Single-Ventricle Circulation: From Parallel Physiology to the Fontan Circulation

Single-ventricle physiology is best understood by comparing it with normal biventricular circulation and then following the staged conversion from a parallel circulation to cavopulmonary series circulation. The defining problem is not simply the presence of one functional ventricle. Before cavopulmonary connection, the same ventricle must support both systemic and pulmonary blood flow. This produces mixing and arterial desaturation, competition between the pulmonary and systemic vascular beds, and chronic ventricular volume overload. The Glenn and Fontan operations progressively remove pulmonary blood flow from the workload of the systemic ventricle, improving ventricular loading and systemic oxygenation, but replacing ventricularly driven pulmonary flow with passive transpulmonary flow [1].

1. Biventricular Circulation Is a Series Circulation

In the normal postnatal heart, pulmonary and systemic circulations are connected in series and powered by two pumps. The left ventricle ejects into the systemic circulation and perfuses the systemic capillary bed. Systemic venous return then enters the right heart, and the right ventricle provides the energy required to propel blood across the pulmonary vascular bed. Oxygenated pulmonary venous blood returns to the left atrium and fills the left ventricle.

In steady state, pulmonary blood flow and systemic blood flow are therefore essentially equal. Each ventricle supports one vascular circuit, and pulmonary blood flow does not compete directly with systemic perfusion for the output of a single pump. The subpulmonary ventricle is particularly important because it maintains low systemic venous pressure while supplying sufficient energy to overcome pulmonary vascular resistance.

2. Native Single-Ventricle Physiology Is a Parallel Circulation

In functionally univentricular hearts, one dominant ventricle supports both circulations. Depending on the anatomy, systemic venous and pulmonary venous blood mix at the atrial, ventricular, or great-arterial level before ventricular ejection. The single ventricle then distributes its output between the systemic and pulmonary vascular beds. The circulation is therefore arranged in parallel rather than in series [1].

Conceptually:

Ventricular output ≈ Qs + Qp

where Qs is systemic blood flow and Qp is pulmonary blood flow.

In a normal series circulation, the same blood sequentially traverses the pulmonary and systemic circuits. In a parallel circulation, pulmonary blood flow returns through the pulmonary veins to the common ventricular inflow and must be pumped again. The ventricle therefore handles effective systemic flow plus recirculated pulmonary flow.

The two characteristic consequences are arterial desaturation from mixing and chronic ventricular volume overload [1]. Higher Qp increases pulmonary venous return and therefore increases the volume repeatedly pumped by the single ventricle.

The clinical importance of this load was demonstrated in 84 patients with a single left ventricle. Before Fontan-type repair, ventricular volumes were approximately two to three times normal, and progressive abnormalities in ventricular geometry and wall stress were associated with deteriorating systolic function and contractility. When a Glenn or Fontan operation was performed before 10 years of age, ventricular dimensions, volumes, and wall stress decreased and contractility improved significantly; recovery was much less evident after surgery beyond 10 years of age [2]. Ventricular unloading is therefore not merely a surgical consequence but a central physiologic goal of staged palliation.

3. Qp/Qs Balance and Pulmonary “Steal”

Because Qp and Qs arise from the same ventricular output, the pulmonary and systemic circulations compete for flow. If total ventricular output cannot increase sufficiently, an increase in Qp may reduce the proportion available for systemic perfusion. This is the physiologic basis of pulmonary “steal.”

Flow distribution depends on the relative resistances of the pulmonary and systemic circuits together with the anatomy of the pulmonary and systemic outflow pathways. A fall in pulmonary vascular resistance, an excessively large systemic-to-pulmonary shunt, or unrestricted pulmonary outflow may produce pulmonary overcirculation. Conversely, excessive restriction of pulmonary blood flow may preserve systemic perfusion but cause severe cyanosis.

The goal before Glenn is therefore not maximal arterial oxygen saturation but an appropriate Qp/Qs balance. Excessive Qp can produce relatively high saturation while causing low systemic diastolic pressure, impaired systemic perfusion, ventricular dilation, atrioventricular valve regurgitation, pulmonary congestion, and increased myocardial work. Chronically excessive pulmonary flow and pressure can also promote pulmonary vascular remodeling, an especially important concern in patients who will later depend on very low pulmonary vascular resistance for cavopulmonary flow [3].

Insufficient Qp causes inadequate pulmonary venous oxygenation and severe systemic desaturation. Saturation must therefore be interpreted together with systemic perfusion, blood pressure, lactate, urine output, hemoglobin, and the clinical estimate of cardiac output.

