Glenn Circulation #5: Why Glenn Circulation Fails Over Time

The bidirectional Glenn circulation is a powerful but intrinsically limited stage of single-ventricle palliation. Its success depends on a low-resistance pathway from the superior vena cava (SVC) to the pulmonary arteries, unobstructed pulmonary arterial anatomy, low pulmonary vascular resistance, preserved ventricular function, and the absence of major collateral pathways. Over time, this balance can deteriorate, producing progressive cyanosis, reduced exercise tolerance, ventricular volume loading, and impaired candidacy for Fontan completion. Your current concept correctly emphasizes four major mechanisms: growth-related reduction in SVC contribution, veno-venous collaterals, pulmonary arteriovenous malformations, and aortopulmonary collaterals.

Historically, late deterioration after superior cavopulmonary connection was recognized soon after the original Glenn experience, with mechanisms including reduced effective pulmonary blood flow, systemic venous collateralization, pulmonary vascular abnormalities, and progressive desaturation [1]. Modern bidirectional Glenn physiology reduces some of the limitations of the classic Glenn, but the central problem remains the same: pulmonary blood flow is passive and depends on venous return rather than ventricular propulsion.

1. Growth Shifts Pulmonary Blood Flow Away From the SVC

After the bidirectional Glenn, pulmonary blood flow is supplied primarily by SVC return. This is physiologically favorable in infancy because the upper body and brain account for a relatively large fraction of total cardiac output. However, this proportion changes with somatic growth.

In normal children, the SVC contribution to total cardiac output is high in early childhood but gradually decreases toward the adult pattern as lower-body mass increases [2]. In Glenn physiology, this maturational change has direct clinical consequences:

Growth → greater IVC contribution → relatively lower SVC return → reduced Glenn pulmonary blood flow → lower systemic oxygen saturation.

Therefore, an anatomically patent Glenn does not guarantee stable long-term oxygenation. The circulation may become progressively mismatched simply because the child grows. The single ventricle continues to supply the systemic circulation, but the amount of venous blood routed passively through the lungs becomes proportionally smaller.

Clinical implication:

A gradual fall in SpO₂ after Glenn does not always indicate surgical obstruction. It may reflect the natural decline in SVC-to-total systemic venous return, especially when no Fontan completion has yet incorporated the IVC and hepatic venous return into the pulmonary circulation.

2. Veno-Venous Collaterals: A Pressure Relief Pathway That Causes Cyanosis

Veno-venous collaterals are one of the most important and treatable causes of desaturation after Glenn. These channels connect the SVC system to the IVC, atrium, pulmonary veins, or other systemic venous pathways. They may develop because of chronically elevated SVC pressure, pressure gradients between venous territories, or persistent embryologic venous channels.

From a hemodynamic standpoint, these collaterals may function as a decompressive “pop-off” pathway. From an oxygenation standpoint, they are harmful:

SVC blood bypasses the pulmonary vascular bed → desaturated venous blood enters systemic circulation → systemic SpO₂ decreases.

McElhinney and colleagues demonstrated that systemic venous collateral channels can cause clinically significant desaturation after bidirectional cavopulmonary anastomosis and that transcatheter occlusion can improve oxygen saturation in selected patients [3]. Heinemann and colleagues similarly reported that systemic venous collaterals after Glenn and Fontan procedures are associated with systemic desaturation and reduced functional performance [4]. More recent data continue to support transcatheter collateral closure as an effective strategy when collaterals are hemodynamically significant and when systemic venous pressure is acceptable [5].

Practical evaluation

Veno-venous collaterals should be suspected when:

• Oxygen saturation is lower than expected for Glenn physiology.
• The Glenn pathway is patent without obvious pulmonary artery obstruction.
• There is no major pulmonary venous obstruction or ventricular dysfunction.
• Desaturation is disproportionate to echocardiographic findings.

Management principle

Collateral closure is most effective when the collateral is a true pathologic shunt rather than a necessary decompression route. If SVC pressure is high because of pulmonary vascular disease, pulmonary artery stenosis, or elevated atrial pressure, closure may worsen venous hypertension. Therefore, the decision to occlude a collateral must be based on catheterization hemodynamics, anatomy, and the pressure gradient across the collateral pathway.

3. Pulmonary Arteriovenous Malformations: Intrapulmonary Shunting From Absent Hepatic Venous Flow

Pulmonary arteriovenous malformations are another major mechanism of late desaturation after Glenn. In classic Glenn physiology, hepatic venous blood from the IVC does not reach the pulmonary vascular bed. This exclusion of hepatic venous effluent is believed to promote abnormal pulmonary vascular remodeling and diffuse microscopic intrapulmonary arteriovenous shunting.

The central concept is:

Absence of hepatic venous flow to the lungs → abnormal pulmonary microvascular remodeling → pulmonary AVMs → intrapulmonary right-to-left shunt → progressive cyanosis.

