Single Ventricle Palliation #2: From Norwood to Fontan Circulation

Single-ventricle palliation is not simply a sequence of operations but a staged transformation of the circulation. In the neonatal circulation, the single systemic ventricle supports both systemic and pulmonary blood flow in parallel. The superior cavopulmonary connection then removes a substantial portion of pulmonary blood flow from the ventricular workload, placing the lungs between the superior vena cava (SVC) and the heart. Fontan completion ultimately incorporates the inferior vena caval return and establishes a total cavopulmonary circulation in which essentially all systemic venous blood reaches the pulmonary arteries without an interposed subpulmonary ventricle.

The fundamental trade-off is progressive separation of systemic and pulmonary blood flow at the cost of progressively greater dependence on passive transpulmonary flow. Stage I is therefore predominantly a problem of Qp/Qs balance; the Glenn circulation is strongly dependent on SVC flow and pulmonary vascular resistance (PVR); and the Fontan circulation is ultimately governed by the interaction among systemic venous pressure, PVR, pulmonary venous/atrial pressure, and ventricular filling.

1. Stage I: Establishing a Stable Parallel Circulation

For patients with hypoplastic left heart syndrome (HLHS) and related lesions with single-ventricle physiology and systemic outflow obstruction, Stage I palliation must accomplish three essential objectives: provide unobstructed systemic outflow, ensure a controlled source of pulmonary blood flow, and establish unrestricted atrial-level communication.

The Norwood operation reconstructs the systemic outflow by connecting the pulmonary root to the systemic arterial pathway and augmenting the hypoplastic aortic arch. Because native antegrade pulmonary flow is thereby interrupted or substantially altered, pulmonary blood flow must be supplied through a separate controlled source, traditionally either a modified Blalock–Taussig–Thomas shunt or a right ventricle-to-pulmonary artery (RV–PA) conduit. Adequate atrial communication is equally important because pulmonary venous return must reach the systemic ventricle without restriction.

Despite the surgical reconstruction, Stage I physiology remains fundamentally a parallel circulation. The systemic ventricle receives combined systemic and pulmonary venous return and must generate both systemic blood flow (Qs) and pulmonary blood flow (Qp):

COₛᵥ ≈ Qs + Qp

Accordingly, pulmonary blood flow remains part of the total ventricular output. Excessive Qp therefore does not simply improve oxygenation; it diverts a larger fraction of available cardiac output toward the lungs, increases pulmonary venous return and ventricular volume loading, and may compromise systemic and coronary perfusion. Conversely, inadequate Qp produces severe hypoxemia. The clinically relevant target is therefore not maximal arterial saturation but an appropriate balance between pulmonary and systemic flow.

The systemic-to-pulmonary shunt and RV–PA conduit create different hemodynamic profiles. A systemic arterial shunt permits continuous pulmonary runoff, including during diastole, whereas the RV–PA conduit substantially reduces diastolic runoff from the systemic circulation. Early observational studies demonstrated higher diastolic pressure and more favorable postoperative hemodynamics with an RV–PA conduit [1]. In the randomized Pediatric Heart Network Single Ventricle Reconstruction trial, transplantation-free survival at 12 months was higher with the RV–PA conduit than with the modified Blalock–Taussig shunt, although the RV–PA group required more unintended cardiovascular interventions and the early survival difference did not persist consistently with longer follow-up [2]. Thus, shunt selection involves physiologic and technical trade-offs rather than a universally superior configuration.

From a physiologic perspective, Stage I management depends on the interaction among shunt resistance, PVR, systemic vascular resistance (SVR), ventricular function, atrioventricular valve competence, and systemic outflow. Falling neonatal PVR can substantially increase Qp, producing pulmonary overcirculation and systemic hypoperfusion even when arterial oxygen saturation appears satisfactory. The interpretation of saturation must therefore be integrated with systemic perfusion, diastolic pressure, lactate, venous saturation, urine output, ventricular performance, and the clinical assessment of Qp/Qs.

2. Stage II: The Bidirectional Glenn as Ventricular Volume Unloading

The bidirectional Glenn, or superior cavopulmonary connection, fundamentally changes the source of pulmonary blood flow. The SVC is connected directly to the pulmonary arteries, allowing upper-body systemic venous return to traverse the lungs without first passing through the ventricle.