Systemic oxygen delivery is better represented by:

DO₂ = Qs × CaO₂

where CaO₂ is arterial oxygen content.

A relatively high saturation can coexist with inadequate systemic oxygen delivery if Qs is low. The true physiologic target is adequate tissue oxygen delivery, not saturation alone.

4. The Glenn Circulation: Partial Ventricular Unloading

The bidirectional Glenn directly connects the superior vena cava to the pulmonary arteries. SVC blood therefore enters both lungs without first returning to the ventricle. Pulmonary flow from the upper body is driven by the pressure gradient between the systemic veins and the pulmonary venous atrial chamber.

The major benefit is ventricular unloading. SVC return no longer enters the common ventricular inflow before traversing the pulmonary vascular bed, reducing pulmonary venous recirculation and ventricular volume load.

The Glenn, however, is not a complete series circulation. Inferior vena caval blood still returns to the heart and mixes with pulmonary venous blood, so systemic arterial desaturation persists. As the child grows and lower-body venous return becomes a larger proportion of total venous flow, this mixing becomes increasingly important.

Glenn pulmonary blood flow depends on low resistance throughout the pathway from the SVC through the pulmonary arteries, pulmonary microcirculation, pulmonary veins, atrium, and ventricular filling compartment. Increased pulmonary vascular resistance, pulmonary artery obstruction, pulmonary venous obstruction, elevated atrial pressure, or ventricular diastolic dysfunction can reduce transpulmonary flow. Unlike a circulation with a right ventricle, the Glenn has little ability to overcome an adverse pressure gradient by increasing subpulmonary pump work.

Additional pulmonary blood flow can modify this physiology. Antegrade pulmonary flow or systemic-to-pulmonary collateral flow may increase arterial saturation but may also restore some ventricular volume loading. In cardiac magnetic resonance data, systemic-to-pulmonary collateral flow was higher before Fontan completion, and Fontan patients with greater collateral flow had larger ventricular end-diastolic volumes [4]. Thus, not all pulmonary blood flow is equivalent from the perspective of ventricular unloading.

5. Fontan Completion: Series Circulation Without a Subpulmonary Ventricle

Fontan completion directs inferior vena caval blood to the pulmonary arteries in addition to the SVC blood already routed through the Glenn connection. In the absence of a fenestration or residual shunt, essentially all systemic venous return must traverse the pulmonary vascular bed before returning to the systemic ventricle. The systemic and pulmonary circulations are therefore placed functionally in series [1,5].

The resulting pathway is:

Systemic ventricle → systemic arteries → systemic capillaries → systemic veins → pulmonary arteries → lungs → pulmonary veins → systemic ventricle

The defining feature is that there is no subpulmonary ventricle.

This produces two major benefits. First, mixing of systemic and pulmonary venous blood is markedly reduced, so systemic arterial oxygenation improves. Second, chronic ventricular volume overload is relieved because systemic venous blood is no longer returned directly to the ventricle before crossing the lungs.

The physiologic cost is that pulmonary blood flow becomes passive and depends predominantly on the pressure gradient across the pulmonary circuit rather than the output of a right ventricle [5]. Fontan circulation therefore requires a relatively elevated systemic venous pressure and extremely low resistance between the caval veins and the pulmonary venous atrium.

6. The Fontan Paradox: Venous Hypertension and Preload Deprivation

The Fontan circulation creates a characteristic paradox: systemic venous pressure is elevated while ventricular preload and cardiac output remain limited. Pulmonary blood flow becomes the principal determinant of ventricular filling. Any factor that restricts transpulmonary flow therefore limits cardiac output upstream of the ventricle.

This explains why modest abnormalities may be poorly tolerated. Increased pulmonary vascular resistance, branch pulmonary artery obstruction, pulmonary venous stenosis, elevated ventricular end-diastolic pressure, atrioventricular valve regurgitation, or loss of atrioventricular synchrony can all reduce forward flow. There is no subpulmonary pump capable of compensating for these abnormalities.

Fontan physiology also introduces an important ventricular-vascular energetic limitation. In an experimental model of 12 anesthetized dogs, creation of Fontan circulation reduced the slope of the end-systolic pressure-volume relationship from 7.5 to 5.3 mm Hg/mL and reduced mechanical efficiency from 0.82 to 0.56, while vascular impedance and ventriculoarterial coupling worsened [6]. These findings support the concept of contractility-afterload mismatch. Volume unloading is beneficial, but the systemic ventricle operates within a circulation characterized by restricted preload, increased effective vascular impedance, and limited mechanical reserve.