McFaul and colleagues reported pulmonary arteriovenous shunts after superior vena cava–right pulmonary artery anastomosis and identified them as an important cause of late clinical deterioration [6]. Subsequent work has reinforced the concept that pulmonary AVMs after superior cavopulmonary connection are strongly related to the absence or maldistribution of hepatic venous blood flow to the lungs [7].

This mechanism is especially important in:

• Classic Glenn physiology
• Kawashima circulation
• Heterotaxy with interrupted IVC
• Unilateral or asymmetric hepatic flow distribution after Fontan completion
• Patients with progressive cyanosis despite unobstructed Glenn anatomy

Clinical behavior

Pulmonary AVMs often present as progressive systemic desaturation without a discrete intracardiac shunt. Contrast echocardiography, catheterization, angiography, and cross-sectional imaging may support the diagnosis, although diffuse microscopic AVMs can be difficult to quantify.

Treatment concept

Unlike focal veno-venous collaterals, diffuse pulmonary AVMs are not usually treated by simple coil occlusion. The physiologic treatment is restoration or redirection of hepatic venous blood to the affected pulmonary vascular bed, most commonly through Fontan completion or revision of hepatic flow distribution.

4. Aortopulmonary Collaterals: Extra Pulmonary Flow With Ventricular Cost

Aortopulmonary collaterals arise from systemic arteries and supply the pulmonary circulation directly. They may develop in response to chronic cyanosis, reduced pulmonary blood flow, low pulmonary arterial pulsatility, previous surgical palliation, or altered pulmonary vascular signaling.

Unlike Glenn flow, which is passive venous flow, APC flow is systemic arterial flow. This creates an inefficient circulation:

Aorta/systemic arteries → pulmonary arteries/lung → pulmonary veins → atrium → single ventricle → systemic arteries again.

This pathway may increase pulmonary blood flow and sometimes improve oxygen saturation superficially, but it does so at the cost of ventricular volume loading.

Triedman and colleagues showed that aortopulmonary collaterals are common after bidirectional Glenn and Fontan procedures, with a higher prevalence after Glenn than after Fontan in their catheterization cohort [8]. Additional pulmonary blood flow at the time of bidirectional Glenn remains physiologically complex; it may improve saturation in some patients but can increase ventricular volume load, elevate pulmonary artery pressures, and complicate Fontan candidacy [9].

Hemodynamic consequences of APCs

Aortopulmonary collaterals may cause:

• Increased pulmonary venous return
• Single-ventricle volume overload
• Elevated ventricular end-diastolic pressure
• Increased pulmonary artery pressure
• Prolonged pleural drainage after Fontan
• Distorted assessment of true pulmonary vascular resistance
• Inefficient recirculation of systemic arterial blood

Thus, APCs are not simply “more pulmonary blood flow.” They represent unregulated pulmonary blood flow with a ventricular penalty.

5. Pulmonary Vascular Remodeling: The Common Downstream Problem

Pulmonary AVMs and aortopulmonary collaterals can be viewed as two different expressions of abnormal pulmonary vascular adaptation in single-ventricle palliation. The Glenn circulation exposes the pulmonary vasculature to nonpulsatile, low-pressure, low-shear venous flow, while excluding hepatic venous effluent from the lungs. These conditions may promote maladaptive vascular remodeling over time [10].

This pulmonary vascular remodeling has two clinically important consequences:

1. Right-to-left intrapulmonary shunting, mainly through pulmonary AVMs
2. Systemic-to-pulmonary runoff, mainly through APCs

Both mechanisms can worsen Glenn performance, but they do so in opposite hemodynamic directions. Pulmonary AVMs reduce effective oxygen uptake by allowing blood to bypass the alveolar capillary bed. APCs increase pulmonary blood flow but impose volume loading on the single ventricle.

6. Additional Factors That Accelerate Glenn Failure

Although collateralization and pulmonary vascular remodeling are central, Glenn deterioration is often multifactorial.

Pulmonary artery stenosis or distortion

Branch pulmonary artery narrowing increases resistance to passive SVC flow. Even mild stenosis can be physiologically important because there is no subpulmonary ventricle to overcome resistance.

Elevated pulmonary vascular resistance

Respiratory disease, atelectasis, hypoventilation, pulmonary venous obstruction, or chronic pulmonary vascular remodeling can reduce Glenn flow. Glenn physiology is particularly vulnerable because flow is driven by venous pressure and respiratory mechanics.

Ventricular dysfunction

Single-ventricle dysfunction raises atrial and pulmonary venous pressures, increasing downstream resistance to Glenn flow. This can reduce effective pulmonary blood flow and worsen systemic venous congestion.

Atrioventricular valve regurgitation

AV valve regurgitation increases atrial pressure and pulmonary venous pressure. In Glenn physiology, this reduces the pressure gradient available for passive pulmonary flow and may accelerate desaturation.

