Platypnea-Orthodeoxia Syndrome Associated With a Right-to-Left Cardiac Shunt Detected by Lung Perfusion Scintigraphy

Introduction

Platypnea-orthodeoxia syndrome (P-OS) is a rare condition characterized by posture-dependent right-to-left shunting at the atrial level, which results in dyspnea in the upright position with improvement when supine [1,2]. In intracardiac shunt disorders, including P-OS, accurate diagnosis requires definitive demonstration of a right-to-left shunt [3], typically achieved through echocardiography with microbubble contrast or transesophageal echocardiography [4]. However, these procedures require patient cooperation – such as breath-holding – and may cause discomfort, making them difficult to perform in older adults or patients with psychiatric disorders. Furthermore, the examinations and their interpretation generally require cardiologist involvement, limiting feasibility in facilities without cardiology support.

In most reported cases, the diagnosis of intracardiac shunt disorders has been triggered by echocardiography [5,6]. In contrast, lung perfusion scintigraphy offers a practical alternative for detecting and quantifying intracardiac shunts, including those associated with P-OS [7]. This technique allows rapid assessment even in patients who cannot cooperate due to psychiatric comorbidities [8,9], and it can be performed without cardiologist involvement. Despite these advantages, lung perfusion scintigraphy remains underused in clinical practice, and its diagnostic value is often underrecognized.

This report describes a 64-year-old woman who displayed P-OS associated with a right-to-left cardiac shunt caused by a patent foramen ovale, which was successfully diagnosed via lung perfusion scintigraphy.

Case Report

A 64-year-old Japanese woman was found unconscious by a passerby and transported to our hospital. Her medical history included hypertension and severe bipolar disorder, for which she was not receiving treatment. Due to her psychiatric condition, she was unable to engage in meaningful communication. There was no notable family history. On arrival, the patient was alert, with a Glasgow Coma Scale score of E4V4M6. Vital signs were as follows: body temperature, 36.6°C; blood pressure, 154/98 mmHg; heart rate, 108 beats/min (regular); and oxygen saturation, 85% while receiving 4 L/min oxygen via face mask in the semi-sitting position. Physical examination revealed no conjunctival pallor. Chest auscultation revealed no murmurs, rubs, or adventitious sounds. No neurological deficits, including paralysis, were observed. Because of persistent hypoxemia, the patient was admitted to the respiratory medicine ward for further evaluation.

Laboratory tests at admission demonstrated mild hepatic enzyme elevation and polycythemia (Table 1), whereas B-type natriuretic peptide levels were within the normal range. Arterial blood gas analysis on room air in the supine position revealed pronounced hypoxemia (PaO2 53.7 mmHg, oxygen saturation 87.0%) (Table 1). Chest radiography showed cardiomegaly and right hilar enlargement, likely secondary to dilation of the aortic root, without evidence of aneurysm, pleural effusion, or pneumonia (Figure 1A). Chest computed tomography (CT) revealed mild post-inflammatory changes in the right middle lobe but no pulmonary arteriovenous malformation. Contrast-enhanced CT showed compression of the right atrium by the dilated aorta (Figure 1B), but pulmonary embolism was absent. Electrocardiography indicated left axis deviation without other notable abnormalities. Transthoracic echocardiography revealed a dilated aortic root (Figure 2A), which obscured visualization of the right atrium, tricuspid valve, and atrial septum; there were no other clinically significant findings. These results excluded heart failure, pneumonia, and pulmonary embolism as causes of hypoxemia. However, a right-to-left shunt remained a possible explanation. Although echocardiographic microbubble testing or transesophageal echocardiography would typically be required [4], these examinations were not feasible because the patient could not follow instructions due to severe bipolar disorder, and cardiology support for transesophageal echocardiography was not immediately available.

