Glenn Circulation #6: Pulsatile Glenn

A pulsatile Glenn refers to a bidirectional Glenn circulation in which antegrade pulmonary blood flow (APBF) is intentionally preserved after the superior vena cava has been connected to the pulmonary arteries. In this setting, pulmonary perfusion is supplied by two sources:

1. Passive cavopulmonary flow from the SVC through the Glenn pathway
2. Pulsatile antegrade flow from the ventricle through the native pulmonary outflow tract or pulmonary artery pathway

This creates a form of dual-source pulmonary perfusion. The concept is physiologically attractive because APBF can increase effective pulmonary blood flow, improve systemic oxygen saturation, and provide a flow stimulus for pulmonary arterial growth. However, it also introduces a competing hemodynamic burden: increased pulmonary venous return, ventricular volume loading, and potential elevation of pulmonary artery and SVC pressures. For this reason, the role of APBF after bidirectional Glenn has remained controversial for decades [1].

1. Core Concept: Why Pulsatile Glenn Is Different

In a conventional bidirectional Glenn, the SVC is disconnected from the heart and anastomosed directly to the pulmonary arteries. Pulmonary blood flow becomes largely passive, driven by the pressure gradient between the systemic venous system and the pulmonary vascular bed.

In contrast, a pulsatile Glenn preserves additional ventricular contribution to pulmonary blood flow. Therefore, the pulmonary circulation receives both:

• Nonpulsatile SVC-derived Glenn flow
• Pulsatile ventricular-derived antegrade pulmonary flow

This additional flow may improve oxygen delivery, but it also means that part of the Glenn circulation remains exposed to the consequences of ventricular output into the pulmonary arteries.

A useful conceptual formula is:

Glenn flow + antegrade pulmonary blood flow = increased pulmonary blood flow, but also increased preload

Thus, pulsatile Glenn should not be interpreted simply as “more flow is better.” The essential issue is whether APBF is controlled, physiologically appropriate, and compatible with future Fontan completion.

2. Potential Benefits of Maintaining APBF

2.1 Improved systemic oxygenation

The most consistent benefit of APBF is higher systemic arterial oxygen saturation. By increasing total pulmonary blood flow, APBF increases pulmonary venous return and improves systemic oxygen content.

Several clinical series have shown that patients with preserved APBF after bidirectional Glenn tend to have higher oxygen saturation than those in whom APBF is eliminated [2–5]. Reported differences are often clinically meaningful, with saturations in the APBF group commonly in the low-to-mid 80% range compared with high 70% to low 80% range in non-APBF patients.

This can be particularly valuable when Glenn-only pulmonary blood flow is limited by:

• Small branch pulmonary arteries
• Borderline pulmonary vascular resistance
• Lower-than-expected SVC contribution
• Excessive systemic desaturation after Glenn
• High-risk or delayed single-ventricle palliation

In this context, APBF may function as a physiologic supplement to the Glenn circulation.

2.2 Promotion of pulmonary artery growth

Another important rationale for maintaining APBF is the possibility of promoting pulmonary artery growth. Pulmonary arteries are flow-sensitive structures, and inadequate pulmonary blood flow during staged palliation may limit branch PA development.

Additional antegrade flow may provide:

• Greater total pulmonary blood flow
• Pulsatile energy transmission
• More symmetric or more robust pulmonary artery growth
• Improved pulmonary artery size before Fontan completion

Ferns and colleagues reported better pulmonary artery growth in patients with pulsatile bidirectional Glenn, including a higher McGoon ratio in the APBF group [4]. Baek and colleagues also found that APBF was associated with larger pulmonary artery size before Fontan operation, although this benefit did not necessarily translate into better long-term survival [8].

Therefore, APBF may improve the anatomic substrate for Fontan completion, but it does not automatically improve the overall Fontan pathway.

2.3 Potential support in selected high-risk patients

In selected patients, especially those with suboptimal pulmonary arteries or borderline oxygenation, APBF may serve as a useful temporary strategy. Calvaruso and colleagues described bidirectional Glenn with APBF as a potentially effective interim palliation before Fontan completion, particularly when the antegrade flow is not excessive [2].

The key point is selection. APBF may be beneficial when it provides enough additional pulmonary blood flow to improve oxygenation and PA growth without causing venous hypertension or ventricular dilation.

3. Hemodynamic Disadvantages

3.1 Ventricular volume overload

The major physiologic cost of APBF is ventricular volume loading. Any additional pulmonary blood flow returns to the atrium and ventricle as pulmonary venous return. Therefore, preserved APBF increases preload to the single ventricle.

This may lead to:

• Ventricular dilation
• Increased ventricular end-diastolic pressure
• Worsening atrioventricular valve regurgitation
• Increased pulmonary venous pressure
• Higher myocardial workload
• Reduced efficiency of staged single-ventricle palliation

The bidirectional Glenn is partly designed to unload the single ventricle. Excessive APBF can partially negate this benefit.

