Glenn Circulation #2: Cavopulmonary Anastomosis

The bidirectional Glenn procedure is a superior cavopulmonary connection in which the superior vena cava is separated from the right atrium and anastomosed to the pulmonary artery. Its central purpose is to redirect upper-body systemic venous return directly into the pulmonary circulation, thereby reducing ventricular volume load and preparing the patient for later Fontan circulation [1, 2].

In this operation, the success of the procedure depends on both surgical geometry and pulmonary vascular physiology. A technically wide anastomosis is not sufficient if pulmonary vascular resistance is high; conversely, favorable physiology cannot compensate for an obstructed or distorted SVC–PA pathway. The Glenn circulation is therefore best understood as a low-resistance, passive-flow system in which venous pressure, pulmonary arterial anatomy, ventricular function, airway mechanics, and systemic oxygen saturation are tightly linked [3, 4].

1. Surgical Concept

The Glenn procedure converts the SVC from a venous inflow channel to the right atrium into a direct pulmonary inflow conduit.

Before Glenn circulation:

• The functional single ventricle supports both systemic and pulmonary circulations.
• Pulmonary blood flow may depend on a systemic-to-pulmonary shunt, ductal stent, native outflow, or residual antegrade pulmonary flow.
• The ventricle is exposed to excessive volume load.

After Glenn circulation:

• SVC blood flows directly into the pulmonary arteries.
• Ventricular volume load is reduced.
• Pulmonary blood flow becomes dependent on a favorable pressure gradient from the SVC to the pulmonary venous atrium.
• Low pulmonary vascular resistance becomes essential for stable circulation.

This transition is important. The Glenn is not simply an anatomic connection between the SVC and pulmonary artery. It is a physiologic conversion from a ventricle-driven pulmonary circulation to a venous pressure-driven pulmonary circulation.

2. Key Operative Steps

2.1 Establish cardiopulmonary bypass and expose the SVC–PA field

After initiating cardiopulmonary bypass, the SVC and right atrium are decompressed. The SVC is mobilized sufficiently to allow a tension-free anastomosis to the right pulmonary artery.

Important technical points include:

• Adequate mobilization of the SVC
• Avoidance of traction injury
• Clear exposure of the right pulmonary artery
• Control of the branch pulmonary arteries
• Preservation of an unobstructed pathway into both pulmonary arteries

The SVC must reach the pulmonary artery naturally. If the anastomosis is created under tension, the connection may narrow, bleed, distort the RPA, or later become obstructed.

2.2 Divide the SVC at the right atrial junction

The SVC is divided near the SVC–RA junction. The atrial stump is oversewn securely, and the cephalad SVC stump is prepared for cavopulmonary anastomosis.

A key anatomical concern is the sinus node region. The sinus node is located near the junction of the SVC and right atrium, and excessive dissection, cautery, traction, or an injudicious suture line may contribute to postoperative sinus node dysfunction. However, clinical data suggest that technical modifications designed to spare the sinus node do not completely eliminate early sinus node dysfunction, which has been reported in approximately 6–9.8% of patients at discharge [5]. Therefore, the practical principle is not that one specific modification guarantees protection, but that the surgeon must maintain constant awareness of the sinus node region during division and oversewing of the atrial stump.

2.3 Prepare the right pulmonary artery

A longitudinal arteriotomy is created on the superior aspect of the right pulmonary artery. The length of this arteriotomy should match the SVC diameter and allow a broad end-to-side connection.

The pulmonary arteriotomy should be:

• Long enough to avoid an anastomotic waist
• Centered to allow balanced right and left pulmonary artery flow
• Free from distortion or narrowing
• Constructed without excessive upward traction on the RPA

A small arteriotomy can create a restrictive connection even if the SVC itself is normal in caliber. This may lead to elevated Glenn pressure, reduced pulmonary blood flow, facial venous congestion, and impaired candidacy for future Fontan completion.

3. Construction of the Cavopulmonary Anastomosis

The standard bidirectional Glenn is an end-to-side SVC–RPA anastomosis. The technical goal is to create a wide, smooth, nonrestrictive pathway from the SVC into both branch pulmonary arteries.

Essential surgical principles

1. Full-caliber anastomosis

The anastomosis should preserve the full diameter of the SVC. A purse-string effect from the running suture must be avoided.

2. Tension-free geometry

The SVC should sit naturally on the RPA without stretch. Tension increases the risk of narrowing, bleeding, and late obstruction.

3. No torsion or kinking

SVC torsion may be subtle externally but can create significant functional obstruction.

4. Balanced pulmonary artery distribution

The Glenn should allow bidirectional flow into both branch pulmonary arteries. Distortion of the PA confluence may compromise future Fontan physiology.

