orthodeoxia

Orthodeoxia is a clinical phenomenon characterized by a decrease in arterial oxygen saturation when a person moves from a supine to an upright position, often accompanied by dyspnea. The desaturation typically improves when the patient returns to a horizontal position. When orthodeoxia occurs together with positional dyspnea, the paired syndrome is called platypnea-orthodeoxia syndrome (POS).

The condition is most commonly associated with right-to-left shunts that become more effective in the upright position. Intracardiac causes include patent foramen ovale and other atrial septal defects, where anatomical or functional changes allow deoxygenated blood to bypass the lungs. Intrapulmonary causes include hepatopulmonary syndrome with intrapulmonary vascular dilatations and pulmonary arteriovenous malformations. Other contributing factors can include chest wall deformities, age-related anatomical changes, or lung diseases that worsen ventilation-perfusion mismatch when upright.

Diagnosis relies on demonstrating a positional drop in oxygenation and identifying the underlying shunt or abnormality. Methods include pulse oximetry or arterial blood gas testing in different positions (supine and upright), echocardiography with a bubble study to detect intracardiac shunts, transesophageal echocardiography for detailed atrial anatomy, contrast-enhanced CT or pulmonary angiography to visualize intrapulmonary shunts or AVMs, and evaluation for liver disease in suspected hepatopulmonary syndrome.

Management targets the underlying cause. Shunts due to intracardiac defects may be treated with percutaneous device closure or surgical repair; pulmonary AVMs are often managed with embolization; hepatopulmonary syndrome may require management of liver disease and, in severe cases, liver transplantation. Supplemental oxygen can provide symptomatic relief. The prognosis varies with etiology and response to targeted therapy.