What is a right-to-left (R-to-L) shunt commonly referred to as in medical terminology?

Right-to-Left Cardiac Shunt

A right-to-left shunt is clinically defined as the abnormal flow of deoxygenated blood from the right side of the heart directly to the left side, bypassing the pulmonary circulation, resulting in systemic arterial desaturation and cyanosis. 1

Pathophysiology and Mechanism

Right-to-left shunts occur when blood flows directly from the right heart chambers or pulmonary arteries to the left heart or systemic circulation without passing through the lungs. This abnormal flow pattern results in:

• Decreased pulmonary perfusion
• Reduced oxygen uptake in the pulmonary circulation
• Systemic arterial desaturation
• Clinical cyanosis

Clinical Contexts

Right-to-left shunts commonly occur in several clinical scenarios:

1. Eisenmenger syndrome: Advanced pulmonary hypertension causing reversal of shunt direction in patients with pre-existing congenital heart defects 2

2. Severe pulmonary hypertension: When pulmonary vascular resistance exceeds systemic resistance, causing right-to-left shunting through any existing septal defect 1

3. Patent foramen ovale with elevated right-sided pressures: Causing intermittent right-to-left shunting 1

4. Pulmonary arteriovenous malformations: Direct connections between pulmonary arteries and veins 3

5. Hepatopulmonary syndrome: Acquired intrapulmonary shunts in patients with cirrhosis 3

Diagnostic Methods

Several diagnostic techniques can identify right-to-left shunts:

• Echocardiography with contrast: The gold standard initial test showing early appearance of contrast bubbles in left heart chambers after right-sided injection 2

• Pulse oximetry: Shows arterial desaturation at rest or with exercise 2

• First-pass radionuclide angiography: Demonstrates early visualization of left heart chambers or aorta 2

• Lung perfusion scanning: Shows appearance of technetium activity in brain and splanchnic viscera 2

• Cardiac catheterization: Confirms reduced pulmonary-to-systemic flow ratio (Qp:Qs <1) 2

• Transesophageal echocardiography: Particularly useful for detecting atrial-level shunts and establishing shunt level 2

Clinical Implications

The presence of a right-to-left shunt has significant clinical consequences:

• Cyanosis: Due to deoxygenated blood entering systemic circulation
• Exercise intolerance: From inability to increase oxygen delivery during exertion
• Risk of paradoxical embolism: Systemic emboli can occur through the shunt
• Contraindication to certain interventions: Closure is contraindicated in severe pulmonary hypertension with fixed right-to-left shunting

Management Considerations

Management depends on the underlying cause and severity:

• Pulmonary vasodilator therapy: May be beneficial in reversible pulmonary hypertension with bidirectional shunting

• Transcatheter closure: Contraindicated in patients with severe pulmonary hypertension and fixed right-to-left shunting 2

• Surgical intervention: May be considered in specific anatomical defects without severe pulmonary hypertension

• Regular follow-up: Essential to monitor for changes in shunt direction and magnitude

Important Caveats

• Never close a right-to-left shunt with severe pulmonary hypertension: The ACC/AHA guidelines explicitly state that ASD closure should not be performed in adults with PA systolic pressure greater than two-thirds systemic, pulmonary vascular resistance greater than two-thirds systemic, and/or a net right-to-left shunt 2

• Contrast echocardiography is more sensitive than Doppler: For detection of low-velocity right-to-left shunts such as ASDs or baffle leaks 2

• Comprehensive hemodynamic assessment is crucial: Before any intervention to avoid worsening right heart failure by closing a "relief valve" 1