Single Ventricle Palliation #4: Stage II Glenn Circulation

Stage II palliation, most commonly performed as a bidirectional Glenn procedure or superior cavopulmonary connection, is a central transition point in the staged management of single ventricle physiology. In Stage I physiology, the systemic ventricle supplies both the systemic and pulmonary circulations. After the Glenn procedure, the superior vena cava (SVC) is connected directly to the pulmonary arteries, allowing upper-body venous return to flow passively into the lungs without passing through the single ventricle [1].

This operation is therefore not simply an anatomic rerouting procedure. It is a major physiologic conversion from a ventricle-driven pulmonary circulation to a passive cavopulmonary circulation. The principal goals are to reduce ventricular volume load, stabilize systemic output, improve cyanosis, and prepare the patient for eventual Fontan completion [2].

1. Physiologic Problem Before Glenn

Before Stage II palliation, the single ventricle must support both:

• Systemic blood flow: Qs
• Pulmonary blood flow: Qp

Thus, the total output of the systemic ventricle is:

COsv = Qp + Qs

This physiology creates two major disadvantages.

First, pulmonary blood flow is effectively taken from systemic ventricular output. In other words, a portion of the single ventricular output is “stolen” into the pulmonary circulation rather than being delivered to the systemic circulation. When pulmonary blood flow is excessive, systemic perfusion may become compromised despite an apparently high total ventricular output.

Second, the single ventricle is exposed to chronic volume overload. Both systemic venous return and pulmonary venous return ultimately pass through the same ventricular chamber. Over time, this can promote ventricular dilation, increased wall stress, atrioventricular valve regurgitation, and deterioration of ventricular efficiency.

The bidirectional Glenn shunt addresses this problem by providing pulmonary blood flow without adding additional volume burden to the ventricle. Early physiologic studies demonstrated that the Glenn procedure can reduce single ventricular volume load while maintaining effective pulmonary blood flow and improving systemic oxygenation [3].

2. Core Concept of the Glenn Procedure

The Glenn procedure removes the SVC component of venous return from the heart and directs it into the pulmonary arteries.

In practical surgical terms:

1. The SVC is disconnected from the right atrium.
2. The cardiac end of the SVC is closed.
3. The SVC is anastomosed to the pulmonary artery, usually the right pulmonary artery.
4. Blood from the upper body flows directly into both branch pulmonary arteries.

After Glenn circulation:

Glenn flow ≈ SVC flow

and the systemic ventricle mainly supports systemic circulation:

COsv ≈ Qs

This reduction in ventricular preload is the central unloading effect of Stage II palliation.

The bidirectional Glenn and hemi-Fontan procedures have become established second-stage operations in the Fontan pathway. Although exact timing varies by institution and patient status, Stage II palliation is often performed at approximately 3 to 6 months of age after Norwood-type Stage I palliation in suitable candidates [4].

3. The Lungs Are Placed Between the SVC and the Single Ventricle

A useful conceptual model is that the Glenn procedure places the lungs directly between the SVC and the single ventricle.

Before Glenn:

SVC/IVC → atrium → single ventricle → pulmonary circulation

After Glenn:

SVC → pulmonary arteries → lungs → pulmonary veins → atrium → single ventricle

This configuration has two major consequences.

First, the single ventricle no longer generates the pulmonary blood flow derived from the SVC. This decreases ventricular volume work and improves ventricular loading conditions.

Second, pulmonary blood flow becomes passive and non-pulsatile. There is no subpulmonary ventricle actively pumping blood through the lungs. Instead, flow is determined by the pressure gradient between the SVC/pulmonary arteries and the pulmonary venous atrium.

Therefore, Glenn physiology is highly sensitive to:

• Pulmonary vascular resistance
• Pulmonary artery anatomy
• Pulmonary venous pressure
• Ventricular diastolic function
• Atrioventricular valve competence
• Intrathoracic pressure and respiratory mechanics

4. Hemodynamic Benefits

4.1 Reduction of ventricular volume overload

The most important benefit of the Glenn procedure is volume unloading of the systemic ventricle. By diverting SVC return directly to the pulmonary arteries, the ventricle no longer receives and ejects the entire pulmonary blood flow volume.

This improves ventricular efficiency and reduces the adverse effects of chronic volume overload. In the staged single ventricle pathway, this step is essential because long-term Fontan candidacy depends heavily on preserved ventricular function and acceptable atrioventricular valve competence.

Large clinical series of second-stage palliation have shown that ventricular dysfunction and atrioventricular valve regurgitation are important adverse factors after Stage II palliation [5]. These findings reinforce the principle that the Glenn procedure is not only a pulmonary blood flow operation, but also a ventricular preservation strategy.

