Lymphatic Circulation — Anatomy, Flow, and Failure

The lymphatic system is a low-pressure circulatory network that maintains interstitial fluid balance, transports dietary lipids, participates in immune surveillance, and returns protein-rich lymph to the systemic venous circulation. In patients with Fontan physiology, this system becomes particularly vulnerable because lymph must drain into a venous circuit that is chronically hypertensive. As a result, the lymphatic system represents one of the major weak points of the Fontan circulation and is increasingly recognized as a central contributor to late Fontan morbidity [1, 2].

1. Normal Lymphatic Circulation: A Low-Pressure Return System

Under normal conditions, plasma filtrate continuously leaves the systemic capillaries and enters the interstitial space. Most of this fluid returns directly to the venous side of the microcirculation, while the remaining fraction is collected by lymphatic capillaries. This lymphatic fraction is relatively small in volume but physiologically essential because it contains proteins, lipids, immune cells, and excess interstitial fluid that cannot be adequately cleared by venous reabsorption alone.

Classically, approximately 85–90% of interstitial fluid returns through the venous system, while the remaining 10–15% is cleared through the lymphatic circulation. After entering the lymphatic capillaries, lymph passes through regional lymph nodes, converges toward larger collecting lymphatics, and eventually drains into the systemic venous circulation.

The central lymphatic pathway is organized around the cisterna chyli and thoracic duct. Lymph from the lower body and abdominal organs converges into the cisterna chyli, then ascends through the thoracic duct. The thoracic duct usually courses through the posterior mediastinum, crosses toward the left side around the upper thoracic level, and empties into the left venous angle, near the junction of the left internal jugular and left subclavian veins [3].

Key anatomical points

1. Peripheral lymphatic capillaries collect protein-rich interstitial fluid.
2. Collecting lymphatic vessels propel lymph through intrinsic contraction and one-way valves.
3. Regional lymph nodes filter lymph and support immune function.
4. The cisterna chyli receives lymph from the abdomen and lower extremities.
5. The thoracic duct is the major central lymphatic conduit.
6. The left venous angle is the usual final drainage site into the systemic venous circulation.

2. What Drives Lymph Flow?

The lymphatic system does not have a single central pump equivalent to the heart. Instead, lymph flow depends on several weak but coordinated mechanisms.

Major mechanisms of lymph propulsion

• Intrinsic lymphatic contraction

Collecting lymphatic vessels contain smooth muscle and contract rhythmically, functioning as serial micro-pumps.

• One-way lymphatic valves

Valves prevent retrograde flow and allow stepwise forward movement toward the venous circulation.

• Skeletal muscle pump

Limb movement and muscle contraction compress lymphatic vessels and promote central lymph return.

• Arterial pulsation

Pulsation from adjacent arteries provides additional external compression.

• Respiratory pump

Negative intrathoracic pressure during inspiration facilitates upward lymph movement through the thoracic duct.

• Low downstream venous pressure

Effective lymphatic drainage requires a favorable pressure gradient from the lymphatic system into the central venous system.

The final point is critical. Lymphatic return is highly pressure-sensitive. Even if lymph production and intrinsic lymphatic contractility are preserved, drainage becomes inefficient when the venous pressure at the lymphatic outlet is elevated. Therefore, the lymphatic system depends not only on its own pumping capacity, but also on a low-pressure venous endpoint.

3. Fontan Circulation: A Hemodynamic Stress Test for the Lymphatic System

Fontan circulation separates systemic and pulmonary venous return without a subpulmonary ventricle. Systemic venous blood flows directly into the pulmonary arteries, and pulmonary blood flow depends on a passive pressure gradient across the pulmonary vascular bed. This circulation requires relatively elevated systemic venous pressure to maintain pulmonary blood flow.

This creates a fundamental conflict:

Fontan physiology requires elevated venous pressure to drive pulmonary blood flow, whereas lymphatic return requires low downstream venous pressure.

In other words, the same hemodynamic condition that allows the Fontan pathway to function also impairs lymphatic drainage. Chronically elevated central venous pressure increases capillary filtration and lymph production while simultaneously reducing the pressure gradient for lymphatic emptying. Over time, this mismatch can overload the thoracic duct and central lymphatic channels, resulting in lymphatic congestion, abnormal lymphatic collateralization, and leakage into low-pressure compartments [1, 2].

