Fontan Procedure #1: Fontan Circulation

The Fontan procedure is the final stage of palliation for patients with functional single-ventricle physiology. Its purpose is to separate the systemic and pulmonary circulations by directing systemic venous return from the SVC and IVC directly into the pulmonary arteries, without an intervening subpulmonary ventricle. As a result, systemic arterial oxygen saturation improves because pulmonary venous blood becomes the dominant source of ventricular preload and systemic output [1].

However, the Fontan circulation is not a normal biventricular circulation. It is a reconstructed circulation in which pulmonary blood flow is no longer actively pumped by a right ventricle. Instead, blood must pass through the pulmonary vascular bed passively, driven by systemic venous pressure and the transpulmonary pressure gradient. This creates the central paradox of Fontan physiology: adequate venous pressure is required to maintain pulmonary blood flow, but chronically elevated venous pressure also produces systemic venous congestion and long-term end-organ injury [1, 2].

1. Core Concept of Fontan Physiology

In a normal circulation, the right ventricle provides hydraulic energy to move blood through the pulmonary arteries, capillaries, and pulmonary veins. In the Fontan circulation, this subpulmonary pump is absent. Therefore, the pulmonary vascular bed becomes the critical bottleneck between systemic venous return and ventricular preload [1].

For the Fontan circulation to function effectively, three conditions are essential:

1. Low pulmonary vascular resistance

Even a small increase in PVR can reduce pulmonary blood flow and cardiac output.

2. An adequate transpulmonary pressure gradient

Flow depends on the pressure difference between the systemic venous compartment and the pulmonary venous atrium.

3. Unobstructed venous, pulmonary arterial, and pulmonary venous pathways

Any narrowing in the Fontan pathway, branch pulmonary arteries, pulmonary veins, or atrial connection can impair the entire circulation.

Thus, the Fontan circulation should be understood as a pressure-dependent and resistance-sensitive system, rather than simply as an anatomic rerouting procedure.

2. Hemodynamic Principle

The fundamental driving force of Fontan pulmonary blood flow is the gradient from central venous pressure to pulmonary venous atrial pressure. Conceptually, Fontan pulmonary blood flow can be expressed as:

Pulmonary blood flow ≈ (CVP − pulmonary venous atrial pressure) / PVR

This simplified relationship explains several key clinical features:

• Higher CVP may improve forward pulmonary flow, but at the cost of systemic venous congestion.
• Lower PVR facilitates passive pulmonary blood flow and improves ventricular preload.
• Higher ventricular end-diastolic pressure reduces the effective transpulmonary gradient, limiting pulmonary blood flow.
• Pulmonary artery stenosis, pulmonary venous obstruction, or Fontan pathway obstruction increases resistance and may rapidly destabilize the circulation.

In practical terms, the Fontan circulation works best when the pulmonary vascular bed is widely open, low-resistance, and free of anatomic obstruction [1, 3].

3. Surgical Configurations of the Fontan Pathway

Several surgical configurations have been used to connect systemic venous return to the pulmonary arteries.

3.1 Extracardiac Fontan

In the extracardiac Fontan, an external conduit connects the IVC to the pulmonary arteries. The SVC is usually already connected to the pulmonary arteries through a prior bidirectional Glenn procedure.

Potential advantages include:

• Avoidance of extensive atrial suture lines
• Reduced atrial distortion
• Lower risk of atrial dilation compared with older atriopulmonary connections
• A relatively streamlined systemic venous pathway

3.2 Lateral Tunnel Fontan

In the lateral tunnel Fontan, an intra-atrial baffle directs IVC flow through the right atrium toward the pulmonary arteries.

Potential advantages include:

• Use of native atrial tissue
• Technical flexibility in selected anatomy
• Easier creation of a fenestration when needed

3.3 Atriopulmonary Connection

The atriopulmonary Fontan connects the right atrium directly to the pulmonary artery. Although historically important, it is now rarely used because long-term right atrial dilation, energy loss, thrombosis, and atrial arrhythmias are common limitations [2, 4].

4. Why Oxygen Saturation Improves

Before Fontan completion, systemic venous blood and pulmonary venous blood often mix within the single-ventricle circulation, producing systemic cyanosis. After Fontan completion, systemic venous blood is routed directly to the pulmonary arteries, while oxygenated pulmonary venous blood returns to the atrium and becomes the main source of ventricular preload.

Therefore, the immediate physiologic benefit of Fontan completion is improved systemic arterial oxygen saturation.

