Systemic-Pulmonary Shunt #1: Modified BTT Shunt

1. Concept and Purpose

A systemic-pulmonary shunt is a palliative surgical connection between the systemic arterial circulation and the pulmonary arterial circulation. Its purpose is to provide a dependable source of pulmonary blood flow when native antegrade pulmonary blood flow is inadequate or unreliable.

The modified Blalock–Taussig–Thomas shunt, commonly called the modified BTT shunt or modified BT shunt, uses an interposition prosthetic graft, usually PTFE, between a systemic artery and a pulmonary artery. In the contemporary modified technique, the subclavian artery or innominate artery is connected to the pulmonary artery through a graft rather than being divided and directly anastomosed to the pulmonary artery [1].

This operation does not correct the underlying cardiac anatomy. Instead, it stabilizes oxygenation, promotes pulmonary artery development, and serves as a bridge toward definitive repair or staged single-ventricle palliation.

2. Basic Anatomy of the Modified BTT Shunt

The modified BTT shunt creates a controlled systemic-to-pulmonary arterial pathway.

Typical anatomical components include:

1. Systemic arterial source
• Right subclavian artery
• Innominate artery
• Less commonly, another systemic arterial source depending on anatomy and surgical exposure
2. Pulmonary arterial target
• Usually the right pulmonary artery
• Occasionally the left pulmonary artery or central pulmonary artery, depending on the lesion and operative strategy
3. Interposition graft
• Usually a small PTFE graft
• Graft diameter is selected to balance adequate pulmonary blood flow against pulmonary overcirculation

The modified technique preserves continuity of the subclavian artery and provides a more reproducible graft-based conduit than the original classic shunt.

3. Physiologic Mechanism

The modified BTT shunt delivers blood from the high-pressure systemic arterial circulation into the lower-pressure pulmonary arterial circulation.

This increases pulmonary blood flow, which leads to:

1. Improved systemic oxygen saturation
• More blood reaches the pulmonary vascular bed for oxygenation.
• Pulmonary venous return to the left atrium increases.
• Systemic arterial oxygen saturation rises.
2. Pulmonary artery growth
• Increased flow through the branch pulmonary arteries promotes pulmonary arterial development.
• This is particularly important when future repair or staged palliation depends on adequate pulmonary artery size.
3. Increased pulmonary venous return and left-sided preload
• Increased pulmonary blood flow returns to the left atrium and left ventricle.
• In selected biventricular lesions, this may help maintain or augment left-sided chamber volume loading.
4. Bridge to the next surgical stage
• In biventricular lesions, the shunt may bridge the patient to complete repair.
• In single-ventricle physiology, it may serve as initial palliation before Glenn and Fontan circulation.

4. Graft Size and Hemodynamic Calibration

The modified BTT shunt is a calibrated hemodynamic intervention. The graft must be large enough to provide adequate pulmonary blood flow, but small enough to avoid excessive left-to-right shunting.

In neonates and small infants with biventricular circulation, a 3.0-mm graft has been reported as a safe and effective option for patients around 3.5 kg, providing adequate pulmonary blood flow while reducing the risk of excessive shunting [2]. However, graft selection must be individualized according to body weight, pulmonary artery size, pulmonary vascular resistance, native antegrade pulmonary blood flow, ductal contribution, ventricular function, and the intended surgical pathway.

Excessive graft size may cause:

• Pulmonary overcirculation
• Low diastolic systemic pressure
• Coronary hypoperfusion
• Systemic hypoperfusion
• Ventricular volume overload
• Pulmonary edema
• Heart failure

Insufficient graft size may cause:

• Persistent cyanosis
• Inadequate pulmonary blood flow
• Poor pulmonary artery growth
• Increased risk of low-flow shunt thrombosis

5. Common Clinical Indications

A modified BTT shunt is considered when pulmonary blood flow is inadequate and the patient requires a reliable surgical source of pulmonary perfusion.

Common indications include:

1. Tetralogy of Fallot with severe RVOT obstruction
• Particularly in small infants, high-risk neonates, or patients not yet suitable for complete repair.
• The shunt improves oxygenation and may allow somatic and pulmonary artery growth before definitive repair.
2. Pulmonary atresia or critical pulmonary stenosis
• Used when antegrade pulmonary blood flow is absent or severely limited.
3. Single-ventricle physiology with reduced pulmonary blood flow
• Used as an initial source of pulmonary blood flow before superior cavopulmonary connection.
• Examples include selected cases of tricuspid atresia, pulmonary atresia with intact ventricular septum, and other single-ventricle variants with inadequate pulmonary blood flow.
4. Staged or hybrid palliation strategies
• In selected high-risk neonatal pathways, a systemic-to-pulmonary shunt may be incorporated into staged management.