5. Meticulous posterior wall suturing

The posterior wall is difficult to inspect and revise after completion. Even spacing, consistent bite depth, and avoidance of tissue inversion are critical.

4. Hemodynamic Logic of Glenn Circulation

After Glenn completion, pulmonary blood flow is driven by the pressure gradient between the SVC and the pulmonary venous atrium. Because there is no subpulmonary ventricle, Glenn flow is highly sensitive to pulmonary vascular resistance.

Mathematical modeling has demonstrated that SVC pressure rises substantially as pulmonary vascular resistance increases; in one model, SVC mean pressure increased from 8.2 to 19.2 mmHg as pulmonary vascular resistance increased from 0.8 to 7.9 Wood units [3]. This illustrates a central principle of Glenn physiology: small increases in pulmonary resistance can produce large increases in systemic venous pressure.

Clinically, the Glenn procedure generally improves systemic oxygenation and reduces ventricular volume load. In early experience with high-risk Fontan candidates, arterial oxygen saturation improved after bidirectional cavopulmonary anastomosis, and the procedure helped stabilize patients who were not immediate Fontan candidates [2]. The Glenn therefore functions as both a physiologic unloading operation and an intermediate staging procedure.

5. Intraoperative Assessment After Glenn Completion

After de-airing and stepwise reperfusion, the operation should be assessed anatomically and physiologically.

Anatomical confirmation

• Wide SVC–RPA anastomosis
• No SVC torsion
• No anastomotic narrowing
• No distortion of the right or left pulmonary artery
• Secure oversewn right atrial stump
• No bleeding from the posterior suture line

Physiologic confirmation

• Acceptable Glenn pressure
• Adequate systemic oxygen saturation
• Stable systemic arterial pressure
• Good ventricular function
• Low airway pressure
• No pulmonary venous obstruction
• No evidence of branch PA obstruction
• Favorable acid-base status and ventilation

In practical terms, a high Glenn pressure should trigger a structured evaluation: anastomotic narrowing, SVC torsion, branch PA stenosis, pulmonary venous obstruction, elevated PVR, excessive antegrade pulmonary blood flow, ventricular dysfunction, AV valve regurgitation, airway pressure, hypercarbia, hypoxia, and acidosis should all be considered.

6. Role of Antegrade Pulmonary Blood Flow

One important area of surgical judgment is whether to preserve or eliminate antegrade pulmonary blood flow at the time of Glenn.

Additional antegrade or pulsatile pulmonary blood flow may improve systemic oxygen saturation and may promote pulmonary artery growth [6–8]. Calvaruso and colleagues reported that bidirectional Glenn with antegrade pulmonary blood flow may provide effective temporary palliation before Fontan completion, with relatively favorable oxygen saturation during follow-up [6]. Demirtürk and colleagues also reported a positive effect of additional pulsatile pulmonary blood flow on main pulmonary artery growth [8].

However, this strategy is not universally beneficial. Excessive additional pulmonary blood flow may increase Glenn pressure, maintain ventricular volume load, worsen AV valve regurgitation, and impair the low-resistance physiology needed for Fontan candidacy. Therefore, the decision should be individualized.

Potential advantages

• Higher systemic oxygen saturation
• More pulsatile pulmonary blood flow
• Potential promotion of pulmonary artery growth
• Possible delay or avoidance of early Fontan conversion in selected patients

Potential disadvantages

• Higher Glenn pressure
• Persistent ventricular volume load
• Risk of pulmonary overcirculation
• Increased pleural effusions
• Less efficient unloading of the single ventricle

The operative question is not simply whether antegrade flow is present. The more important question is whether the additional flow is controlled, physiologically useful, and not increasing Glenn pressure or ventricular burden.

7. Bilateral Bidirectional Glenn: Special Considerations

In patients with bilateral SVCs or complex systemic venous anatomy, bilateral bidirectional Glenn may be required. This anatomy is common in heterotaxy and right atrial isomerism.

Bilateral Glenn circulation introduces additional considerations:

• SVC obstruction may occur more frequently.
• Pulmonary blood flow distribution may be asymmetric.
• Central pulmonary artery development may be affected by the geometry of the anastomoses.
• Future Fontan pathway design may be more complex.

Imai and colleagues reported SVC obstruction in 13% of patients after bilateral bidirectional Glenn, particularly in patients with right atrial isomerism; however, SVC obstruction was not identified as a direct risk factor for mortality or Fontan completion in that cohort [9]. Other studies suggest that bilateral bidirectional Glenn may be associated with more complex Fontan completion and potentially less favorable pre-Fontan risk profiles [10]. These findings reinforce the importance of careful anastomotic geometry and long-term surveillance of the cavopulmonary pathway.

