What is platypnea-orthodeoxia?

Platypnea-Orthodeoxia Syndrome: Definition and Clinical Characteristics

Platypnea-orthodeoxia syndrome (POS) is a rare clinical disorder characterized by dyspnea (platypnea) and arterial oxygen desaturation (orthodeoxia) that occurs specifically in the upright position and improves when lying supine—the opposite of typical orthopnea. 1, 2

Core Clinical Features

The syndrome requires two essential components to manifest:

• Platypnea: Shortness of breath that worsens when standing or sitting upright and improves in the recumbent position 1, 3
• Orthodeoxia: Arterial oxygen desaturation (decreased oxygen saturation) that occurs in the upright position and normalizes or improves when supine 2, 4

Pathophysiological Mechanism

POS fundamentally results from positional right-to-left shunting that requires both an anatomical defect and a functional component 2, 3:

Anatomical Defects (The Shunt Pathway)

Intracardiac causes are most common:

• Patent foramen ovale (PFO) is the most frequently reported anatomical substrate 3, 5
• Atrial septal defect (ASD) 3
• Atrial septal aneurysm (ASA), often in combination with PFO 3, 5
• Prominent Eustachian valve directing blood flow toward the interatrial septum 5

Intrapulmonary causes include:

• Pulmonary arteriovenous malformations (PAVMs) that create direct connections between pulmonary arteries and veins, bypassing the capillary bed 6
• Intrapulmonary vascular dilations, particularly in hepatopulmonary syndrome 7

Functional/Positional Component

The anatomical defect alone is insufficient—a functional mechanism must promote preferential right-to-left shunting in the upright position 2, 3:

• Gravitational redistribution of blood flow: In the upright position, blood flow preferentially shifts to the lung bases where vascular dilations or shunts may predominate 6, 7
• Altered cardiac geometry: Positional changes in atrial pressure gradients or anatomical relationships that favor shunt flow when upright 5
• Secondary anatomic abnormalities: Most patients with intracardiac shunts demonstrate additional structural or functional defects that facilitate the positional shunting 3

Clinical Context and Associated Conditions

POS has been reported in association with:

• Hepatopulmonary syndrome: Where intrapulmonary vascular dilations worsen in the upright position due to gravitational blood flow redistribution to lung bases 7
• Pulmonary arteriovenous malformations: Particularly those located in the lower lobes (65-83% of PAVMs), causing gravity-dependent hypoxemia 6
• Post-cardiac surgery or structural heart disease: Creating anatomical substrates for shunting 3
• Thromboembolic disease: As illustrated by cases of right atrial thrombus causing functional obstruction and shunt flow through PFO 2

Diagnostic Approach

Orthostatic testing is crucial and involves measuring oxygen saturation in both supine and upright positions to document the positional change 6, 5:

• Arterial oxygen saturation typically normalizes in supine position but drops significantly (often to 83% or lower) when upright 5
• Standard pulse oximetry in a single position may miss the diagnosis entirely 6

Confirmation of the anatomical substrate requires:

• Contrast echocardiography (transthoracic and transesophageal) to identify intracardiac shunts 5
• Right and left heart catheterization with serial oxygen saturation measurements in different positions 5
• Contrast echocardiography or pulmonary perfusion scintigraphy for intrapulmonary vascular dilations 7

Treatment Implications

For cardiac POS with PFO, percutaneous closure is safe and effective, immediately relieving orthodeoxia and symptoms with improvement in oxygen saturation from approximately 83% to 93% in the upright position 5

For hepatopulmonary syndrome, liver transplantation is the only curative treatment, resulting in complete reversal or significant improvement in >85% of patients 7

Critical Clinical Pitfall

POS is frequently under-diagnosed because clinicians may not consider positional testing when evaluating hypoxemia 5. The syndrome must be actively investigated whenever position-dependent hypoxemia is suspected, as it is often treatable once recognized 2, 5.