What are the types of heart shunts?

Types of Heart Shunts

Heart shunts are classified into two major categories: left-to-right shunts and right-to-left shunts, with each having distinct anatomical locations and physiological consequences. 1, 2

Left-to-Right Shunts

Left-to-right shunts occur when blood flows from the higher-pressure left heart chambers or aorta to the lower-pressure right heart chambers or pulmonary circulation.

Anatomical Types:

1. Pre-tricuspid shunts:

   • Atrial Septal Defects (ASDs):
     • Ostium secundum (most common)
     • Sinus venosus
     • Ostium primum
   • Anomalous pulmonary venous connections (partial or total)

2. Post-tricuspid shunts:

   • Ventricular Septal Defects (VSDs)
   • Patent Ductus Arteriosus (PDA)

3. Combined shunts:

   • Multiple defects occurring simultaneously

4. Complex congenital heart defects:

   • Complete atrioventricular septal defect
   • Truncus arteriosus
   • Single ventricle physiology with unobstructed pulmonary blood flow
   • Transposition of the great arteries with VSD

Physiological Consequences:

Left-to-right shunts increase pulmonary blood flow, which can lead to:

• Right heart chamber enlargement
• Pulmonary overcirculation
• Congestive heart failure
• Eventual development of pulmonary arterial hypertension
• If untreated, potential progression to Eisenmenger syndrome 1

Right-to-Left Shunts

Right-to-left shunts occur when blood flows from the right heart chambers or pulmonary circulation to the left heart chambers or systemic circulation, bypassing the lungs.

Common Causes:

1. Eisenmenger syndrome: Advanced pulmonary hypertension causing reversal of a previously left-to-right shunt 1

2. Cyanotic congenital heart defects:

   • Tetralogy of Fallot
   • Tricuspid atresia
   • Pulmonary atresia
   • Ebstein's anomaly with atrial communication

3. Complex single ventricle lesions with pulmonary stenosis or atresia 1

Physiological Consequences:

Right-to-left shunts result in:

• Decreased pulmonary perfusion
• Systemic arterial desaturation
• Cyanosis
• Exercise intolerance
• Risk of paradoxical embolism 2

Palliative and Therapeutic Shunts

These are surgically created shunts to improve circulation in certain congenital heart defects:

1. Systemic-to-pulmonary artery shunts:

   • Modified Blalock-Taussig shunt (subclavian artery to pulmonary artery)
   • Central shunt (ascending aorta to main/right pulmonary artery) 1

2. Cavopulmonary connections:

   • Bidirectional Glenn (superior vena cava to right pulmonary artery)
   • Bidirectional Glenn with additional pulmonary blood flow 1

3. Modified Fontan procedures:

   • Extracardiac conduit (inferior vena cava to pulmonary artery)
   • Intra-atrial conduit
   • Intracardiac lateral tunnel
   • Atriopulmonary connection (rarely used now) 1

Diagnostic Evaluation

Non-invasive Methods:

• Echocardiography: First-line imaging to assess location, size, direction of shunting
• Pulse oximetry: Particularly useful for detecting right-to-left shunts
• CMR/CTA: Recommended for evaluation of complex shunts and anomalous pulmonary venous connections 1, 2

Invasive Methods:

• Cardiac catheterization: Provides definitive hemodynamic assessment
• Oximetry run: Detects step-up in oxygen saturation to identify left-to-right shunts
• Pulmonary-to-systemic flow ratio (Qp:Qs): Quantifies shunt magnitude 1, 3

Management Considerations

Management depends on shunt type, size, and hemodynamic impact:

• Left-to-right shunts: Closure (surgical or transcatheter) is indicated when Qp:Qs ≥1.5:1 with right heart enlargement and pulmonary artery systolic pressure <50% of systemic pressure 1

• Right-to-left shunts: Management focuses on the underlying cause; closure is contraindicated in Eisenmenger physiology 1

• Palliative shunts: Used as staged procedures toward definitive repair in complex congenital heart disease 1

Important Clinical Pitfalls

• Closing shunts in patients with severe pulmonary hypertension (PA pressure >2/3 systemic) can lead to right heart failure and increased mortality 1

• Failing to assess pulmonary vascular resistance before closure can lead to inappropriate interventions in patients with Eisenmenger physiology 2

• Overlooking multiple shunts can result in incomplete treatment and persistent symptoms 2

• Exercise testing is crucial as some patients may have normal saturations at rest but desaturate with activity, indicating dynamic shunt reversal 1

Understanding the type, direction, and hemodynamic significance of cardiac shunts is essential for appropriate management decisions and timing of interventions.