Fenestrated Fontan #1: Concept and Physiology

1. Overview

The Fontan operation establishes a circulation in which systemic venous return reaches the pulmonary arteries without a subpulmonary ventricle. Superior and inferior vena caval blood are directed into the pulmonary vascular bed, while the single systemic ventricle receives pulmonary venous return and ejects into the systemic circulation. The operation converts a parallel single-ventricle circulation into a series circulation, but places the pulmonary vascular bed directly between systemic venous return and ventricular preload.

This arrangement is intrinsically energy-limited. Pulmonary blood flow is passive and depends on a favorable pressure gradient from the systemic veins to the pulmonary veins and atrium. The Fontan pathway therefore creates a physiologic bottleneck characterized by upstream systemic venous hypertension and reduced downstream ventricular filling. The systemic ventricle remains the pump, but cannot independently draw sufficient blood across a restrictive pulmonary vascular bed [1].

A Fontan fenestration is an intentionally created communication between the Fontan pathway and the atrium. It permits a controlled right-to-left shunt and functions as a pressure-responsive “safety valve” or “pop-off,” decompressing the Fontan circuit and preserving ventricular preload in selected patients.

2. Hemodynamics of the Non-Fenestrated Fontan

In a non-fenestrated Fontan circulation, nearly all systemic venous return must cross the pulmonary vascular bed before reaching the heart. Pulmonary and systemic blood flow are closely coupled, and ventricular filling is limited by the amount of blood that can traverse the pulmonary circuit.

The relationship can be conceptualized as:

Pulmonary blood flow ≈ (Fontan pressure − atrial pressure) / pulmonary vascular resistance

A rise in pulmonary vascular resistance requires a higher systemic venous pressure to maintain flow. An increase in pulmonary venous or atrial pressure similarly reduces the transpulmonary pressure gradient. Elevated atrial pressure may result from ventricular diastolic dysfunction, atrioventricular valve regurgitation, pulmonary venous obstruction, or elevated ventricular filling pressure.

When the pulmonary pathway becomes restrictive, ventricular preload and cardiac output fall while systemic venous pressure rises. The consequences include hepatic congestion, ascites, pleural effusions, lymphatic dysfunction, edema, renal impairment, and impaired intestinal perfusion. Complete separation of systemic and pulmonary venous blood therefore comes without a direct decompression route when passive pulmonary blood flow is inadequate.

3. Anatomical Construction and Flow Direction

The fenestration connects the Fontan pathway with the atrium receiving pulmonary venous return. In a lateral tunnel Fontan, it is created through the intracardiac baffle. In an extracardiac conduit Fontan, a communication is established between the conduit and the atrium, either directly or through a short interposed connection.

Because Fontan pathway pressure normally exceeds atrial pressure, flow is predominantly right-to-left. Its magnitude depends on the pressure difference, fenestration diameter, channel geometry, blood viscosity, and atrial pressure. Shunt flow increases when Fontan pressure rises and may become minimal when pulmonary blood flow is efficient.

The opening must be sufficiently patent to provide meaningful decompression but sufficiently restrictive to avoid excessive cyanosis. There is no universally optimal diameter because the required decompression depends on patient size, pulmonary resistance, ventricular filling characteristics, and the intended duration of fenestration.

4. Mechanism of Hemodynamic Benefit

The principal benefit is the creation of a low-resistance pathway in parallel with the pulmonary vascular bed. A portion of systemic venous return bypasses the lungs and enters the atrium directly. This reduces the effective resistance faced by venous return, lowers systemic venous pressure, and increases the blood volume available to fill the systemic ventricle.

The immediate effects are:

1. Decompression of the Fontan pathway.
2. Preservation of ventricular preload.
3. Support of systemic cardiac output.
4. Reduction of venous and lymphatic congestion.

Computational modeling calibrated to clinical Fontan data demonstrates the central trade-off: increasing fenestration flow can increase cardiac output and decrease systemic venous pressure, but lowers arterial oxygen saturation. In some high-risk physiologic states, a range of fenestration sizes may reduce venous pressure while maintaining relatively stable systemic oxygen availability [2].

In a fenestrated Fontan, systemic blood flow may exceed effective pulmonary blood flow by the amount of right-to-left shunt. The ventricle can therefore deliver a higher output than would be possible if all venous return were forced through a restrictive pulmonary vascular bed.

5. Oxygen Saturation Versus Oxygen Delivery

The predictable cost is arterial oxygen desaturation. Systemic venous blood crosses into the atrium and mixes with pulmonary venous blood without undergoing gas exchange. The resulting saturation depends on the shunt fraction, systemic venous saturation, pulmonary venous saturation, and total cardiac output.