This converts the circulation from one in which the systemic ventricle pumps both Qp and Qs toward one in which its output more closely approximates systemic flow:

Before Glenn: COₛᵥ ≈ Qp + Qs

After Glenn: COₛᵥ ≈ Qs

The most important physiologic benefit is therefore ventricular volume unloading. Pulmonary blood flow derived from the SVC is no longer a recirculating component of ventricular output. This transition reduces preload and chronic volume work on the single ventricle while retaining adequate pulmonary blood flow for oxygenation. The superior cavopulmonary connection is consequently a crucial intermediate stage rather than merely a preparatory anatomical step before Fontan completion [3].

At the same time, the pulmonary circuit becomes substantially passive. In an uncomplicated Glenn without important additional antegrade pulmonary flow:

Qp ≈ Qₛᵥ꜀

The lungs have effectively been placed between the SVC and the pulmonary venous atrium. There is no subpulmonary ventricular pump generating a large pressure gradient across this circuit. Pulmonary blood flow is therefore particularly sensitive to PVR, pulmonary artery distortion or obstruction, pulmonary venous pressure, intrathoracic pressure, and the pressure available within the SVC.

This explains two major determinants of Glenn oxygenation. First, saturation depends on the proportion of total systemic venous return represented by SVC flow. Younger infants have a relatively large contribution of upper-body flow, whereas with somatic growth an increasing fraction of systemic venous return arises from the IVC. Consequently, progressive desaturation may occur during prolonged Glenn physiology even in the absence of an anatomical obstruction. Second, increased PVR directly limits passive pulmonary blood flow. Clinical studies have demonstrated that maturational and hemodynamic factors influence arterial oxygenation after the bidirectional cavopulmonary connection [4].

The Glenn circulation also illustrates why positive-pressure ventilation can adversely affect passive pulmonary flow. Excessive mean airway pressure raises intrathoracic pressure and can impede systemic venous return and transpulmonary flow. Spontaneous inspiration, conversely, may augment cavopulmonary flow through reductions in intrathoracic pressure. These effects are particularly relevant during perioperative management of patients without an effective additional source of pulmonary blood flow.

Additional antegrade or shunt-derived pulmonary blood flow may improve oxygen saturation and promote pulmonary arterial growth in selected patients, but it reintroduces ventricular volume loading and may increase cavopulmonary pressures. Observational data suggest that such strategies can be useful in carefully selected borderline patients, but their application remains individualized rather than universally indicated [5].

3. Stage III: Fontan Completion and Total Cavopulmonary Connection

Fontan completion incorporates IVC return into the pulmonary arterial circulation, most commonly through an extracardiac conduit or intracardiac lateral tunnel. The result is total cavopulmonary connection: both SVC and IVC blood are directed to the pulmonary arteries without an intervening subpulmonary ventricle.

The immediate physiologic advantages are substantial. Systemic and pulmonary circulations are functionally placed in series, systemic arterial oxygen saturation approaches normal or near-normal levels, and the single ventricle is relieved of the chronic volume overload intrinsic to parallel circulation. Contemporary extracardiac Fontan series demonstrate excellent early and intermediate survival in appropriately selected patients [6].

However, Fontan physiology solves the problem of cyanosis and ventricular volume loading by creating a fundamentally abnormal circulatory arrangement. Without a subpulmonary pump, blood must cross the pulmonary vascular bed using the residual pressure energy in the systemic venous circulation. The Fontan pathway therefore behaves as a critical resistance or impedance interposed upstream of ventricular filling [7].

A simplified representation is:

Transpulmonary flow ∝ (CVP − atrial/pulmonary venous pressure) / pulmonary vascular resistance

The exact relationship is more complex because vascular impedance, respiratory mechanics, collateral flow, ventricular diastolic properties, and conduit geometry contribute to the circulation, but the principle is clinically important. Adequate pulmonary blood flow requires:

• sufficiently low PVR and unobstructed pulmonary arteries;
• low pulmonary venous and atrial pressure;
• an adequate pressure gradient from the systemic veins through the pulmonary circulation;
• satisfactory ventricular relaxation and compliance; and
• favorable respiratory mechanics.

Thus, an apparently well-functioning systemic ventricle cannot independently compensate for impaired transpulmonary flow. In Fontan physiology, ventricular preload itself becomes restricted by the pulmonary circuit. Gewillig and Brown described this as a fundamental Fontan “bottleneck”: resistance upstream of the ventricle produces both systemic venous congestion and reduced downstream ventricular filling [7].

4. The Fontan Paradox: Adequate Preload Requires Elevated Venous Pressure

The defining hemodynamic paradox of the Fontan circulation is that systemic venous pressure must be sufficiently elevated to drive blood through the lungs, yet excessive venous pressure is itself harmful.