3.2 Increased PA and SVC pressure

Because the Glenn pathway connects the SVC directly to the pulmonary arteries, any increase in pulmonary artery pressure can be transmitted backward into the SVC.

Excessive APBF may therefore increase:

• Mean pulmonary artery pressure
• Glenn pressure
• SVC pressure
• Cerebral venous pressure
• Pleural effusion risk
• Chylothorax risk
• Duration of chest tube drainage

This has been observed clinically. Mainwaring and colleagues reported higher morbidity when accessory pulmonary blood flow was maintained [1]. Ferns and colleagues also found that although APBF improved pulmonary artery growth, it was associated with increased postoperative morbidity, including effusions and longer drainage [4]. Yan and colleagues similarly reported that APBF increased oxygen saturation after late bidirectional Glenn but prolonged pleural effusion and hospital stay [7].

Thus, a higher saturation should not be interpreted in isolation. It must be weighed against venous pressure, effusions, ventricular size, and AV valve function.

3.3 Reduced cavopulmonary efficiency

A Glenn circulation is most efficient when pulmonary blood flow is passive, low-resistance, and unobstructed. Excessive APBF may create unfavorable flow competition within the pulmonary arteries.

Potential consequences include:

• Increased energy loss within the pulmonary arterial confluence
• Disturbed distribution of flow between the branch pulmonary arteries
• Elevated Glenn pathway pressure
• Less efficient SVC drainage
• Increased risk of venous congestion
• Greater burden on the future Fontan circulation

Computational hemodynamic studies have supported this physiologic concern by showing that additional pulmonary blood flow can improve oxygen delivery but may also worsen energy efficiency and pressure conditions within Glenn and Fontan-type circulations [9].

4. Controlled Versus Uncontrolled APBF

The most important distinction is not simply whether APBF is present or absent. The key distinction is whether APBF is controlled or uncontrolled.

Controlled APBF

Controlled APBF means that antegrade flow is present but limited. This may occur through:

• Native pulmonary stenosis
• Prior pulmonary artery banding
• Restrictive pulmonary outflow
• Intentional surgical regulation of pulmonary blood flow

In this setting, APBF may improve oxygen saturation and pulmonary artery growth without excessive ventricular preload.

Uncontrolled APBF

Uncontrolled APBF means that pulmonary blood flow remains too large relative to the patient’s ventricular capacity and pulmonary vascular bed. This may result in:

• Ventricular dilation
• AV valve regurgitation progression
• Elevated PA/SVC pressure
• Persistent effusions
• Delayed Fontan completion
• Higher long-term morbidity

Zhang and colleagues specifically highlighted the adverse consequences of uncontrolled antegrade pulmonary blood flow after bidirectional Glenn, including ventricular enlargement, worsening AV valve regurgitation, and delayed Fontan completion [6].

Therefore, the surgical question is not:

“Should APBF always be preserved?”

The more precise question is:

“Can APBF be preserved at a controlled level that supports oxygenation and pulmonary artery growth without compromising ventricular loading or Glenn pressure?”

5. Impact on Fontan Pathway

The Fontan circulation requires a low-resistance pulmonary vascular bed, well-developed pulmonary arteries, low ventricular filling pressure, good ventricular function, and minimal AV valve regurgitation. APBF can support some of these requirements but can harm others.

Potential Fontan advantages

APBF may contribute to:

• Higher pre-Fontan oxygen saturation
• Better pulmonary artery size
• Improved pulmonary artery growth indices
• Potentially better tolerance of the interstage period

Potential Fontan disadvantages

However, APBF may also cause:

• Ventricular volume loading
• Elevated end-diastolic pressure
• AV valve regurgitation
• Higher PA pressure
• Pleural effusions
• Delayed Fontan completion
• Worse transplant-free survival in some cohorts

Baek and colleagues reported that maintaining APBF was associated with higher oxygen saturation and larger pulmonary artery size before Fontan, but also with more effusion-related morbidity and worse overall transplant-free survival until APBF was eliminated [8]. This finding emphasizes the central paradox of pulsatile Glenn: better saturation and larger pulmonary arteries do not necessarily equal better long-term outcome.

6. Practical Surgical Decision-Making

At the time of bidirectional Glenn, the surgeon generally has three options:

1. Preserve APBF
2. Restrict APBF
3. Eliminate APBF

The decision should be individualized based on anatomy, hemodynamics, and the anticipated Fontan pathway.

