Shunts and Collaterals #1: Systemic-to-Pulmonary Shunts

Systemic-to-pulmonary shunts and collaterals are vascular pathways that direct blood from the systemic arterial circulation into the pulmonary arterial circulation.

Because systemic arterial pressure is usually higher than pulmonary arterial pressure, these channels generally create a left-to-right shunt, increasing pulmonary blood flow. This physiology may be beneficial when native pulmonary blood flow is inadequate, but it becomes harmful when the additional flow exceeds the capacity of the pulmonary vascular bed or the ventricle [1].

In simple terms:

Systemic artery → Pulmonary artery → Lungs → Pulmonary veins → Left heart

The extra pulmonary venous return increases left atrial and ventricular preload. When excessive, this produces pulmonary overcirculation, ventricular volume loading, systemic steal, and congestive heart failure [1, 2].

2. Hemodynamic Principle

The central hemodynamic effect is an increase in Qp, or pulmonary blood flow.

When systemic-to-pulmonary flow becomes significant:

• Qp increases
• Pulmonary venous return increases
• Left atrial and left ventricular volume load increase
• Pulmonary vascular congestion may occur
• Effective systemic output may decrease because part of systemic flow is “recycled” through the lungs

This explains why even relatively small vessels may become clinically important when the pressure gradient is large, when multiple collateral channels exist, or when the circulation is already vulnerable, such as in single-ventricle physiology [1, 3].

3. Representative Pathways

3.1 Patent Ductus Arteriosus

A patent ductus arteriosus is a persistent fetal connection between the aorta and pulmonary artery. After pulmonary vascular resistance falls, ductal flow usually becomes left-to-right, producing pulmonary overcirculation and left heart volume overload [2].

Clinically significant PDA may cause:

• Tachypnea and increased work of breathing
• Feeding difficulty and poor weight gain
• Pulmonary edema
• Left atrial and left ventricular dilation
• Systemic hypoperfusion in large shunts

3.2 Surgical Systemic-to-Pulmonary Shunts

A surgically created systemic-to-pulmonary shunt, such as a modified Blalock–Taussig–Thomas shunt, is intentionally used to provide pulmonary blood flow in cyanotic congenital heart disease with inadequate pulmonary perfusion.

However, the physiology is delicate. An undersized shunt may cause persistent cyanosis, whereas an oversized shunt may cause pulmonary overcirculation, systemic hypotension, coronary steal, acidosis, and ventricular overload [3].

Thus, the surgical shunt is not simply a “connection.” It is a calibrated source of pulmonary blood flow.

3.3 Major Aortopulmonary Collateral Arteries

Major aortopulmonary collateral arteries, or MAPCAs, are systemic arterial vessels arising from the aorta or its branches and supplying portions of the pulmonary vascular bed [4].

They are commonly associated with:

• Pulmonary atresia with VSD
• Tetralogy of Fallot with severe pulmonary stenosis or pulmonary atresia
• Hypoplastic or discontinuous native pulmonary arteries
• Segmental pulmonary blood supply

MAPCAs may provide essential pulmonary perfusion, but they may also expose the pulmonary vascular bed to systemic pressure, producing segmental overcirculation, stenosis, pulmonary vascular disease, or ventricular volume loading [4].

3.4 Acquired Systemic-to-Pulmonary Collaterals

Acquired systemic-to-pulmonary collaterals may develop after cyanosis, prior surgical palliation, altered pulmonary blood flow, or cavopulmonary connections. They are particularly important in Glenn and Fontan pathways, where additional collateral flow can impose a volume load on a circulation that is designed to function without a subpulmonary ventricle [5, 6].

4. Physiologic Consequences

A. Pulmonary Overcirculation

Excess systemic-to-pulmonary flow increases total pulmonary blood flow. This may cause pulmonary congestion, increased respiratory effort, impaired feeding, prolonged ventilation, and heart failure symptoms.

In biventricular physiology, the major burden usually returns to the left atrium and left ventricle.

In single-ventricle physiology, the same collateral flow becomes a direct volume burden on the systemic ventricle [1, 5].

B. Ventricular Volume Loading

The additional pulmonary venous return increases ventricular preload.

Over time, this can lead to:

• Atrial dilation
• Ventricular dilation
• Increased myocardial oxygen demand
• Atrioventricular valve regurgitation
• Reduced cardiac efficiency
• Heart failure progression

In Fontan candidates and Fontan patients, significant aortopulmonary collaterals have been associated with increased postoperative morbidity, longer recovery, and, in selected series, a marker of eventual cardiac failure [5, 6].

C. Systemic Steal

Large systemic-to-pulmonary pathways may divert systemic arterial blood into the pulmonary circulation, reducing effective systemic perfusion.

This is particularly relevant in neonates, infants, and shunt-dependent physiology, where excessive pulmonary runoff may reduce:

• Diastolic blood pressure
• Coronary perfusion
• Mesenteric perfusion
• Renal perfusion
• Systemic oxygen delivery

Therefore, the problem is not simply “too much pulmonary blood flow.” It is maldistribution of systemic cardiac output.

5. Diagnostic Assessment

The key diagnostic question is not only whether collaterals exist, but whether they are hemodynamically significant.

Evaluation may include:

1. Echocardiography
• Ventricular size and function
• AV valve regurgitation
• Pulmonary venous return pattern
• Indirect evidence of volume loading
2. CT angiography or MR angiography
• Origin and course of collateral vessels
• Native pulmonary artery anatomy
• Segmental pulmonary blood supply
• Relationship between MAPCAs and native pulmonary arteries
3. Cardiac catheterization
• Direct angiographic mapping
• Pulmonary artery pressure
• Pulmonary vascular resistance
• Test occlusion when needed
• Potential transcatheter embolization
4. Cardiac MRI flow quantification
• Systemic-to-pulmonary collateral flow can be quantified noninvasively and has been associated with acute post-Fontan outcomes in clinical studies [7].