A relatively high central venous pressure is therefore not merely a consequence of Fontan failure; some elevation is intrinsic to the circulation. The problem arises when a greater pressure is required to maintain adequate pulmonary flow because of increased PVR, pulmonary artery obstruction, elevated atrial or ventricular filling pressure, or unfavorable pathway energetics.

Increasing venous pressure has limited ability to compensate. Once systemic venous pressure rises excessively, hepatic congestion, ascites, peripheral edema, venovenous collateral formation, and impairment of lymphatic drainage may develop. At the same time, if transpulmonary flow remains restricted, ventricular preload and systemic cardiac output remain inadequate.

PVR is therefore especially consequential in the Fontan circulation. A modest increase that might be readily overcome by a normal right ventricle can substantially reduce Fontan flow because no subpulmonary pump exists to generate additional pressure [7,8]. Pulmonary vasodilator therapy has consequently been investigated, but current evidence does not support routine treatment of every Fontan patient; potential benefit likely depends on the underlying hemodynamic phenotype [8].

5. Systemic Venous and Lymphatic Consequences

The long-term burden of Fontan physiology extends beyond the heart. Chronic elevation of systemic venous pressure is transmitted directly to the hepatic venous system and lymphatic circulation. Fontan-associated liver disease develops progressively in many patients, while abnormal lymphatic pressures and flow can contribute to protein-losing enteropathy, plastic bronchitis, pleural effusions, and other lymphatic complications.

The American Heart Association emphasizes that Fontan circulation should therefore be regarded as a multisystem condition rather than an isolated cardiac reconstruction. Long-term surveillance requires assessment of ventricular and valve function, rhythm, Fontan pathway patency, exercise capacity, hepatic disease, renal function, nutritional status, thrombosis, and lymphatic complications [9].

Large contemporary extracardiac Fontan cohorts demonstrate favorable survival but also illustrate the accumulation of late morbidity. In a 500-patient series, late complications included arrhythmias, protein-losing enteropathy, thromboembolism, and progressive evidence of hepatic abnormalities despite excellent overall survival [6]. These findings reinforce the distinction between successful Fontan completion and a physiologically normal circulation.

6. The Physiologic Continuum of Staged Palliation

The three stages can be understood as a progressive redistribution of responsibility for pulmonary blood flow.

During Stage I, the ventricle directly supplies both circulations, and pulmonary blood flow competes with systemic blood flow for ventricular output. The dominant problem is flow balance.

After the Glenn, SVC blood bypasses the ventricle and flows passively through the lungs. The ventricle is partially volume-unloaded, but oxygenation depends increasingly on SVC flow and low pulmonary resistance.

After Fontan completion, essentially all systemic venous return must traverse the pulmonary circuit passively. Ventricular volume loading is minimized and oxygenation improves, but cardiac output becomes strongly dependent on transpulmonary flow and adequate ventricular preload. The physiologic price is chronic systemic venous hypertension and vulnerability to venous and lymphatic congestion.

This progression can therefore be summarized conceptually as:

Parallel circulation → partial cavopulmonary circulation → total cavopulmonary circulation

or, from the ventricular perspective:

Qp + Qs workload → Qs-dominant workload → preload-limited systemic pump

Understanding this transition is essential when interpreting saturation, central venous pressure, ventricular filling, PVR, and cardiac output at each stage. The same numerical value may have very different significance in the Norwood, Glenn, and Fontan circulations.

Key Surgical and Clinical Principles

• Stage I requires unobstructed systemic outflow, controlled pulmonary blood flow, and unrestricted atrial communication.
• After Norwood palliation, Qp remains part of systemic ventricular output; excessive Qp can therefore produce systemic hypoperfusion and ventricular volume overload despite higher oxygen saturation.
• Glenn palliation redirects SVC blood directly to the pulmonary arteries and thereby provides major single-ventricle volume unloading.
• Glenn oxygenation depends on both the amount of SVC return and resistance to passive pulmonary blood flow.
• Fontan completion improves systemic oxygenation and eliminates most residual ventricular volume overload, but it does not restore normal biventricular physiology.
• Successful Fontan flow requires low PVR, low pulmonary venous/atrial pressure, unobstructed cavopulmonary pathways, and adequate systemic venous driving pressure.
• Chronic systemic venous hypertension and reduced ventricular preload are intrinsic consequences of Fontan physiology and underlie many late hepatic, lymphatic, exercise, and circulatory complications.

References

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