Factors favoring preservation or partial preservation

APBF may be useful when:

• Pulmonary arteries are small
• Oxygen saturation is expected to be low after Glenn
• Pulmonary vascular resistance is acceptable
• Ventricular function is good
• AV valve regurgitation is trivial or mild
• APBF is naturally restrictive
• Glenn/SVC pressure remains acceptable
• Fontan completion is anticipated after further growth

Factors favoring restriction or elimination

APBF should be restricted or eliminated when there is:

• High Glenn/SVC pressure
• Excessive pulmonary blood flow
• Ventricular dilation
• Elevated ventricular end-diastolic pressure
• Moderate or worse AV valve regurgitation
• Persistent pleural effusion or chylothorax
• Pulmonary overcirculation
• Concern for delayed Fontan completion
• Poor ventricular compliance

In practice, mild restrictive antegrade flow may be helpful, whereas large unrestricted antegrade flow is usually harmful.

7. Key Teaching Point

A pulsatile Glenn is best understood as a controlled physiologic compromise.

It preserves additional antegrade pulmonary blood flow to improve systemic oxygenation and support pulmonary artery growth. However, because this flow returns to the ventricle and can elevate pulmonary artery and SVC pressures, it may also undermine the very goals of staged single-ventricle palliation.

The central principle is:

APBF is beneficial only when it is appropriately balanced.

Too little pulmonary blood flow may result in cyanosis and poor pulmonary artery growth. Too much pulmonary blood flow may cause ventricular volume overload, venous hypertension, effusions, AV valve deterioration, and impaired Fontan readiness.

Thus, the goal is not simply to preserve pulsatility.

The goal is to preserve useful pulsatility without excessive volume load or venous pressure.

Revised Slide-Style Summary

Pulsatile Glenn preserves antegrade pulmonary blood flow after bidirectional Glenn, creating dual-source pulmonary perfusion: passive SVC-derived Glenn flow plus pulsatile ventricular pulmonary flow.

The benefit is improved systemic oxygenation and potential pulmonary artery growth, particularly when pulmonary arteries are small or Glenn-only flow is insufficient.

The trade-off is ventricular volume loading and possible elevation of PA/SVC pressure; uncontrolled APBF may increase effusions, worsen AV valve regurgitation, delay Fontan completion, and compromise long-term outcomes.

References

[1] Mainwaring RD, Lamberti JJ, Uzark K, Spicer RL. Bidirectional Glenn. Is accessory pulmonary blood flow good or bad? Circulation. 1995;92(9 Suppl):II294-II297.

[2] Calvaruso DF, Rubino A, Ocello S, Salviato N, Guardì D, Petruccelli DF, Cipriani A, Fattouch K, Agati S, Mignosa C, Zannini L, Marcelletti C. Bidirectional Glenn and antegrade pulmonary blood flow: temporary or definitive palliation? Ann Thorac Surg. 2008;85(4):1389-1395.

[3] Gray RG, Altmann K, Mosca RS, Prakash A, Williams IA, Quaegebeur JM, Chen JM. Persistent antegrade pulmonary blood flow post-Glenn does not alter early post-Fontan outcomes in single-ventricle patients. Ann Thorac Surg. 2007;84(3):888-893.

[4] Ferns SJ, El Zein C, Multani K, Sajan I, Subramanian S, Polimenakos AC, Ilbawi MN. Is additional pulsatile pulmonary blood flow beneficial to patients with bidirectional Glenn? J Thorac Cardiovasc Surg. 2013;145(2):451-454.

[5] Chen Q, Tulloh R, Caputo M, Stoica S, Kia M, Parry AJ. Does the persistence of pulsatile antegrade pulmonary blood flow following bidirectional Glenn procedure affect long term outcome? Eur J Cardiothorac Surg. 2015;47(1):154-158.

[6] Zhang T, Shi Y, Wu K, Hua Z, Li S, Hu S, Zhang H. Uncontrolled antegrade pulmonary blood flow and delayed Fontan completion after the bidirectional Glenn procedure: real-world outcomes in China. Ann Thorac Surg. 2016;101(4):1530-1538.

[7] Yan T, Tong G, Zhang B, Yan F, Zhou X, Wang X, Lu H, Ma T, Wang X, Yu H, Sun Z, Zhang W. The effect of antegrade pulmonary blood flow following a late bidirectional Glenn procedure. Interact Cardiovasc Thorac Surg. 2018;26(3):454-459.

[8] Baek JS, Park CS, Choi ES, Yun TJ, Kwon BS, Yu JJ, Kim YH. The impact of additional antegrade pulmonary blood flow at bidirectional Glenn shunt on long-term outcomes. J Thorac Cardiovasc Surg. 2021;162(5):1346-1355.e4.

[9] Chen X, Yuan H, Liu J, Zhang N, Zhou C, Huang M, Jian Q, Zhuang J. Hemodynamic Effects of Additional Pulmonary Blood Flow on Glenn and Fontan Circulation. Cardiovasc Eng Technol. 2020;11(3):268-282.