Fontan Circulation — Pressure Gradient

1. Conceptual Overview

In the Fontan circulation, all systemic venous blood is directed to the pulmonary arteries without a subpulmonary ventricle. The cavopulmonary connection creates a “critical bottleneck”: blood must cross the pulmonary vascular bed driven only by a small pressure gradient from the systemic veins → pulmonary arteries → left atrium → single ventricle, rather than by a pumping chamber.[1]

Because of this design, cardiac output is determined primarily by pulmonary vascular impedance. Even modest increases in pulmonary vascular resistance (PVR) markedly reduce flow, while the single ventricle can do little to “suck” blood through the lungs.[1,5] The hemodynamic hallmark of Fontan physiology is therefore a combination of:

• Systemic venous hypertension, and
• Chronically reduced cardiac output.[2]

Over time, this low‐output, high‐venous-pressure state produces chronic venous and lymphatic congestion, progressive ventricular diastolic dysfunction, and multi-organ injury—the “Janus-faced” nature of the Fontan circulation.[3,4]

2. Key Pressure Levels in the Fontan Circuit

In a well-functioning Fontan, pressures fall in small steps from the peripheral veins to the ventricle. Typical resting values are:

2.1 Peripheral Venous Pressure (PVP)

• Normal biventricular circulation: ~5–10 mmHg
• Fontan: usually 15–20 mmHg → systemic venous hypertension

This is the “entry pressure” into the Fontan circuit. The elevated PVP is not merely a complication; it is the necessary upstream load that provides enough energy to push blood across the pulmonary vascular bed in the absence of a right ventricle.[1,2]

2.2 Central Venous Pressure (CVP; Fontan Conduit)

• Typical Fontan range at rest: 12–18 mmHg

CVP reflects pressure within the extracardiac conduit or lateral tunnel and is effectively the driving head for pulmonary blood flow.

• If CVP is too low, pulmonary blood flow and cardiac output fall.
• If CVP is too high, venous congestion of the liver, gut, kidneys, and lymphatic system worsens, while pulmonary flow increases little once PVR becomes the rate-limiting step.[1,2]

2.3 Mean Pulmonary Artery Pressure (mPAP)

• Slightly lower than CVP, modulated by respiration (negative intrathoracic pressure augments venous return).

In Fontan patients, mPAP must remain low. Any increase—due to pulmonary vascular disease, small pulmonary arteries, or anatomic obstruction—reduces the pressure drop across the lungs and directly limits cardiac output.[1,5]

2.4 Atrial Pressure (CAP / Left Atrial Pressure)

• Typical range: 5–10 mmHg

This is the “exit pressure” of the pulmonary circuit. When LA pressure rises (e.g., from AV-valve regurgitation, impaired ventricular relaxation, pulmonary venous obstruction, or high ventricular end-diastolic pressure), the difference between CVP and LA pressure narrows, reducing pulmonary blood flow and ventricular preload.[2,4]

2.5 Transpulmonary Gradient (TPG = CVP − CAP/LA)

• Normal Fontan range: 3–6 mmHg

TPG represents the net pressure drop across the pulmonary vascular bed and is a convenient surrogate for PVR:

• TPG 3–6 mmHg: compliant, low-resistance pulmonary circulation.
• TPG > 6–8 mmHg: suggests elevated PVR or anatomic obstruction and is often an early warning sign of Fontan failure.[1,5]

Because there is no subpulmonary pump, even mild elevation of PVR (and thus TPG) in the setting of low cardiac index identifies patients at highest risk of Fontan failure.[5]

2.6 Ventricular End-Diastolic Pressure (EDP)

• Normal: ≤10 mmHg

The single ventricle must remain highly compliant. When EDP rises—due to hypertrophy, fibrosis, or chronic preload deprivation—this increase is transmitted backward to the atrium and pulmonary veins, then to the Fontan pathway and systemic veins, collapsing the effective pressure gradient that drives flow through the lungs.[1–3]

3. Putting the Gradient Together

A simplified resting profile in a well-compensated Fontan might be:

• PVP: 15–20 mmHg
• CVP (conduit): 12–18 mmHg
• mPAP: slightly below CVP
• LA/CAP: 5–10 mmHg
• EDP: <10 mmHg

Pressures step down gradually:

PVP → CVP → PAP → LA → Ventricular EDP

As long as:

• each downstream chamber is a few mmHg lower than the previous one, and
• the TPG remains modest (≈3–6 mmHg),

blood can traverse the lungs passively and fill the ventricle adequately, allowing reasonable cardiac output for daily activities.[1,2]

4. Why the Fontan Circuit Is So Fragile

The Fontan circulation behaves as a single series circuit. Disturbance at any point—venous, pulmonary, atrial, or ventricular—propagates through the entire loop.[1–4]

4.1 Venous Side — PVP/CVP Elevated

• Causes:
• Obstruction or kinking of the Fontan pathway
• Elevated PVR (e.g., hypoplastic pulmonary arteries, pulmonary vascular remodeling)
• High intrathoracic pressure or excessive positive-pressure ventilation
• Consequences:
• Progressive hepatic and splanchnic congestion, Fontan-associated liver disease, renal dysfunction
• Lymphatic failure with protein-losing enteropathy, plastic bronchitis, pleural effusions
• Peripheral edema and impaired exercise tolerance[2–4]

4.2 Pulmonary Bed — TPG and PVR Increased

• Elevated PVR (fixed or reactive) becomes the true bottleneck of the system.[1,5]
• Even small increases in PVR markedly reduce preload and cardiac output, despite already high CVP.
• Over years, adverse pulmonary vascular remodeling further accelerates hemodynamic attrition.[3,5]

4.3 Atrial / Ventricular Side — LA Pressure or EDP Elevated

• Mechanisms:
• AV-valve regurgitation or stenosis
• Systolic or diastolic ventricular dysfunction (often from chronic preload deprivation and fibrosis)
• Pulmonary venous obstruction
• Effects:
• LA and EDP rise, erasing the Fontan gradient.
• Systemic venous pressures increase further, while forward flow falls—producing the classic picture of low cardiac output with severe venous congestion.[2–4]

5. Clinical Implications

From a practical standpoint, understanding these pressure relationships helps to:

1. Interpret catheter data in Fontan patients (e.g., distinguishing predominant venous vs pulmonary vs ventricular problems).
2. Target interventions—relieving anatomic obstruction, aggressively protecting the pulmonary vascular bed, optimizing AV-valve function and ventricular diastolic properties.[1,2,4,5]
3. Explain Fontan fragility to trainees and families: the circulation works only as long as a delicate, stepwise pressure gradient is preserved; once any segment fails, the entire series circuit is compromised.

References

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

[2] Van De Bruaene A, Claessen G, Salaets T, Gewillig M. Late Fontan circulatory failure. What drives systemic venous congestion and low cardiac output in adult Fontan patients? Front Cardiovasc Med. 2022;9:825472.

[3] Ridderbos FJS, Hoendermis ES, Berger RMF, van Melle JP. The Janus-faced Fontan circulation: unravelling its elusive pathophysiology. Eur J Heart Fail. 2019;21(6):810–812.

[4] Rychik J, Atz AM, Celermajer DS, et al. 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] Egbe AC, Connolly HM, Miranda WR, et al. Hemodynamics of Fontan failure: the role of pulmonary vascular disease. Circ Heart Fail. 2017;10(12):e004515.