6. Management Strategy

Management depends on whether the vessel is useful, excessive, or harmful.

6.1 Observation

Observation may be appropriate when collateral flow is small, clinically silent, and not associated with ventricular dilation, pulmonary overcirculation, elevated pulmonary artery pressure, or impaired postoperative recovery.

Not every collateral vessel requires closure.

6.2 Catheter Occlusion

Transcatheter embolization is useful for abnormal systemic-to-pulmonary vessels or surgical shunts that produce excessive pulmonary blood flow, interfere with perioperative management, or contribute to heart failure [8].

Potential devices include:

• Coils
• Vascular plugs
• Duct occluders
• Particle embolization in selected diffuse networks

The goal is to reduce unnecessary pulmonary runoff while preserving essential pulmonary perfusion.

6.3 Surgical Ligation or Division

Surgical ligation may be considered when the collateral is accessible during planned repair or when catheter occlusion is not feasible. This may be relevant for large collateral vessels, prior shunts, or vessels encountered during unifocalization.

6.4 Unifocalization

In pulmonary atresia/VSD/MAPCAs, MAPCAs may represent the primary blood supply to portions of the lung. In this setting, simple occlusion may be inappropriate.

Instead, the surgical goal may be unifocalization: incorporation of MAPCAs into a reconstructed pulmonary arterial system, ideally creating a central, low-resistance pulmonary vascular bed suitable for complete repair [9, 10].

This strategy requires careful assessment of:

• Native pulmonary artery size
• Number and origin of MAPCAs
• Segmental lung perfusion
• Dual-supply segments
• Stenosis within collateral vessels
• Expected pulmonary vascular resistance after reconstruction

7. Important Surgical Perspective

Systemic-to-pulmonary shunts and collaterals should be interpreted as part of the patient’s overall circulatory design.

The essential surgical questions are:

• Is this vessel necessary for oxygenation?
• Does it supply a lung segment without native pulmonary artery flow?
• Is it creating excessive Qp?
• Is it causing ventricular volume loading?
• Is it contributing to pulmonary hypertension or heart failure?
• Should it be preserved, occluded, ligated, or incorporated into reconstruction?

The same vessel may be lifesaving in one patient and harmful in another.

8. Practical Summary

Systemic-to-pulmonary shunts and collaterals connect systemic arterial pressure to the pulmonary vascular bed.

They may improve oxygenation when pulmonary blood flow is inadequate, but excessive flow can produce pulmonary overcirculation, ventricular volume loading, systemic steal, prolonged postoperative recovery, and heart failure.

The clinical objective is not simply to “close” or “preserve” these vessels. The objective is to define whether the pathway provides balanced, useful, and sustainable pulmonary blood flow within the patient’s circulation.

References

[1] Geva T. Quantification of systemic-to-pulmonary artery collateral flow: challenges and opportunities. Circ Cardiovasc Imaging. 2012;5(2):175-177.

[2] Philip R, Nathaniel Johnson J, Naik R, Kimura D, Boston U, Chilakala S, et al. Effect of patent ductus arteriosus on pulmonary vascular disease. Congenit Heart Dis. 2019;14(1):37-41.

[3] Ohye RG, Devaney EJ, Hirsch JC, Bove EL. The modified Blalock-Taussig shunt versus the right ventricle-to-pulmonary artery conduit for the Norwood procedure. Pediatr Cardiol. 2007;28(2):116-121.

[4] Alex A, Ayyappan A, Valakkada J, Kramadhari H, Sasikumar D, Menon S. Major aortopulmonary collateral arteries. Radiol Cardiothorac Imaging. 2022;4(1):e210157.

[5] Kanter KR, Vincent RN, Raviele AA. Importance of acquired systemic-to-pulmonary collaterals in the Fontan operation. Ann Thorac Surg. 1999;68(3):969-974.

[6] Kanter KR, Vincent RN. Management of aortopulmonary collateral arteries in Fontan patients: occlusion improves clinical outcome. Semin Thorac Cardiovasc Surg Pediatr Card Surg Annu. 2002;5:48-54.

[7] Glatz AC, Rome JJ, Small AJ, Gillespie MJ, Dori Y, Harris MA, et al. Systemic-to-pulmonary collateral flow, as measured by cardiac magnetic resonance imaging, is associated with acute post-Fontan clinical outcomes. Circ Cardiovasc Imaging. 2012;5(2):218-225.

[8] Sharma S, Kothari SS, Krishnakumar R, Saxena A, Sharma R, Taneja K, et al. Systemic-to-pulmonary artery collateral vessels and surgical shunts in patients with cyanotic congenital heart disease: perioperative treatment by transcatheter embolization. AJR Am J Roentgenol. 1995;164(6):1505-1510.

[9] Mainwaring RD, Reddy VM, Peng LF, Kuan C, Palmon M, Hanley FL. Hemodynamic assessment after complete repair of pulmonary atresia with major aortopulmonary collaterals. Ann Thorac Surg. 2013;95(4):1397-1403.

[10] Bauser-Heaton H, Ma M, Wise-Faberowski L, Asija R, Shek J, Zhang Y, et al. Outcomes after initial unifocalization to a shunt in complex tetralogy of Fallot with MAPCAs. Ann Thorac Surg. 2019;107(6):1807-1815.