Therefore, lung perfusion scintigraphy, which can be performed without patient cooperation or cardiology involvement, was conducted on the second hospital day. Ventilation scans excluded ventilation-perfusion mismatch suggestive of pulmonary embolism. However, substantial technetium-99m uptake was noted in the thyroid and kidneys, consistent with a right-to-left shunt (Figure 2B). Based on these findings, the respiratory medicine team referred the patient to cardiology for further evaluation. On the fifth hospital day, sedated cardiac catheterization was performed with family consent. A guidewire was advanced smoothly from the right atrium to the left atrium, confirming patent foramen ovale. Right atriography demonstrated compression of the right atrium by the dilated aorta (Figure 2C). Pulmonary vascular resistance and right-sided heart pressures were within normal limits, but a right-to-left shunt was confirmed (Table 2). Coronary angiography revealed no clinically significant coronary artery disease. Collectively, these findings established a diagnosis of P-OS caused by right-to-left shunting through a patent foramen ovale. The mechanism was attributed to anatomical alteration from right atrial compression by the dilated aorta, which facilitated posture-dependent shunting.

The definitive treatment for P-OS is shunt closure [10]. Although invasive investigations were performed with family consent, our institution did not have a specialist qualified to perform percutaneous shunt closure; therefore, closure was not undertaken during catheterization. Because of the patient’s psychiatric instability and lack of symptom awareness while supine, surgical intervention was not pursued; treatment was deferred until symptom progression clearly warranted intervention. Subsequently, the patient’s psychiatric condition deteriorated, and she became largely bedridden in the psychiatric ward. At 12 months of follow-up, shunt closure had not been performed.

Discussion

This case highlights the diagnostic value of lung perfusion scintigraphy for P-OS, a rare disorder characterized by right-to-left shunting. A 64-year-old woman with severe bipolar disorder presented with unexplained hypoxemia and posture-dependent dyspnea that required further evaluation for a right-to-left shunt. Diagnostic procedures such as echocardiographic microbubble testing or transesophageal echocardiography, which require patient cooperation and cardiology support, were not feasible. Instead, lung perfusion scintigraphy demonstrated a right-to-left shunt, and subsequent cardiac catheterization confirmed the diagnosis of P-OS.

P-OS is a rare condition in which dyspnea and hypoxemia worsen in the upright position and improve when supine [1–3]. This phenomenon is attributed to postural changes that affect interatrial shunting [2,3]. Right-to-left shunting in P-OS results from both anatomical and mechanical factors. Anatomically, a patent foramen ovale or atrial septal defect provides a channel for shunting. Mechanically, elongation or dilation of the aorta may compress the right atrium, producing septal deformation in the upright position [11,12]. In the present case, both anatomical (a patent foramen ovale) and mechanical (right atrial compression by a dilated aorta of uncertain cause) factors were present, likely contributing to P-OS development. In most previously reported cases, the intracardiac shunt was readily detected because patients could cooperate with echocardiographic testing [5,6]. Although right atrial compression was evident on chest CT, the patent foramen ovale was not visualized on transthoracic echocardiography; microbubble testing was not possible due to the patient’s psychiatric condition and lack of immediate cardiology support. Lung perfusion scintigraphy revealed a right-to-left shunt, whereas chest CT excluded intrapulmonary shunting, thus guiding subsequent invasive evaluation via cardiac catheterization.

Lung perfusion scintigraphy enables the detection and quantification of intracardiac and intrapulmonary right-to-left shunts through intravenous administration of technetium-99m-labeled macroaggregated albumin [7–9]. Particles larger than 10 μm are usually trapped within the pulmonary capillary bed. In the presence of a right-to-left shunt, however, some particles bypass the lungs and accumulate in systemic organs such as the thyroid and kidneys [7–9]. In the present case, clear uptake in these organs provided definitive evidence of systemic tracer distribution and confirmed the presence of a right-to-left shunt. This modality is rapid, minimally invasive, and does not require heavy sedation, making it particularly valuable when an intracardiac shunt is suspected but microbubble testing or transesophageal echocardiography cannot be performed. Such situations include older or uncooperative patients [9,13,14]; those with contagious respiratory infections such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); patients with a pronounced gag reflex [8,9]; and individuals with oral, pharyngeal; or esophageal abnormalities that preclude transesophageal echocardiography [8,9].

Conclusions

Lung perfusion scintigraphy represents a noninvasive and effective diagnostic tool for detecting intracardiac shunting in patients with unexplained hypoxemia when microbubble testing or transesophageal echocardiography is not feasible. This case highlights the importance of including lung perfusion scintigraphy in the diagnostic evaluation of intracardiac shunts such as P-OS and reaffirms the value of this imaging modality in complex clinical scenarios.