Shunts and Collaterals in Congenital Heart Disease

Shunts and collateral vessels are frequent and clinically important in congenital heart disease, particularly after staged single-ventricle palliation. Their physiologic consequences are determined by the vascular compartments connected, the pressure gradient that drives flow, the circulation bypassed, and the chamber or vascular bed that receives the additional volume. In Glenn and Fontan physiology, collateral pathways may arise as adaptive responses to cyanosis, reduced effective pulmonary blood flow, venous hypertension, or regional exclusion of hepatic venous effluent. The same vessel may therefore provide short-term compensation while creating long-term circulatory inefficiency [1].

Three patterns should be distinguished: systemic-to-pulmonary arterial pathways, pulmonary arteriovenous malformations, and systemic venous collaterals. Systemic-to-pulmonary arterial flow generally increases pulmonary blood flow and ventricular preload. Pulmonary arteriovenous malformations bypass the alveolar capillary network and create an intrapulmonary right-to-left shunt. Systemic venous collaterals divert venous return away from the intended cavopulmonary pathway or directly into the pulmonary venous atrium. Precise anatomical classification is essential because their indications for closure are not interchangeable.

1. Hemodynamic Framework

After a bidirectional Glenn or Fontan operation, pulmonary blood flow is generated without a subpulmonary ventricle. Forward flow depends on a low-resistance pathway from the systemic veins through the pulmonary arteries and lungs to an atrium with low filling pressure. Consequently, pulmonary artery stenosis, elevated pulmonary vascular resistance, ventricular diastolic dysfunction, atrioventricular valve regurgitation, or cavopulmonary pathway obstruction can magnify the effects of collateral vessels.

Each abnormal pathway should be analyzed using four questions:

1. Which vascular territories are connected?
2. What pressure gradient determines the direction of flow?
3. Does the pathway add blood to, or remove blood from, the effective pulmonary circulation?
4. Is the pathway primarily pathologic, compensatory, or essential to regional perfusion?

This framework prevents a common error: treating the angiographic appearance of a collateral without addressing the hemodynamic substrate that promoted its formation.

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

2. Anatomy and Classification

Systemic-to-pulmonary arterial pathways deliver systemic arterial blood to the pulmonary arterial circulation. They include a modified Blalock–Thomas–Taussig shunt, a patent ductus arteriosus, major aortopulmonary collateral arteries, residual native antegrade pulmonary blood flow, and acquired collateral arteries arising from bronchial, intercostal, internal thoracic, subclavian, or other systemic arteries.

These vessels have different developmental and clinical roles. A surgical shunt or patent ductus may be intentionally maintained to provide essential pulmonary blood flow. Major aortopulmonary collateral arteries may supply pulmonary segments that lack adequate central pulmonary arterial perfusion. Acquired aortopulmonary collateral arteries after Glenn or Fontan palliation usually enlarge in association with chronic cyanosis, reduced regional pulmonary perfusion, previous thoracic surgery, and angiogenic stimulation.

Systemic-to-pulmonary collaterals were detected in 59% of patients at follow-up catheterization after bidirectional cavopulmonary anastomosis in one retrospective series. Previous systemic-to-pulmonary shunting and operative variables were associated with their development, although the presence of collaterals alone did not predict prolonged Fontan effusions or poor outcome in that cohort [2].

3. Hemodynamic Consequences

Because systemic arterial pressure exceeds pulmonary arterial pressure, acquired systemic-to-pulmonary collateral flow is usually left-to-right. Blood enters the pulmonary arteries, returns through the pulmonary veins, and increases preload to the systemic ventricle. In single-ventricle physiology, the sole ventricle must handle this additional recirculating volume.

Moderate collateral flow may improve oxygen saturation or support pulmonary artery growth. Excessive flow, however, can produce pulmonary overcirculation, ventricular dilation, atrioventricular valve regurgitation, elevated atrial pressure, and reduced ventricular efficiency. During Fontan completion, uncontrolled collateral flow may increase pulmonary venous return while cardiopulmonary bypass and surgical exposure limit effective decompression. Postoperatively, it may contribute to elevated pulmonary artery pressure, heart failure, persistent pleural drainage, or increased transfusion requirements.

In a series of 137 children undergoing fenestrated lateral-tunnel Fontan completion, 38% underwent preoperative collateral occlusion and 22% required postoperative coil occlusion for elevated pulmonary artery pressure, heart failure, or prolonged chest-tube drainage. Aggressive control of hemodynamically significant vessels was associated with good early and intermediate survival, although the need for postoperative embolization also identified a subgroup at risk for later cardiac failure [3].

4. Diagnostic Assessment and Treatment

Echocardiography may suggest excessive collateral burden through ventricular enlargement, increased pulmonary venous return, atrioventricular valve regurgitation, or persistent antegrade pulmonary flow, but diffuse collateral flow is commonly underestimated. Catheter angiography defines vessel origin, course, and supplied territory. Cardiac magnetic resonance imaging can quantify collateral burden by comparing pulmonary venous return with pulmonary arterial forward flow and can assess differential pulmonary perfusion.

