ABSTRACT
<p><b>INTRODUCTION</b>Orthodeoxia is a rare clinical syndrome characterised by dyspnoea and arterial deoxygenation that accompanies a change from a supine to erect position.</p><p><b>CLINICAL PICTURE</b>We describe an unusual case of "paradoxical orthodeoxia" in a 70-year-old man with a thoracic aortic aneurysm: arterial desaturation when supine that improved when erect.</p><p><b>TREATMENT AND OUTCOME</b>Non-invasive imaging revealed compression of the left pulmonary artery by the aneurysm (thoracic computed tomography) and patent foramen ovale (transesophageal echocardiography). Nuclear studies show decreased relative left lung perfusion attributable to the former, and right-to-left atrial shunt attributable to the latter. The degree of right-to-left shunt increases in the supine position: nuclear pulmonary shunt study shows shunt extent of 21% when supine versus 10% erect.</p><p><b>CONCLUSION</b>A physioanatomical explanation is proposed.</p>