4.2 Stabilization of pulmonary blood flow

Before Glenn, pulmonary blood flow may depend on a systemic-to-pulmonary shunt, ductal physiology, or an RV–PA conduit. These sources can create excessive pulmonary blood flow, systemic runoff, or unstable Qp/Qs balance.

After Glenn, pulmonary blood flow is derived primarily from SVC return. This creates a more controlled and lower-volume pulmonary blood flow source. Clinical series have shown that Glenn circulation can improve cyanosis and allow a substantial proportion of patients to progress to Fontan completion, although attrition between Glenn and Fontan remains clinically important [6].

4.3 Preparation for Fontan completion

The Glenn circulation is an intermediate step toward total cavopulmonary connection. It partially separates systemic venous return by directing SVC blood to the pulmonary arteries while leaving IVC return to enter the heart.

This provides a period of ventricular unloading and physiologic stabilization before Fontan completion, when IVC flow is also routed to the pulmonary arteries.

5. Determinants of Oxygen Saturation After Glenn

Systemic oxygen saturation after Glenn is lower than normal because only the SVC return passes directly through the pulmonary circulation. The IVC return continues to bypass the pulmonary arteries and mixes within the heart until Fontan completion.

Therefore, systemic arterial oxygen saturation depends on the balance between:

• Oxygenated pulmonary venous return generated by SVC-driven pulmonary blood flow
• Desaturated IVC return entering the systemic ventricle
• Pulmonary vascular resistance
• Pulmonary artery size and symmetry
• Pulmonary venous pressure
• Systemic venous collateral burden

Typical oxygen saturations after Glenn are often in the mid-70s to mid-80s, although the exact value varies according to anatomy, age, pulmonary vascular resistance, collateral circulation, ventricular function, and the presence or absence of additional pulmonary blood flow.

6. Why Desaturation Occurs After Glenn

Desaturation after Glenn can be understood through two broad mechanisms: flow limitation and resistance limitation.

6.1 Flow-related desaturation

Because Glenn pulmonary blood flow is derived from the SVC, systemic oxygen saturation depends heavily on the amount of upper-body venous return.

As the child grows, the relative proportion of IVC return increases. This means that a larger fraction of systemic venous return bypasses the lungs until Fontan completion.

In simple terms:

Less SVC contribution → less Glenn flow → less pulmonary blood flow → lower saturation

This is one reason Glenn circulation is usually an intermediate stage rather than the final palliation.

6.2 Resistance-related desaturation

Because Glenn flow is passive, even modest increases in pulmonary vascular resistance can reduce pulmonary blood flow.

Important causes include:

• Atelectasis
• Hypoxia
• Hypercarbia
• Acidosis
• Elevated mean airway pressure
• Pulmonary edema
• Branch pulmonary artery stenosis
• Pulmonary venous obstruction
• Elevated atrial pressure from ventricular dysfunction or AV valve regurgitation

In Glenn physiology:

Higher PVR → lower pulmonary blood flow → lower oxygen saturation

This principle explains why perioperative respiratory management is critical. Ventilation strategy, lung recruitment, avoidance of hypercarbia or hypoxia, and reduction of excessive intrathoracic pressure can directly influence Glenn flow [7].

7. Additional Pulmonary Blood Flow: Benefit or Burden?

One controversial issue in Glenn physiology is whether to preserve or eliminate additional antegrade pulmonary blood flow.

Potential advantages include:

• Higher systemic oxygen saturation
• Improved pulmonary artery growth
• More pulsatile pulmonary blood flow
• Possible delay of Fontan completion in selected patients

Potential disadvantages include:

• Persistent ventricular volume load
• Elevated pulmonary artery pressure
• Pleural effusions
• Venovenous collateral formation
• Increased postoperative complications
• Reduced suitability for Fontan completion if pulmonary pressures remain high

Earlier clinical experience demonstrated that additional pulmonary blood flow may be useful in selected patients but that its overall role remains complex and patient-specific [8]. Larger series of bidirectional Glenn with antegrade pulmonary blood flow have suggested that it can serve as effective temporary palliation, particularly in patients with restricted but not critically reduced pulmonary blood flow [9]. However, studies comparing pulsatile and non-pulsatile Glenn physiology have shown that although pulsatility may promote pulmonary artery growth, it may also increase postoperative complications and pulmonary artery pressure [10].

Thus, the decision to preserve antegrade pulmonary blood flow should not be automatic. It should be individualized according to ventricular function, pulmonary artery size, pulmonary vascular resistance, oxygen saturation, and the intended timing of Fontan completion.