The result may be adequate resting hemodynamics but limited capacity to augment cardiac output during exercise or physiologic stress.

7. Respiration, Exercise, and Perioperative Management

Because Fontan pulmonary blood flow is passive, respiration becomes part of the cardiovascular pump. Spontaneous inspiration lowers intrathoracic pressure and can augment systemic venous return and transpulmonary flow. Positive-pressure ventilation, especially excessive mean airway pressure or positive end-expiratory pressure, may impede venous return and pulmonary blood flow.

These cardiopulmonary interactions contribute to the distinctive limitations of exercise in Fontan patients [7]. Exercise requires increased venous return and pulmonary blood flow to augment ventricular preload, but no subpulmonary ventricle exists to generate the necessary pressure increase.

Perioperative management should therefore minimize resistance throughout the cavopulmonary circuit:

• Maintain low pulmonary vascular resistance.
• Avoid hypoxemia, significant hypercarbia, acidosis, and excessive airway pressures.
• Maintain adequate but not excessive intravascular volume.
• Preserve sinus rhythm and atrioventricular synchrony when possible.
• Identify pulmonary artery, Fontan pathway, or pulmonary venous obstruction.
• Minimize ventricular end-diastolic and atrial pressures.
• Address significant atrioventricular valve regurgitation and ventricular dysfunction.
• Recognize that cardiac output may be limited by pulmonary blood flow even when ventricular systolic function appears satisfactory.

The Fontan circulation should therefore be evaluated as a complete hydraulic circuit, not as ventricular function alone.

8. Long-Term Consequences and Clinical Perspective

Cavopulmonary connection substantially reduces chronic ventricular volume loading, but it does not recreate normal biventricular physiology. Long-term success depends on preserved ventricular function, low pulmonary vascular resistance, unobstructed cavopulmonary pathways, competent atrioventricular valves, and stable rhythm.

Historical adult data illustrate the consequences of prolonged single-ventricle physiology without Fontan completion. Among 50 adults managed with cavopulmonary or aortopulmonary shunts, survival from entry into the adult congenital program was 51.9% at 20 years, and arrhythmias developed in 25 patients. Poor ventricular function was associated with ventricular arrhythmia and adverse clinical outcomes. Cavopulmonary palliation was associated with better preservation of ventricular function than aortopulmonary shunts, reinforcing the importance of reducing ventricular volume loading [8].

Fontan physiology itself, however, produces chronic systemic venous hypertension and limited cardiac output reserve. These mechanisms contribute to exercise intolerance, hepatic congestion and fibrosis, lymphatic dysfunction, protein-losing enteropathy, plastic bronchitis, venovenous collaterals, arrhythmias, and eventually Fontan circulatory failure.

The Glenn and Fontan operations should therefore not be viewed simply as procedures that increase oxygen saturation. Their deeper physiologic purpose is to reorganize pulmonary blood flow so that it no longer imposes chronic volume work on the single systemic ventricle.

9. Conceptual Summary

The evolution of single-ventricle palliation is a staged solution to two competing problems: inadequate oxygenation and excessive ventricular workload.

Before cavopulmonary connection, the single ventricle supports systemic and pulmonary blood flow in parallel. Pulmonary blood recirculates through the heart, producing chronic volume overload, while admixture causes arterial desaturation. Qp and Qs compete for the same ventricular output, making balanced pulmonary flow essential.

The bidirectional Glenn connects the SVC directly to the pulmonary arteries. This partially removes pulmonary circulation from ventricular work and substantially reduces volume loading, but IVC blood continues to mix within the heart, so cyanosis persists.

Fontan completion directs both SVC and IVC return through the lungs. Systemic and pulmonary circulations are thereby placed in series, ventricular volume load is further reduced, and systemic oxygenation improves.

The price of this transformation is the absence of a subpulmonary ventricle. Pulmonary blood flow becomes passive, systemic venous pressure must remain elevated, and ventricular preload becomes restricted. The systemic ventricle is therefore volume-unloaded but operates with limited preload reserve and potentially unfavorable ventriculoarterial coupling.

The defining concept is: staged single-ventricle palliation progressively removes pulmonary blood flow from the workload of the systemic ventricle while preserving oxygenation. Glenn accomplishes this partially; Fontan completes the separation. The resulting Fontan circulation is a series circulation without a subpulmonary ventricle.

References

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