Mechanisms of platypnea-orthodeoxia syndrome

Platypnea orthodeoxia syndrome (POS) is a clinical entity described in the middle of the last century. It is characterized by dyspnea and hypoxemia triggered by standing and relieved with recumbency. The diagnosis is predominately clinical. The degree of hypoxemia is variable; however, the diagnostic criteria include the decrease in arterial oxygen pressure more than 4 mmHg or oxygen saturation more than 5%. Even though many diseases cause this syndrome, there are only two responsible mechanisms, intracardiac, and intrapulmonary shunts. The coexistence of diverse structural and physiological abnormalities joined to gravitational forces that induce blood shunt after standing is crucial in each mechanism. The intracardiac mechanism is characterized by right to left blood shunt through atrial septal communications and, the right atrium pressure could be normal or increased. In addition, some patients have one or more coexistent aortic, spinal, or intracardiac alterations. The intrapulmonary mechanism is less frequent and is caused by parenchymal or vascular pathologies. Transthoracic echocardiogram is the first diagnostic modality; however, understanding the pathophysiology is the key for a rational diagnostic approach and subsequent diagnostic studies. Treatment is possible and effective in the majority of intracardiac mechanisms and some intrapulmonary. This review focuses on the pathophysiologic mechanisms of POS and their diagnostic workup.