Embolization should be directed toward vessels that impose a demonstrable volume or pressure burden. Coils are suitable for smaller tortuous branches, whereas vascular plugs may be preferable for larger, straighter vessels with high flow. Surgical ligation may be used when vessels are inaccessible by catheter or encountered during reoperation.

Closure must remain selective. Before occlusion, the operator must establish that a vessel is not the sole supply to a pulmonary segment and is not required to maintain acceptable oxygenation. Aortopulmonary collateral arteries should not be treated as a uniform lesion; the decision depends on anatomy, supplied lung territory, ventricular volume loading, cavopulmonary pressure, and the timing of definitive repair or Fontan completion.

Shunts and Collaterals #2: Pulmonary Arteriovenous Malformations

5. Definition and Mechanism

Pulmonary arteriovenous malformations are abnormal communications between pulmonary arteries and pulmonary veins that bypass the alveolar capillary network. Desaturated pulmonary arterial blood therefore returns to the systemic ventricle without effective gas exchange, producing a true intrapulmonary right-to-left shunt.

Diffuse pulmonary arteriovenous malformations are strongly associated with superior cavopulmonary connections. Their development has been attributed principally to exclusion of hepatic venous effluent from the affected pulmonary bed and, secondarily, to reduced pulmonary arterial pulsatility. Abnormal angiogenic signaling is considered a probable final common pathway, although the specific circulating “hepatic factor” has not been identified [4].

Historical physiologic studies also demonstrated abnormal distribution of pulmonary perfusion after Glenn and Fontan operations. In one series, reduced upper-to-lower lobe perfusion was observed in 65% of Glenn patients and correlated with time after surgery; confirmed pulmonary arteriovenous fistulae were less common, emphasizing that abnormal perfusion and established arteriovenous shunting are related but distinct findings [5]. Pulmonary arteriovenous malformations have also been described after modified Fontan operations without a previous Glenn, indicating that nonpulsatile flow and other Fontan-related factors may contribute independently of superior caval diversion [6].

6. Clinical Consequences and Diagnosis

The principal manifestation is progressive systemic desaturation despite adequate ventilation and unobstructed central pulmonary arteries. Supplemental oxygen may provide limited improvement when the shunt fraction is substantial. Patients may develop exercise intolerance, secondary erythrocytosis, digital clubbing, and impaired systemic oxygen delivery. Because the pulmonary capillary filter is bypassed, clinically important malformations may also increase the risk of paradoxical embolization and cerebral infection.

Contrast echocardiography is a sensitive screening method. Delayed appearance of agitated-saline microbubbles in the systemic atrium or ventricle after passage through the lungs supports an intrapulmonary shunt, whereas immediate opacification suggests an intracardiac communication. Catheterization may demonstrate diffuse pulmonary vascular blushing and rapid pulmonary arterial-to-venous transit. Computed tomography is useful for discrete macroscopic lesions but may underestimate diffuse microscopic disease.

Evaluation should include hepatic venous anatomy, Fontan conduit geometry, pulmonary artery morphology, and differential hepatic flow distribution. A technically patent Fontan pathway does not guarantee balanced delivery of hepatic effluent. Preferential streaming toward one lung can leave the contralateral lung functionally deprived and allow unilateral pulmonary arteriovenous malformations to persist.

7. Management

Discrete focal lesions may be treated by transcatheter embolization. Diffuse Glenn-associated disease is not amenable to complete coil occlusion because innumerable small vascular connections are involved. The definitive strategy is restoration or redistribution of hepatic venous blood to the affected lung.

Approaches include Fontan completion, revision of a malaligned extracardiac conduit, creation of a hepatic vein-to-azygos connection, or another operation designed to balance hepatic venous effluent between the branch pulmonary arteries. Clinical improvement may require months because oxygenation improves only as abnormal vascular channels regress. Persistent desaturation after reconstruction should prompt reassessment of hepatic flow streaming, residual venous collaterals, pulmonary artery obstruction, and ventricular filling pressure.

Shunts and Collaterals #3: Veno-Venous Collaterals

8. Anatomical Subtypes

The term systemic venous collateral includes at least two physiologically distinct groups.

Systemic venous-to-systemic venous collaterals connect the higher-pressure superior caval or cavopulmonary territory with the inferior caval system or another lower-pressure systemic venous compartment. Common routes involve the azygos and hemiazygos systems, intercostal veins, paravertebral veins, lumbar veins, internal thoracic veins, and mediastinal channels.

Systemic venous-to-pulmonary venous collaterals drain systemic venous blood directly into a pulmonary vein, the pulmonary venous atrium, or the systemic atrium. These vessels produce a direct right-to-left shunt and should not be conflated with systemic venous decompression into the inferior caval system.

9. Development and Physiologic Effects

After Glenn or Fontan palliation, elevated central venous pressure may recruit or recanalize embryologically preformed venous channels. A pressure difference between the cavopulmonary system and a lower-pressure venous or atrial compartment then sustains flow [1,7].

