Why is a Glenn shunt (Glenn procedure) performed before a Blalock-Taussig (BT) shunt?

Why Glenn Shunt is Done Prior to BT Shunt

The Glenn shunt is not typically performed before a Blalock-Taussig (BT) shunt; rather, the BT shunt is usually performed first as an initial palliative procedure, followed by the Glenn shunt as part of a staged approach toward Fontan completion in patients with single ventricle physiology.

Correct Sequence of Palliation in Single Ventricle Physiology

Initial Palliation

• The BT shunt (systemic-to-pulmonary artery shunt) is typically the first palliative procedure performed in neonates with restricted pulmonary blood flow to increase pulmonary blood flow and improve oxygen saturation 1
• This initial shunt is usually performed from the ascending aorta to the main or right pulmonary artery 1
• However, the BT shunt causes volume overload on the single ventricle, which can lead to ventricular dilation and failure over time 1

Second Stage: Glenn Procedure

• The Bidirectional Glenn (BDG) or bidirectional cavopulmonary anastomosis (BDCPA) is most commonly performed in infancy or early childhood as the second stage of palliation 1
• The Glenn procedure involves an end-to-side anastomosis of the divided superior vena cava to the undivided right pulmonary artery 1
• This provides a stable source of pulmonary blood flow without volume loading the single ventricle, which is a significant advantage over the BT shunt 1

Benefits of the Glenn Procedure

• The Glenn shunt provides pulmonary blood flow without volume loading the single ventricle, unlike the BT shunt 1
• It results in superior vena cava blood being directed to both right and left pulmonary arteries 1
• The 20-year survival rate after Glenn shunt is approximately 50%, compared to the same survival rate for systemic-PA shunts but with less ventricular strain 1

Complications and Considerations

BT Shunt Complications

• Systemic ventricular dilatation and failure 1
• Atrial fibrillation/flutter 1
• Higher hospital mortality (15% in single ventricle patients vs. 3% in biventricular patients) 2
• Risk factors for mortality after BT shunt include need for cardiopulmonary bypass, unplanned cardiac reoperation, pulmonary atresia, and shunt size/weight ratio 2

Glenn Shunt Complications

• Progressive cyanosis may occur due to relatively greater inferior vena cava flow versus superior vena cava flow 1
• Development of pulmonary arteriovenous fistulas, particularly with classic Glenn procedures 1
• Bidirectional Glenn shunts have fewer pulmonary arteriovenous malformations compared to classic Glenn procedures 1

Pre-Glenn Evaluation

Hemodynamic Assessment

• Cardiac catheterization is indicated before Glenn procedure to assess pulmonary artery obstruction and restore maximal continuous, effective systemic venous flow to pulmonary artery segments 1
• Assessment and elimination of systemic-to-pulmonary vein collaterals and systemic-to-pulmonary artery connections are necessary 1
• For patients with existing systemic-to-pulmonary shunts, the potential for perioperative transcatheter shunt exclusion should be examined 1

Optimal Conditions for Glenn Procedure

• Low pulmonary vascular resistance is critical for successful Glenn physiology 3, 4
• Early extubation provides correct performance of the shunt 4
• Surgeons with training and expertise in congenital heart disease should perform these operations 1

Special Considerations

Additional Pulmonary Blood Flow

• In some cases, additional pulmonary blood flow may be maintained along with the Glenn shunt 1
• The most reliable source is via the native right ventricular outflow tract with native pulmonary stenosis or with a pulmonary artery band 1
• A concomitant systemic-to-pulmonary artery shunt may be added if increased systemic oxygenation is required, but this increases volume load on the single ventricle 1

Path to Fontan Completion

• The Glenn procedure serves as an intermediate step toward eventual Fontan completion in single ventricle patients 1
• The Fontan procedure has approximately 90% 10-year survival rate in the absence of risk factors 1
• Late complications after Fontan include atrial arrhythmias, thrombus formation, protein-losing enteropathy, progressive systemic ventricular failure, and progressive AV valve regurgitation 1

Clinical Pitfalls to Avoid

• Performing Glenn procedure in patients with high pulmonary vascular resistance can lead to failure of the shunt 4
• Inadequate assessment of pulmonary artery anatomy before Glenn procedure can result in suboptimal outcomes 1
• Failure to identify and address collateral vessels can compromise Glenn shunt function 1
• Delayed extubation after Glenn procedure may negatively affect shunt performance 4