Residual or excessive antegrade pulmonary blood flow

Additional antegrade pulmonary blood flow may improve saturation early but can increase ventricular volume loading and pulmonary artery pressure. Its benefit depends on the balance between oxygenation, ventricular loading, and pulmonary vascular conditioning [9].

Practical Framework: Why Glenn Circulation Fails Over Time

Late Glenn deterioration should be understood as a failure of flow distribution, vascular adaptation, and ventricular efficiency, not simply as failure of the surgical anastomosis.

The major mechanisms are:

1. Growth-related reduction in SVC contribution

The child’s lower body grows, IVC return increases, and the proportion of cardiac output passing through the Glenn pathway decreases.

2. Veno-venous collateral formation

SVC blood decompresses into systemic venous or atrial pathways, bypassing the lungs and causing systemic desaturation.

3. Pulmonary arteriovenous malformations

Absence of hepatic venous flow to the lungs promotes intrapulmonary right-to-left shunting.

4. Aortopulmonary collateral formation

Systemic arterial blood enters the pulmonary circulation, increasing pulmonary venous return and imposing volume load on the single ventricle.

5. Increased resistance to passive pulmonary flow

Pulmonary artery stenosis, elevated PVR, pulmonary venous hypertension, ventricular dysfunction, or AV valve regurgitation can all reduce Glenn efficiency.

Key Surgical and Clinical Message

The Glenn circulation is best understood as a transitional circulation. It unloads the single ventricle compared with neonatal systemic-to-pulmonary shunt physiology, but it remains dependent on a limited source of pulmonary blood flow: the SVC.

Over time, somatic growth shifts venous return toward the IVC, systemic venous pressure may drive collateral formation, the absence of hepatic venous effluent may promote pulmonary AVMs, and chronic cyanosis or low-pulsatility flow may stimulate aortopulmonary collaterals. These mechanisms progressively reduce the efficiency of passive pulmonary blood flow and can lead to systemic desaturation, ventricular volume loading, and impaired Fontan readiness.

In short:

Glenn failure is not usually a single anatomic event.It is the cumulative result of growth, passive-flow limitation, collateral escape, pulmonary vascular remodeling, and ventricular loading.

References

[1] Bargeron LM Jr, Karp RB, Barcia A, Kirklin JW, Hunt D, Deverall PB. Late deterioration of patients after superior vena cava to right pulmonary artery anastomosis. Am J Cardiol. 1972;30(3):211-216. (PubMed)

[2] Salim MA, DiSessa TG, Arheart KL, Alpert BS. Contribution of superior vena caval flow to total cardiac output in children. A Doppler echocardiographic study. Circulation. 1995;92(7):1860-1865. (PubMed)

[3] McElhinney DB, Reddy VM, Hanley FL, Moore P. Systemic venous collateral channels causing desaturation after bidirectional cavopulmonary anastomosis: evaluation and management. J Am Coll Cardiol. 1997;30(3):817-824. (PubMed)

[4] Heinemann M, Breuer J, Steger V, Steil E, Sieverding L, Ziemer G. Incidence and impact of systemic venous collateral development after Glenn and Fontan procedures. Thorac Cardiovasc Surg. 2001;49(3):172-178. (PubMed)

[5] Ali YA, Nour El-Deen NES, Elshahed GS. Management of collaterals after Glenn procedure and its impact on patients with a single ventricle: a single-center study. REC Interv Cardiol. 2024. (PubMed)

[6] McFaul RC, Tajik AJ, Mair DD, Danielson GK, Seward JB. Development of pulmonary arteriovenous shunt after superior vena cava-right pulmonary artery (Glenn) anastomosis. Report of four cases. Circulation. 1977;55(1):212-216. (PubMed)

[7] Kavarana MN, Jones JA, Stroud RE, Bradley SM, Ikonomidis JS, Mukherjee R. Pulmonary arteriovenous malformations after the superior cavopulmonary shunt: mechanisms and clinical implications. Expert Rev Cardiovasc Ther. 2014;12(6):703-713. (PubMed)

[8] Triedman JK, Bridges ND, Mayer JE Jr, Lock JE. Prevalence and risk factors for aortopulmonary collateral vessels after Fontan and bidirectional Glenn procedures. J Am Coll Cardiol. 1993;22(1):207-215. (PubMed)

[9] McElhinney DB, Marianeschi SM, Reddy VM. Additional pulmonary blood flow with the bidirectional Glenn anastomosis: does it make a difference? Ann Thorac Surg. 1998;66(2):668-672. (PubMed)

[10] Spearman AD, Ginde S. Pulmonary vascular sequelae of palliated single ventricle circulation: arteriovenous malformations and aortopulmonary collaterals. J Cardiovasc Dev Dis. 2022;9(9):309. (PubMed)