However, postoperative oxygen saturation is not always completely normal. Mild desaturation may persist because of:

• Fontan fenestration
• Venovenous collaterals
• Pulmonary arteriovenous malformations
• Residual intracardiac or extracardiac shunting
• Uneven hepatic venous flow distribution to the pulmonary arteries

Thus, near-normal oxygen saturation represents the ideal physiologic goal, but actual saturation depends on the presence of fenestration, collateral burden, pulmonary vascular physiology, and pulmonary arteriovenous malformation risk [4].

5. The Single Ventricle After Fontan Completion

After Fontan completion, the single ventricle is relieved from the volume overload that characterizes earlier stages of palliation. This may improve ventricular efficiency and reduce cyanosis. However, the ventricle becomes chronically preload-limited because systemic venous return must first traverse the pulmonary circulation passively [1].

This creates several important constraints:

1. Reduced preload reserve

The ventricle cannot easily increase filling during exercise, fever, dehydration, or stress.

2. Dependence on low filling pressure

Elevated ventricular end-diastolic pressure reduces the transpulmonary gradient and impairs Fontan flow.

3. Sensitivity to AV valve regurgitation

AV valve regurgitation raises atrial pressure, reduces effective forward flow, and worsens venous congestion.

4. Limited exercise capacity

Cardiac output augmentation is constrained because pulmonary blood flow cannot be actively increased by a subpulmonary ventricle.

For this reason, many Fontan patients appear stable at rest but have limited physiologic reserve during illness, exercise, arrhythmia, dehydration, or perioperative stress [1, 4].

6. Pulmonary Vascular Resistance as the Central Determinant

Pulmonary vascular resistance is one of the most important determinants of Fontan performance. Because there is no right ventricle to overcome resistance, even modest increases in PVR can reduce pulmonary blood flow, ventricular preload, and systemic cardiac output [1, 3].

Clinically important causes of increased PVR include:

• Hypoxia
• Hypercarbia
• Acidosis
• Atelectasis
• Positive-pressure ventilation
• Pulmonary artery stenosis
• Pulmonary venous obstruction
• Elevated ventricular filling pressure
• Chronic pulmonary vascular remodeling

In Fontan physiology, the lungs are not only the organ of gas exchange; they are the main resistor between systemic venous return and ventricular preload. This is why perioperative and long-term management focus heavily on maintaining low PVR, avoiding acidosis and hypercarbia, optimizing ventilation, preserving pulmonary artery growth, and treating anatomic obstruction [3, 4].

7. The Fontan Circulation as a Chronic Venous Pressure–Loaded System

The Fontan circulation requires chronically elevated systemic venous pressure to drive blood through the pulmonary vascular bed. This is fundamentally different from normal physiology, where venous pressure remains low and the right ventricle supplies the pulmonary driving force.

This creates a narrow hemodynamic window:

• If venous pressure is too low, pulmonary blood flow and cardiac output fall.
• If venous pressure is too high, systemic venous congestion progresses.

Over time, this chronic venous pressure load can affect multiple organ systems, including the liver, lymphatic system, gastrointestinal tract, kidneys, coagulation system, and pulmonary vasculature [4, 5]. Therefore, Fontan physiology should be viewed as a systemic circulatory condition, not merely as a congenital cardiac repair.

8. Lymphatic Failure and Multiorgan Complications

A major modern concept in Fontan physiology is the central role of the lymphatic system. Chronically elevated systemic venous pressure impairs lymphatic drainage and increases lymphatic afterload. When lymphatic compensation fails, several characteristic Fontan complications may develop [4, 5].

Important lymphatic and venous congestion–related complications include:

1. Pleural effusions and chylothorax

Elevated venous and lymphatic pressure can impair postoperative drainage and delay recovery.

2. Protein-losing enteropathy

Intestinal lymphatic congestion and abnormal lymphatic flow contribute to enteric protein loss, edema, immune dysfunction, and malnutrition.

3. Plastic bronchitis

Abnormal lymphatic leakage into the airways can produce obstructive bronchial casts.

4. Fontan-associated liver disease

Chronic hepatic venous congestion can lead to fibrosis, cirrhosis, portal hypertension, and, in selected patients, hepatocellular carcinoma risk.

5. Peripheral edema and ascites

These findings reflect systemic venous congestion and impaired lymphatic compensation.

These complications demonstrate that the Fontan circulation is not a static surgical endpoint. It is a lifelong physiology with progressive circulatory, lymphatic, and end-organ consequences [4, 5].