8. Requirements for a Good Glenn Circulation

A successful Glenn circulation requires more than a technically patent SVC-to-PA anastomosis. The entire cavopulmonary circuit must be favorable.

Important requirements include:

1. Low pulmonary vascular resistance

Passive pulmonary blood flow can only occur through a low-resistance pulmonary vascular bed.

2. Unobstructed branch pulmonary arteries

Branch PA stenosis increases resistance and can produce asymmetric pulmonary blood flow.

3. Unobstructed pulmonary venous return

Pulmonary venous obstruction raises downstream pressure and reduces transpulmonary flow.

4. Good ventricular systolic and diastolic function

Poor ventricular compliance increases atrial pressure and reduces the pressure gradient across the lungs.

5. Acceptable atrioventricular valve competence

Significant AV valve regurgitation elevates atrial pressure, worsens pulmonary venous hypertension, and compromises Glenn flow.

6. Unobstructed systemic outflow

Residual arch obstruction or systemic outflow obstruction increases ventricular workload and reduces systemic reserve.

7. Absence of major systemic venous collaterals

Venovenous collaterals can decompress the SVC but worsen systemic desaturation by bypassing the pulmonary circulation.

9. Clinical Significance

The Glenn procedure is best understood as a ventricular unloading and pulmonary blood flow conversion operation.

Its goals are to:

• Remove the SVC component of pulmonary blood flow from the single ventricle
• Reduce ventricular volume overload
• Stabilize systemic cardiac output
• Improve oxygenation compared with unstable Stage I physiology
• Protect ventricular function and AV valve competence
• Prepare the patient for Fontan completion

However, Glenn physiology remains incomplete. The IVC return still bypasses the pulmonary arteries, systemic oxygen saturation remains limited, and pulmonary blood flow remains dependent on low resistance rather than ventricular propulsion.

For this reason, the Glenn circulation is a delicate but powerful intermediate physiology: it reduces ventricular work, improves circulatory efficiency, and creates the physiologic platform for eventual total cavopulmonary connection.

Summary

Stage II palliation with the bidirectional Glenn procedure diverts SVC blood directly into the pulmonary arteries, converting upper-body venous return into passive pulmonary blood flow. This eliminates a major source of “stolen” pulmonary blood flow from the systemic ventricle and substantially reduces single ventricular volume overload. After Glenn, pulmonary blood flow depends on SVC return, low pulmonary vascular resistance, unobstructed pulmonary arteries, unobstructed pulmonary venous return, and low atrial pressure. Desaturation reflects either insufficient SVC-derived pulmonary flow or increased resistance across the pulmonary vascular bed. The Glenn procedure is therefore a resistance-sensitive bridge between Stage I palliation and Fontan completion, with success determined by both surgical anatomy and cardiopulmonary physiology.

References

[1] Rao PS. Single Ventricle-A Comprehensive Review. Children (Basel). 2021;8(6):441.

[2] Bridges ND, Jonas RA, Mayer JE, Flanagan MF, Keane JF, Castaneda AR. Bidirectional cavopulmonary anastomosis as interim palliation for high-risk Fontan candidates. Early results. Circulation. 1990;82(5 Suppl):IV170-IV176.

[3] Allgood NL, Alejos J, Drinkwater DC, Laks H, Williams RG. Effectiveness of the bidirectional Glenn shunt procedure for volume unloading in the single ventricle patient. Am J Cardiol. 1994;74(8):834-836.

[4] Herrmann JL, Brown JW. The Superior Cavopulmonary Connection: History and Current Perspectives. World J Pediatr Congenit Heart Surg. 2019;10(2):216-222.

[5] Lee TM, Aiyagari R, Hirsch JC, Ohye RG, Bove EL, Devaney EJ. Risk factor analysis for second-stage palliation of single ventricle anatomy. Ann Thorac Surg. 2012;93(2):614-618; discussion 619.

[6] Alsoufi B, Manlhiot C, Awan A, Alfadley F, Al-Ahmadi M, Al-Wadei A, McCrindle BW, Al-Halees Z. Current outcomes of the Glenn bidirectional cavopulmonary connection for single ventricle palliation. Eur J Cardiothorac Surg. 2012;42(1):42-49.

[7] Choi RS, DiNardo JA, Brown ML. Superior Cavopulmonary Connection: Its Physiology, Limitations, and Anesthetic Implications. Semin Cardiothorac Vasc Anesth. 2020;24(4):337-348.

[8] 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.

[9] Calvaruso DF, Rubino AS, 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; discussion 1395-1396.

[10] 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.