Arterial saturation should not be interpreted as the sole measure of circulatory adequacy:

Systemic oxygen delivery = cardiac output × arterial oxygen content

A patient with a functioning fenestration may have lower saturation but higher cardiac output. Tissue oxygen delivery may therefore be maintained despite cyanosis. Conversely, eliminating the shunt may increase saturation while reducing preload and systemic output. The goal is adequate oxygen delivery with acceptable systemic venous pressure, not maximal saturation at any cost.

Excessive shunting may produce clinically important cyanosis, reduce effective pulmonary blood flow, promote secondary erythrocytosis, increase blood viscosity, and impair exercise capacity. Persistent right-to-left communication also creates a potential pathway for paradoxical systemic embolization.

6. Patient Selection

Fenestration is most relevant when the newly completed Fontan circulation is expected to function near its hemodynamic limit. High-risk features include elevated pulmonary vascular resistance, small or distorted pulmonary arteries, elevated pulmonary artery pressure, ventricular systolic or diastolic dysfunction, elevated filling pressure, and moderate-to-severe atrioventricular valve regurgitation.

Elevated pulmonary vascular resistance directly restricts passive pulmonary flow and raises the systemic venous pressure required to maintain preload. Small or distorted pulmonary arteries may produce additional resistance and energy loss. Ventricular diastolic dysfunction or atrioventricular valve regurgitation raises atrial pressure, reducing the effective gradient across the pulmonary vascular bed. In each setting, the fenestration provides an alternative pathway for venous return and reduces the hemodynamic consequences of this bottleneck.

Historical strategies have considered a mean pulmonary arterial pressure of approximately 15 mmHg or higher and moderate-to-severe atrioventricular valve regurgitation as markers favoring fenestration. Practice varies, and some centers have extended fenestration beyond conventionally high-risk patients because of favorable early postoperative effects [3,4].

Fenestration should not conceal a surgically correctable lesion. Branch pulmonary artery stenosis, pulmonary venous obstruction, conduit obstruction, severe atrioventricular valve regurgitation, or major ventricular dysfunction must be treated directly. Fenestration modifies the pressure-flow relationship; it does not correct the underlying abnormality.

7. Early Postoperative Outcomes

The early postoperative period is the phase in which fenestration provides its clearest clinical benefit. After cardiopulmonary bypass, pulmonary vascular resistance may be transiently elevated, ventricular compliance reduced, and positive-pressure ventilation may impede passive pulmonary blood flow. A functioning fenestration allows greater tolerance of these temporary abnormalities.

The strongest prospective evidence comes from a randomized trial of 49 standard-risk Fontan patients, with 25 assigned to fenestration and 24 to a non-fenestrated pathway. Fenestration was associated with 55% less total chest tube drainage, a 41% shorter total hospital stay, and 67% fewer additional postoperative procedures [3]. These findings support venous decompression even in patients without extreme preoperative risk.

The likely mechanisms include lower systemic venous pressure, improved lymphatic drainage, reduced pleural fluid formation, and preserved ventricular filling. Clinically, this may translate into fewer pleural effusions, reduced ascites, improved renal and hepatic perfusion, and a lower probability of postoperative low cardiac output.

These benefits do not eliminate the need for meticulous management. Pulmonary vascular resistance should be minimized through adequate oxygenation, ventilation, lung recruitment, acid-base control, analgesia, and avoidance of excessive mean airway pressure. Sinus rhythm and atrioventricular synchrony should be maintained whenever possible.

8. Assessment of Fenestration Function

Postoperative evaluation should determine whether the fenestration is patent, appropriately restrictive, or excessively large. Echocardiography can demonstrate flow direction and estimate the pressure gradient. It should also assess pathway obstruction, branch pulmonary artery anatomy, ventricular function, atrioventricular valve regurgitation, and pulmonary venous flow.

Catheterization may be required when the clinical course suggests elevated Fontan pressure, inadequate output, excessive cyanosis, or a reversible obstruction. Measurements should include Fontan pressure, pulmonary artery pressure, atrial pressure, ventricular end-diastolic pressure, systemic output, and oxygen saturations. Angiography may identify conduit narrowing, pulmonary artery distortion, collateral vessels, or pulmonary venous obstruction.

Failure to decompress may indicate a small, obstructed, or compressed fenestration. Excessive shunting may reflect a large communication or significant downstream restriction forcing blood through the fenestration.

9. Fenestration Closure

Closure is considered when the early adaptive period has passed and the circulation appears capable of maintaining pulmonary blood flow without a pop-off. Closure may improve arterial saturation, but removes the low-resistance pathway supporting preload.

In 14 patients studied after fenestrated Fontan completion, temporary occlusion increased arterial saturation but decreased systemic blood flow and systemic oxygen transport, while oxygen extraction increased [5]. Improved saturation therefore does not necessarily indicate improved global physiology.

During test occlusion, changes in Fontan pressure, atrial pressure, systemic output, arterial saturation, and oxygen delivery should be assessed. A marked rise in venous pressure or important fall in output argues against closure. The decision should be based on integrated hemodynamics rather than cyanosis alone.

