What is the pathophysiology of intrapulmonary shunting diagnosed by agitated saline (saline solution) on echocardiogram (echo), also known as contrast echocardiography?

From the Guidelines

Intrapulmonary shunting diagnosed by agitated saline on echocardiogram is characterized by abnormal vascular communications within the lungs, allowing deoxygenated blood to bypass normal gas exchange and enter the systemic circulation, as supported by the most recent guidelines 1. The pathophysiology involves direct channels between pulmonary arteries and veins, created by conditions such as pulmonary arteriovenous malformations (PAVMs), hepatopulmonary syndrome, and pulmonary telangiectasias. During an agitated saline (bubble) study, microbubbles are injected intravenously and normally would be filtered by the pulmonary capillaries. However, in intrapulmonary shunting, these bubbles appear in the left heart chambers after a delay of 3-6 cardiac cycles, indicating blood is bypassing the alveolar-capillary interface where oxygen exchange normally occurs. Key points to consider in the diagnosis and management of intrapulmonary shunting include:

• The use of contrast (microbubble) echocardiography to characterize hepatopulmonary syndrome (HPS) and detect intrapulmonary shunting 1
• The importance of assessing the severity of HPS, as patients with severe hypoxemia may be at risk of irreversible respiratory failure and high perioperative mortality 1
• The role of pulmonary angiography in diagnosing and treating arteriovenous communications that are amenable to embolization 1
• The use of echocardiography with contrast to detect intracardiac or transpulmonary right-to-left shunts, and to identify unusual venous connections or acquired intrapulmonary shunts 1
• The estimation of systolic pulmonary artery pressure (PAP) based on peak tricuspid regurgitation velocity (TRV) and right atrial pressure (RAP), and the limitations of Doppler-derived pressure estimation 1. Overall, the diagnosis and management of intrapulmonary shunting require a comprehensive approach, incorporating clinical evaluation, imaging studies, and hemodynamic assessments to guide treatment decisions and improve patient outcomes.

From the Research

Pathophysiology of Intrapulmonary Shunting

The pathophysiology of intrapulmonary shunting diagnosed by agitated saline on echocardiogram involves the presence of intrapulmonary vascular abnormalities, such as arteriovenous malformations and capillary dilatations, which can result in right-to-left shunting and hypoxemia 2, 3, 4. These abnormalities can be found in patients with severe liver disease, including those with end-stage hepatic disease and hepatopulmonary syndrome 2, 3.

Causes and Associations

Intrapulmonary shunting can be associated with various diseases, including:

• Hereditary hemorrhagic telangiectasia (HHT) 5
• Hepatopulmonary syndrome 3, 4
• Congenital heart defects after partial or complete cavopulmonary anastomosis 5
• Pulmonary hypertension 6

Detection and Diagnosis

Transthoracic echocardiography with agitated saline contrast injection can be used to diagnose intrapulmonary shunting 3, 4, 5. The presence of contrast bubbles in the left atrium or pulmonary veins can indicate right-to-left shunting 4. The severity of shunting can be graded based on the amount of contrast bubbles present 5.

Clinical Implications

Intrapulmonary shunting can have significant clinical implications, including:

• Hypoxemia 2, 3, 6
• Paradoxical embolization and neurologic complications 5
• Increased risk of brain abscesses after procedures with risk for bacteremia 5
• Need for antibiotic prophylaxis and additional diagnostic testing, such as chest computed tomography 5