In a contemporary cohort of 586 patients, veno-venous collaterals were identified between the Glenn and Fontan stages in 12.3%. Patients with collaterals had higher pre-Glenn mean pulmonary artery pressure and transpulmonary gradient, although long-term survival after total cavopulmonary connection was similar to that of patients without collaterals [8].

When superior caval blood is diverted into the inferior caval system or atrium, it bypasses the pulmonary bed, reducing effective pulmonary blood flow and systemic oxygen saturation. The vessel may simultaneously decompress the upper-body venous circulation. This dual effect is critical: an apparently undesirable collateral may protect against severe superior vena caval hypertension when pulmonary vascular resistance, pulmonary artery obstruction, or ventricular filling pressure is elevated.

An earlier cohort found systemic venous collaterals in 20.2% of patients after Glenn or Fontan procedures. The vessels were associated with desaturation and reduced ventricular function, and saturation improved in all five patients treated specifically for cyanosis by catheter embolization or surgical closure [7].

Systemic venous-to-pulmonary venous collaterals are particularly prevalent in adult Fontan survivors. In a study of 66 adults, these channels were present in 58%, most commonly arising from the brachiocephalic vein, azygos vein, or superior vena cava. Patients selected for transcatheter closure had lower baseline saturation than untreated patients. At six months, ambulatory saturation increased from 85.6% to 91.8%, with minimal procedural risk reported [9].

10. Evaluation, Test Occlusion, and Closure

Unexpected desaturation after Glenn or Fontan palliation should prompt systematic evaluation for systemic venous collaterals, pulmonary arteriovenous malformations, fenestration flow, pulmonary venous obstruction, residual intracardiac shunting, and low cardiac output. Cross-sectional imaging can define complex paravertebral or mediastinal pathways, but selective venography remains the most direct method for identifying the origin, course, drainage site, and competing venous channels.

Hemodynamic measurements are mandatory before closure. Glenn or Fontan pressure, pulmonary artery pressure, transpulmonary gradient, ventricular end-diastolic pressure, and systemic output should be assessed. Temporary balloon occlusion is valuable when a collateral may be functioning as an essential decompressive route. A marked rise in caval pressure, worsening venous congestion, or reduction in cardiac output argues against immediate permanent closure.

Transcatheter occlusion with coils or vascular plugs is the principal treatment for anatomically suitable vessels causing clinically important desaturation. Surgical closure is reserved for vessels that are inaccessible, very large, or encountered during another operation. The procedure should be accompanied by correction of the underlying substrate whenever possible, including pulmonary artery stenosis, cavopulmonary pathway obstruction, elevated pulmonary vascular resistance, atrioventricular valve regurgitation, ventricular dysfunction, or unfavorable Fontan geometry.

11. Integrated Clinical Interpretation

Systemic-to-pulmonary arterial collaterals add pulmonary flow and increase systemic ventricular preload. Pulmonary arteriovenous malformations permit pulmonary arterial blood to bypass alveolar gas exchange. Systemic venous collaterals remove blood from the effective cavopulmonary pathway or deliver it directly to the systemic side. All can alter oxygen saturation, but only the first typically causes pulmonary overcirculation; the latter two primarily create right-to-left shunting.

The presence of a collateral is therefore a diagnostic clue rather than an automatic indication for closure. The clinician must determine whether the vessel is essential to pulmonary perfusion, compensatory for venous hypertension, or independently responsible for clinically important cyanosis or volume loading. Catheter-based occlusion produces meaningful improvement when the lesion is anatomically suitable and the circulation tolerates removal of the pathway. Durable management, however, requires correction of the pressure gradient, obstruction, hepatic flow exclusion, or ventricular dysfunction that allowed the collateral to develop.

References

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[6] Moore JW, Kirby WC, Madden W, Gaither NS. Development of pulmonary arteriovenous malformations after modified Fontan operations. J Thorac Cardiovasc Surg. 1989. doi:10.1016/S0022-5223(19)34317-X.

[7] Heinemann M, Breuer J, Steger V, Steil E, Sieverding L, Ziemer G. Incidence and impact of systemic venous collateral development after Glenn and Fontan procedures. Thorac Cardiovasc Surg. 2001. doi:10.1055/S-2001-14339.

[8] Nguyen Cong MBH, Schaeffer T, Osawa T, Palm J, Niedermaier C, Piber N, Matsubara M, Heinisch P, Georgiev S, Hager A, Ewert P, Hörer J, Ono M. Systemic venous collaterals between Glenn and Fontan: prevalence, therapy, and impact on outcomes. Cardiol Young. 2025. doi:10.1017/S104795112403662X.

[9] Lluri G, Levi DS, Aboulhosn J. Systemic to pulmonary venous collaterals in adults with single ventricle physiology after cavopulmonary palliation. Int J Cardiol. 2015. doi:10.1016/j.ijcard.2015.04.065.