8. Technical Pitfalls

8.1 Anastomotic stenosis

A restrictive anastomosis may result from:

• Short RPA arteriotomy
• Size mismatch
• Excessive running-suture tension
• Purse-string narrowing
• Tissue inversion
• Posterior wall narrowing

Even mild narrowing can be physiologically significant because Glenn flow is passive and pressure-dependent.

8.2 SVC torsion

The SVC should not be twisted when brought to the RPA. Torsion can produce functional obstruction despite an apparently adequate external anastomosis.

8.3 Pulmonary artery distortion

Excessive traction on the RPA or an eccentric anastomosis can distort the pulmonary artery confluence. This may impair bilateral pulmonary perfusion and complicate later Fontan completion.

8.4 Sinus node dysfunction

Although surgical attention to the sinus node region is important, sinus node dysfunction may still occur even with modifications intended to protect the sinus node [5]. The surgeon should avoid unnecessary injury around the SVC–RA junction, but postoperative rhythm surveillance remains essential.

8.5 Excessive residual pulmonary blood flow

Residual antegrade flow can be useful when controlled, but excessive flow may raise Glenn pressure and maintain ventricular volume loading. This balance should be assessed intraoperatively and during postoperative follow-up.

9. Practical Surgical Interpretation

The Glenn procedure appears anatomically simple: divide the SVC, oversew the right atrial stump, and connect the SVC to the right pulmonary artery. However, its physiology is unforgiving. Because pulmonary blood flow is passive, the circulation depends on a low-resistance pathway from the SVC through the pulmonary arteries, pulmonary capillary bed, pulmonary veins, atrium, and ventricle.

A good Glenn operation is therefore defined by three conditions:

1. Unobstructed anatomy

The SVC–PA pathway must be wide, straight, and tension-free.

2. Low-pressure physiology

Glenn pressure should remain acceptable, reflecting low pulmonary resistance and no mechanical obstruction.

3. Adequate systemic oxygen delivery

Systemic oxygen saturation should be sufficient, ventricular function should remain stable, and the circulation should tolerate the transition from ventricular-driven to passive pulmonary flow.

The surgeon’s objective is not merely to create a connection, but to create a durable, energy-efficient cavopulmonary pathway. A technically elegant Glenn preserves the full caliber of the SVC, avoids PA distortion, minimizes energy loss, and establishes the physiologic foundation for eventual Fontan circulation.

References

[1] Mazzera E, Corno A, Picardo S, Donato RM, Marino B, Costa D, Marcelletti C. Bidirectional cavopulmonary shunts: clinical applications as staged or definitive palliation. Ann Thorac Surg. 1989;47(3):415-420.

[2] Bridges ND, Jonas RA, Mayer JE Jr, 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] Pennati G, Migliavacca F, Dubini G, Pietrabissa R, de Leval MR. A mathematical model of circulation in the presence of the bidirectional cavopulmonary anastomosis in children with a univentricular heart. Med Eng Phys. 1997;19(3):223-234.

[4] Choi R, DiNardo JA, Brown ML. Superior cavopulmonary connection: its physiology, limitations, and anesthetic implications. Semin Cardiothorac Vasc Anesth. 2020;24(3):217-227.

[5] Cohen MI, Bridges ND, Gaynor JW, Hoffman TM, Wernovsky G, Vetter VL, Spray TL, Rhodes LA. Modifications to the cavopulmonary anastomosis do not eliminate early sinus node dysfunction. J Thorac Cardiovasc Surg. 2000;120(5):891-900.

[6] 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):1389-1396.

[7] 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.

[8] Demirtürk OS, Güvener M, Coşkun İ, Yıldırım SV. Results of additional pulsatile pulmonary blood flow with bidirectional Glenn cavopulmonary anastomosis: positive effect on main pulmonary artery growth and less need for Fontan conversion. Heart Surg Forum. 2013;16(1):E30-E34.

[9] Imai K, Hoashi T, Okuda N, Ohuchi H, Kurosaki K, Ichikawa H. Impact of bilateral bidirectional Glenn anastomosis on staged Fontan strategy and Fontan circulation. Eur J Cardiothorac Surg. 2021;60(4):930-938.

[10] Keizman E, Tejman-Yarden S, Mishali D, Levine S, Borik S, Pollak U, Katz U, Serraf A. The bilateral bidirectional Glenn operation as a risk factor prior to Fontan completion in complex congenital heart disease patients. World J Pediatr Congenit Heart Surg. 2019;10(2):174-181.