4. Pathophysiology of Lymphatic Failure in Fontan Circulation

Fontan-associated lymphatic failure can be understood as a mismatch between lymphatic load and lymphatic drainage capacity.

4.1 Increased lymphatic load

Elevated systemic venous pressure increases capillary hydrostatic pressure. This promotes movement of fluid from the intravascular space into the interstitium, increasing the amount of fluid that must be cleared by the lymphatic system.

4.2 Impaired central lymphatic drainage

Because the thoracic duct ultimately drains into the systemic venous circulation, elevated Fontan pressure creates an unfavorable downstream pressure. Lymphatic vessels may continue to generate forward flow, but their outlet is congested.

4.3 Loss of effective pressure gradient

Normal lymphatic return benefits from low venous pressure, respiratory variation, and cyclic changes in cardiac filling. In Fontan physiology, these favorable pressure conditions are blunted. The lymphatic system therefore faces a chronically elevated outflow pressure.

4.4 Abnormal lymphatic decompression

When central lymphatic pressure rises, lymph may decompress through abnormal channels into adjacent low-pressure spaces. Depending on the anatomical route of leakage, this may produce gastrointestinal, pulmonary, pleural, or systemic manifestations.

5. Clinical Manifestations of Fontan-Associated Lymphatic Failure

Fontan-associated lymphatic disease is not a single clinical entity. It represents a spectrum of disorders caused by lymphatic overload, abnormal lymphatic anatomy, and impaired lymphatic drainage.

5.1 Protein-Losing Enteropathy

Protein-losing enteropathy occurs when protein-rich lymph leaks into the gastrointestinal tract. This results in hypoalbuminemia, edema, ascites, diarrhea, malabsorption, immune dysfunction, and nutritional compromise.

PLE after Fontan operation is reported in approximately 4–13% of patients, although estimates vary by cohort, definition, and duration of follow-up [4, 5]. The mechanism is multifactorial and includes elevated central venous pressure, lymphatic congestion, abnormal intestinal lymphatic drainage, altered mesenteric circulation, epithelial barrier dysfunction, inflammation, and immune disturbance [5].

Clinically, PLE should not be viewed as an isolated intestinal problem. It is often a systemic manifestation of Fontan circulatory stress and lymphatic failure.

5.2 Plastic Bronchitis

Plastic bronchitis is characterized by formation of branching bronchial casts that may obstruct the airways. In Fontan patients, these casts are often related to abnormal pulmonary lymphatic flow with leakage of protein-rich lymphatic material into the bronchial tree.

Plastic bronchitis is less common than PLE but can be acutely life-threatening. It may present with cough, wheezing, hypoxemia, recurrent respiratory distress, or expectoration of bronchial casts. The shared pathophysiologic basis of plastic bronchitis and PLE is increasingly understood as lymphatic disruption caused by elevated central venous pressure, increased lymph production, inflammation, and abnormal drainage into low-pressure circuits such as the airways or duodenum [4].

5.3 Chylothorax

Chylothorax occurs when chyle accumulates in the pleural space. In the Fontan setting, chylothorax may result from elevated central lymphatic pressure, abnormal thoracic duct anatomy, postoperative lymphatic injury, or abnormal lymphatic channels that decompress into the pleural cavity [6, 7].

Persistent chylothorax is clinically important because it may cause respiratory compromise, malnutrition, lymphocyte depletion, immunologic vulnerability, electrolyte disturbance, and prolonged hospitalization.

6. Imaging: Making the Lymphatic System Visible

Historically, lymphatic complications were difficult to understand because the lymphatic system was largely invisible with conventional cardiovascular imaging. This has changed substantially with modern lymphatic imaging.

Major imaging modalities

• T2-weighted MR lymphatic mapping

Provides noncontrast visualization of lymphatic anatomy and lymphatic congestion.

• Intranodal lymphangiography

Allows contrast injection through inguinal lymph nodes to visualize central lymphatic flow.

• Dynamic contrast-enhanced MR lymphangiography

Demonstrates lymphatic flow dynamics and abnormal reflux or leakage pathways.

• Intrahepatic and intramesenteric lymphangiography

Helps identify hepatoduodenal, mesenteric, and intestinal lymphatic abnormalities relevant to PLE.

• Conventional fluoroscopic lymphangiography

Can be combined with interventional procedures such as embolization.

These techniques have shifted Fontan-associated lymphatic disease from a largely inferred complication to a visible, anatomically definable, and sometimes treatable disorder [6–9].

