Right-to-left shunt across a patent foramen ovale caused by cardiac tamponade: diagnosis by transesophageal echocardiography. Cardiac tamponade can manifest as profound hypoxemia from intracardiac shunting across a patent foramen ovale. As a consequence, pulmonary embolus can be erroneously diagnosed. As demonstrated in the case described herein, transesophageal echocardiography can be useful in determining the correct diagnosis, especially if transthoracic echocardiography is technically limited. In our patient, the findings on transesophageal echocardiography also helped determine the appropriate treatment. The relative inaccessibility of the pericardial effusion to needle drainage prompted open surgical drainage. 