6. Role in Tetralogy of Fallot

In Tetralogy of Fallot, pulmonary blood flow is limited by right ventricular outflow tract obstruction, which may involve the infundibulum, pulmonary valve, main pulmonary artery, or branch pulmonary arteries.

A modified BTT shunt can be used when:

• Cyanosis is severe.
• Hypercyanotic spells are recurrent or difficult to control.
• The patient is too small or clinically unstable for complete repair.
• Pulmonary arteries require growth before definitive repair.

The shunt increases pulmonary blood flow independently of the native RVOT. This improves oxygen saturation and may create better conditions for later intracardiac repair.

However, the shunt also creates a continuous systemic-to-pulmonary runoff. Excessive flow can lead to pulmonary overcirculation, ventricular volume loading, low diastolic pressure, systemic hypoperfusion, and heart failure. Therefore, shunt size and postoperative management are central determinants of outcome.

7. Role in Single-Ventricle Palliation

In single-ventricle physiology with inadequate pulmonary blood flow, the modified BTT shunt may serve as the first-stage palliation.

The goal is not to create normal circulation, but to establish a controlled source of pulmonary blood flow until the patient is ready for the next stage, usually a bidirectional Glenn procedure.

The staged pathway may include:

1. Initial palliation
• Modified BTT shunt or another source of pulmonary blood flow
2. Superior cavopulmonary connection
• Bidirectional Glenn or hemi-Fontan
3. Fontan completion
• Total cavopulmonary connection

Single-ventricle physiology is consistently associated with higher risk after modified BTT shunt placement. In one neonatal series, hospital mortality was substantially higher in single-ventricle patients than in biventricular patients, emphasizing that the same shunt physiology carries different risk depending on the underlying circulation [3].

8. Outcomes and Risk Factors

Although the modified BTT shunt remains an essential palliative operation, it is not a benign procedure. Reported early mortality varies widely according to patient anatomy, institutional practice, era, body weight, and perioperative management.

Important risk factors for death or failure to progress include:

• Single-ventricle physiology
• Low birth weight, particularly ≤2.5 kg
• Pulmonary atresia
• Use of cardiopulmonary bypass
• Small or distorted pulmonary arteries
• Restrictive interatrial communication in selected single-ventricle lesions
• Postoperative pulmonary overcirculation or low systemic output

In a neonatal cohort, mortality was approximately 3% in biventricular patients and 15% in single-ventricle patients, illustrating the importance of underlying anatomy in determining risk [3]. Protocol-based postoperative management has been associated with improved outcomes, with one study reporting lower mortality after implementation of standardized management pathways for systemic-to-pulmonary shunts [4].

Shunt occlusion remains a major complication, but contemporary series suggest that careful surgical technique, appropriate graft sizing, and effective antithrombotic management can reduce clinically significant graft blockage [5].

9. Surgical Principles

Key technical principles include:

1. Appropriate graft size
• The graft must provide enough pulmonary blood flow for oxygenation.
• It must avoid excessive runoff and pulmonary overcirculation.
• Neonatal grafts are commonly in the 3.0–4.0 mm range, depending on body weight, anatomy, and institutional strategy.
2. Tension-free graft course
• The graft should lie without kinking, twisting, compression, or excessive tension.
• The course should account for lung expansion, mediastinal geometry, and future re-entry.
3. Protection of branch pulmonary arteries
• The pulmonary artery anastomosis should avoid stenosis or distortion.
• Branch pulmonary artery growth is one of the major goals of palliation.
4. Avoidance of excessive competitive flow
• Native antegrade flow, ductal flow, and shunt flow must be considered together.
• In some patients, ductal closure or adjustment of other pulmonary blood flow sources is necessary after shunt placement.
5. Meticulous hemostasis and thrombotic prevention
• Neonatal vessels and small PTFE grafts are technically unforgiving.
• Bleeding, thrombosis, or anastomotic narrowing can rapidly become clinically significant.

