Wisconsin (CHW) after 17 days of fever of unknown origin. An echocardiogram revealed a small pericardial effusion only. Serial echocardiography demonstrated an increased size of this effusion with collapse of the right atrium during diastole. A subxiphoid open pericardiostomy was performed to relieve the impending tamponade and to assist in the diagnosis of his fever. The diagnostic criteria for cardiac tamponade are discussed.