7. Interventional and Therapeutic Perspective

Management of lymphatic complications should begin with a full reassessment of Fontan hemodynamics. Important targets include:

1. Fontan pathway obstruction
2. Branch pulmonary artery stenosis
3. Elevated pulmonary vascular resistance
4. Ventricular dysfunction
5. Atrioventricular valve regurgitation
6. Arrhythmia
7. Loss of fenestration or inadequate decompression
8. Elevated systemic venous pressure
9. Abnormal lymphatic anatomy or flow

In selected patients, lymphatic-directed therapy may be required. Contemporary strategies include selective lymphatic duct embolization, thoracic duct embolization, liver lymphatic embolization, and thoracic duct decompression. These approaches aim either to occlude abnormal lymphatic leakage pathways or to decompress the lymphatic system into a lower-pressure compartment [6, 8–10].

Early reports and small series suggest that these interventions can improve symptoms in selected patients with plastic bronchitis, PLE, or chylothorax. However, the current evidence base remains limited. Much of the available literature consists of case reports, small retrospective series, and expert-center experience. Long-term durability, patient selection, timing of intervention, and integration with Fontan revision or transplantation pathways remain areas of active investigation [9, 10].

8. Conceptual Summary

The lymphatic system normally functions as a low-pressure return pathway. It clears protein-rich interstitial fluid, transports lipids, filters lymph through lymph nodes, and returns lymph to the systemic venous circulation.

Fontan physiology challenges this system at its most vulnerable point: the venous outlet. Chronically elevated central venous pressure increases lymphatic production while impairing central lymphatic drainage. When lymphatic load exceeds lymphatic reserve, lymph may decompress through abnormal pathways, producing protein-losing enteropathy, plastic bronchitis, or chylothorax.

Therefore, Fontan failure should not be understood only as ventricular failure, valve failure, arrhythmia, or pathway obstruction. It may also represent lymphatic failure under chronic venous hypertension.

References

[1] Menon S, Chennapragada M, Ugaki S, Sholler GF, Ayer J, Winlaw D. The lymphatic circulation in adaptations to the Fontan circulation. Pediatr Cardiol. 2017;38(5):886-892.

[2] Kreutzer C, Kreutzer G. The lymphatic system: the Achilles heel of the Fontan-Kreutzer circulation. World J Pediatr Congenit Heart Surg. 2017;8(5):613-623.

[3] Sung C, Bass JL, Berry JM, Shepard CW, Lindgren B, Kochilas LK. The thoracic duct and the Fontan patient. Echocardiography. 2017;34(9):1347-1352.

[4] Mackie AS, Veldtman GR, Thorup L, Hjortdal VE, Dori Y. Plastic bronchitis and protein-losing enteropathy in the Fontan patient: evolving understanding and emerging therapies. Can J Cardiol. 2022;38(7):988-1001.

[5] Alsaied T, Lubert A, Goldberg DJ, Schumacher K, Rathod RH, Katz DA, Opotowsky AR, Jenkins M, Smith CL, Rychik J, Amdani S, Lanford LM, Cetta F, Kreutzer C, Feingold B, Goldstein BH. Protein losing enteropathy after the Fontan operation. Int J Cardiol Congenit Heart Dis. 2022;7:100338.

[6] Itkin M, Pizarro C, Radtke W, Spurrier E, Rabinowitz DA. Lymphatic management in single-ventricle patients. Semin Thorac Cardiovasc Surg Pediatr Card Surg Annu. 2020;23:41-47.

[7] Dori Y, Smith CL. Lymphatic disorders in patients with single ventricle heart disease. Front Pediatr. 2022;10:828107.

[8] Roché Rodríguez M, Dinardo JA. The lymphatic system in the Fontan patient-pathophysiology, imaging, and interventions: what the anesthesiologist should know. J Cardiothorac Vasc Anesth. 2022;36(1):225-231.

[9] Bauer C, Dori Y, Scala M, Tulzer A, Tulzer G. Current diagnostic and therapeutic strategies for the management of lymphatic insufficiency in patients with hypoplastic left heart syndrome. Front Pediatr. 2023;11:1058567.

[10] Bauer C, Mair R, Mair R, Tulzer G. Thoracic duct decompression and jugular vein banding—an effective treatment option for protein-losing enteropathy and plastic bronchitis in severe failing Fontan circulation: a case report. Eur Heart J Case Rep. 2020;4(6):1-6.