10. Postoperative Physiology and Monitoring

After modified BTT shunt placement, the patient has a surgically created systemic-to-pulmonary runoff. Postoperative management requires continuous assessment of the balance between pulmonary and systemic blood flow.

Important postoperative targets include:

• Adequate systemic oxygen saturation
• Preserved systemic perfusion
• Acceptable diastolic blood pressure
• No evidence of excessive pulmonary blood flow
• Stable ventricular function
• Patent shunt flow on echocardiography
• No significant branch pulmonary artery distortion
• Appropriate antiplatelet or anticoagulation strategy according to institutional protocol

A falling oxygen saturation may suggest:

• Shunt obstruction or thrombosis
• Increased pulmonary vascular resistance
• Inadequate shunt size
• Branch pulmonary artery stenosis
• Reduced systemic output

A very high oxygen saturation, especially with low diastolic pressure, tachycardia, pulmonary edema, or rising lactate, may suggest pulmonary overcirculation and systemic steal.

11. Contemporary Context: Surgical Shunt Versus Ductal Stenting

For ductal-dependent pulmonary blood flow, ductal stenting has become an important alternative to surgical systemic-to-pulmonary shunting in selected patients. Multicenter and meta-analytic data suggest that ductal stenting may offer comparable early mortality and, in some series, improved medium-term outcomes, but it may also carry a higher risk of reintervention depending on ductal anatomy and institutional expertise [6,7].

Therefore, the decision between modified BTT shunt and ductal stenting should be individualized. Surgical shunting remains particularly relevant when ductal anatomy is unfavorable for stenting, when branch pulmonary artery anatomy requires surgical management, when concomitant surgical procedures are needed, or when institutional strategy favors operative palliation.

12. Key Clinical Message

The modified BTT shunt is a systemic-to-pulmonary arterial shunt that provides controlled pulmonary blood flow when native antegrade pulmonary blood flow is inadequate.

Its physiologic benefits are:

• Improved oxygenation
• Promotion of pulmonary artery growth
• Increased pulmonary venous return
• Stabilization before definitive repair or staged palliation

Its major risk is imbalance. Too little flow causes cyanosis and inadequate pulmonary perfusion; too much flow causes pulmonary overcirculation, systemic hypoperfusion, and ventricular volume loading.

Therefore, the modified BTT shunt should be understood not simply as a conduit, but as a calibrated hemodynamic intervention whose success depends on anatomy, graft size, operative precision, and postoperative flow management.

References

[1] Yuan SM, Shinfeld A, Raanani E. The Blalock-Taussig shunt. J Card Surg. 2009;24(2):101-108.

[2] Shibata M, Itatani K, Oka N, Yoshii T, Nakamura Y, Kitamura T, Horai T, Miyaji K. Optimal graft size of modified Blalock-Taussig shunt for biventricular circulation in neonates and small infants. Int Heart J. 2015;56(5):533-536.

[3] Alsoufi B, Gillespie S, Mori M, Clabby M, Kanter K, Kogon B. Factors affecting death and progression towards next stage following modified Blalock-Taussig shunt in neonates. Eur J Cardiothorac Surg. 2016;50(1):169-177.

[4] Ismail SR, Almazmi MM, Khokhar R, AlMadani W, Hadadi A, Hijazi O, Kabbani MS, Shaath G, Elbarbary M. Effects of protocol-based management on the post-operative outcome after systemic to pulmonary shunt. Egypt Heart J. 2018;70(4):271-278.

[5] Li D, Wang Y, Lin K, An Q. Modified Blalock-Taussig shunt: a single-center experience and follow-up. Heart Surg Forum. 2020;23(1):E053-E057.

[6] Bentham JR, Zava NK, Harrison WJ, Shauq A, Kalantre A, Derrick G, et al. Duct stenting versus modified Blalock-Taussig shunt in neonates with duct-dependent pulmonary blood flow: associations with clinical outcomes in a multicenter national study. Circulation. 2018;137(6):581-588.

[7] Alsagheir A, Koziarz A, Makhdoum A, Contreras J, Alraddadi H, Abdalla T, et al. Duct stenting versus modified Blalock-Taussig shunt in neonates and infants with duct-dependent pulmonary blood flow: a systematic review and meta-analysis. J Thorac Cardiovasc Surg. 2021;161(2):379-390.e8.