9. Fontan Failure: A Progressive and Multifactorial Process

Fontan failure is not caused by a single mechanism. It may result from ventricular dysfunction, elevated PVR, AV valve regurgitation, arrhythmia, Fontan pathway obstruction, pulmonary artery distortion, lymphatic failure, or progressive multiorgan disease [2, 4].

Major mechanisms include:

• Systolic or diastolic ventricular dysfunction
• Elevated ventricular end-diastolic pressure
• AV valve regurgitation
• Pulmonary artery stenosis or distortion
• Elevated PVR
• Atrial or ventricular arrhythmia
• Fontan pathway obstruction
• Venovenous collaterals or aortopulmonary collaterals
• Protein-losing enteropathy or plastic bronchitis
• Fontan-associated liver disease

Long-term survival after Fontan surgery has improved substantially, but late morbidity remains significant. Meta-analytic data suggest that most patients who survive the early postoperative period remain alive at 20 years; however, late sequelae such as protein-losing enteropathy, ventricular dysfunction, and need for pacing are associated with higher late mortality risk [6]. This reinforces the need for lifelong surveillance rather than viewing Fontan completion as definitive cure.

10. Practical Surgical and Clinical Implications

For surgeons and perioperative teams, a successful Fontan circulation requires both favorable anatomy and favorable physiology.

Key requirements include:

1. Unobstructed Fontan pathway

The SVC, IVC, conduit, baffle, and cavopulmonary connections must remain widely patent.

2. Well-developed pulmonary arteries

Branch pulmonary artery stenosis or distortion increases resistance and reduces pulmonary blood flow.

3. Low pulmonary vascular resistance

The pulmonary vascular bed must accept passive systemic venous flow.

4. Good ventricular systolic and diastolic function

The ventricle must receive adequate preload at low filling pressure.

5. Minimal AV valve regurgitation

AV valve regurgitation increases atrial pressure and reduces forward Fontan flow.

6. Controlled collateral circulation

Venovenous collaterals may cause desaturation, while aortopulmonary collaterals may increase volume load.

7. Preserved rhythm stability

Sinus rhythm and coordinated atrioventricular filling are important for maintaining ventricular preload and cardiac output.

11. Take-Home Message

The Fontan procedure transforms a cyanotic single-ventricle circulation into a circulation with improved systemic oxygenation. However, this improvement is achieved by removing the volume load from the single ventricle and replacing the subpulmonary pump with a passive venous pathway.

The essential feature of Fontan physiology is therefore a hemodynamic bottleneck: systemic venous blood must cross the pulmonary vascular bed without ventricular assistance. This makes the circulation highly dependent on low PVR, low ventricular filling pressure, unobstructed pulmonary pathways, and adequate systemic venous pressure [1–4].

The Fontan circulation is efficient only within a narrow physiologic range. When that range is exceeded, the patient becomes vulnerable to low cardiac output, venous congestion, lymphatic failure, arrhythmia, ventricular dysfunction, and multiorgan complications. Understanding Fontan physiology therefore requires moving beyond the surgical diagram and recognizing the circulation as a lifelong pressure-dependent system with limited reserve.

References

[1] Gewillig M, Brown SC. The Fontan circulation after 45 years: update in physiology. Heart. 2016;102(14):1081-1086.

[2] Gewillig M, Goldberg DJ. Failure of the Fontan circulation. Heart Fail Clin. 2014;10(1):105-116.

[3] Hauck A, Porta N, Lestrud S, Berger S. The pulmonary circulation in the single ventricle patient. Children (Basel). 2017;4(8):71.

[4] Rychik J, Atz AM, Celermajer DS, Deal BJ, Gatzoulis MA, Gewillig MH, Hsia TY, Hsu DT, Kovacs AH, McCrindle BW, Newburger JW, Pike NA, Rodefeld M, Rosenthal DN, Schumacher KR, Marino BS, Stout K, Veldtman G, Younoszai AK, d'Udekem Y. Evaluation and management of the child and adult with Fontan circulation: a scientific statement from the American Heart Association. Circulation. 2019;140(6):e234-e284.

[5] Mazza GA, Gribaudo E, Agnoletti G. The pathophysiology and complications of Fontan circulation. Acta Biomed. 2021;92(5):e2021260.

[6] Poh CL, d'Udekem Y. Life after surviving Fontan surgery: a meta-analysis of the incidence and predictors of late death. Heart Lung Circ. 2018;27(5):552-559.