Transcatheter closure is commonly used when the fenestration is no longer required. Catheter-based treatment can also address Fontan pathway stenosis, pulmonary artery obstruction, collateral vessels, and other residual lesions that influence whether closure is tolerated [6].

10. Long-Term Implications

The long-term benefit of routine fenestration remains uncertain. Available studies consistently support lower early postoperative morbidity, but have not established improved overall survival or freedom from Fontan failure. A narrative review of nine studies, including eight primary studies and one meta-analysis, found no consistent survival advantage or reduction in late Fontan failure. Persistent arterial oxygen desaturation was the most reproducible long-term finding, while evidence concerning thromboembolism and Fontan-associated liver disease was mixed [7].

Fenestration is therefore best regarded as an early hemodynamic risk-modification strategy rather than a proven intervention for preventing late Fontan failure. Late outcomes remain dominated by chronically elevated systemic venous pressure, limited preload reserve, low cardiac output, ventricular and valvar dysfunction, pulmonary vascular abnormalities, arrhythmia, lymphatic failure, and end-organ injury.

A fenestration may remain valuable in a borderline circulation, but persistent patency should be periodically reassessed. Closure may be appropriate when hemodynamics are favorable; continued patency may be preferable when the communication remains necessary to preserve output or limit venous hypertension.

11. Practical Principles

1. A Fontan fenestration is an intentional right-to-left shunt from the Fontan pathway to the atrium.
2. Its primary purpose is to lower systemic venous pressure and preserve ventricular preload and cardiac output.
3. Its benefit is obtained at the cost of lower arterial oxygen saturation.
4. Early advantages include reduced venous congestion, pleural drainage, additional procedures, and hospital stay.
5. Fenestration is most relevant with elevated pulmonary resistance, impaired ventricular filling, small pulmonary arteries, or significant atrioventricular valve regurgitation.
6. It should not substitute for correction of anatomical obstruction or severe valve and ventricular dysfunction.
7. Oxygen delivery, not arterial saturation alone, should guide interpretation.
8. Closure requires comprehensive anatomical and hemodynamic assessment.
9. Long-term survival and Fontan-failure benefits remain unproven.

Conclusion

The fenestrated Fontan deliberately accepts controlled cyanosis to improve the pressure-flow relationship of the Fontan circulation. By providing a parallel low-resistance pathway from the Fontan circuit to the atrium, it lowers systemic venous pressure, preserves ventricular preload, and supports cardiac output when pulmonary blood flow is restricted or ventricular filling is fragile.

The strongest evidence supports improved early postoperative recovery, including reduced chest tube drainage, shorter hospitalization, and fewer additional procedures. The cost is persistent arterial desaturation, and long-term superiority over a non-fenestrated Fontan has not been demonstrated. Fenestration should therefore be used as a targeted hemodynamic strategy, with later closure determined by integrated assessment of venous pressure, cardiac output, oxygen delivery, and underlying anatomy.

References

[1] Gewillig M, Brown SC. The Fontan circulation after 45 years: update in physiology. Heart. 2016;102(14):1081-1086. doi:10.1136/heartjnl-2015-307467.

[2] Ahmad Z, Jin L, Penny DJ, Rusin CG, Peskin CS, Puelz C. Optimal fenestration of the Fontan circulation. Front Physiol. 2022;13:867995. doi:10.3389/fphys.2022.867995.

[3] Lemler MS, Scott WA, Leonard SR, Stromberg D, Ramaciotti C. Fenestration improves clinical outcome of the Fontan procedure: a prospective, randomized study. Circulation. 2002;105(2):207-212. doi:10.1161/HC0202.102237.

[4] Corno AF, Koerner TS, Salazar JD. The pendulum of Fontan fenestration. Transl Pediatr. 2023. doi:10.21037/tp-22-562.

[5] Hijazi ZM, Fahey JT, Kleinman CS, Kopf GS, Hellenbrand WE. Hemodynamic evaluation before and after closure of fenestrated Fontan: an acute study of changes in oxygen delivery. Circulation. 1992;86(1):196-202. doi:10.1161/01.CIR.86.1.196.

[6] Jalal Z, Gewillig M, Boudjemline Y, Guérin P, Pilati M, Butera G, Malekzadeh-Milani S, Avesani M, Thambo JB. Transcatheter interventions in patients with a Fontan circulation: current practice and future developments. Front Pediatr. 2022;10:965989. doi:10.3389/fped.2022.965989.

[7] Nowakowski M, Gwałt P, Musioł A, Lukasz K, Marzec J, Rakuś M. Does Fontan fenestration improve long-term outcomes in single-ventricle patients? A narrative review of survival, Fontan failure, thromboembolic risk, and FALD. Int J Curr Res Rev. 2026. doi:10.31782/ijcrr.2026.18502.