Systemic-Pulmonary Shunt #2: Modified BTT Shunt Technique

1. Concept of the Modified Blalock-Taussig-Thomas Shunt

The modified Blalock-Taussig-Thomas shunt is a systemic-to-pulmonary artery shunt constructed with an interposed PTFE graft. It is used to augment pulmonary blood flow in neonates and infants with ductal-dependent or severely restricted pulmonary circulation, particularly in cyanotic congenital heart disease.

Unlike the classic Blalock-Taussig shunt, in which the subclavian artery is directly anastomosed to the pulmonary artery, the modified technique places a prosthetic conduit between a systemic artery and a branch pulmonary artery. This preserves the continuity of the subclavian artery while allowing calibrated systemic arterial flow into the pulmonary circulation.

The essential surgical objective is simple but technically demanding:

A short, straight, non-kinked PTFE conduit must be constructed from the subclavian or innominate artery to the branch pulmonary artery using wide, non-stenotic end-to-side anastomoses.

In neonatal series, the modified BTT shunt has demonstrated effective palliation with improved oxygenation and acceptable early patency, but mortality and morbidity remain clinically important because the circulation becomes highly dependent on the balance between shunt flow, systemic perfusion, and pulmonary vascular resistance [1–3].

2. Surgical Anatomy

2.1 Systemic Arterial Inflow

The proximal inflow for a modified BTT shunt is usually obtained from one of the following vessels:

1. Right subclavian artery
2. Innominate artery
3. Less commonly, the left subclavian artery or another systemic arterial source depending on anatomy and surgical exposure

In a right-sided shunt, the right subclavian artery or innominate artery provides systemic-pressure arterial inflow. The choice depends on patient size, vessel caliber, branch pulmonary artery anatomy, prior operations, and the intended route of the graft.

2.2 Pulmonary Artery Target

The distal anastomosis is usually constructed to the ipsilateral branch pulmonary artery, most commonly the right pulmonary artery for a right-sided shunt. The target pulmonary artery must be dissected sufficiently to permit accurate placement of the graft without tension, twisting, or compression.

Important anatomic considerations include:

1. Diameter and quality of the branch pulmonary artery
2. Proximity to the ductus arteriosus or ductal tissue
3. Relationship to the ascending aorta, superior vena cava, and airway
4. Risk of distortion or narrowing of the branch pulmonary artery after graft implantation
5. Need to preserve future pulmonary artery growth and future reconstructive options

Branch pulmonary artery geometry is not a minor technical issue. Although the shunt is intended to improve pulmonary blood flow, it may also influence subsequent pulmonary artery development. In one neonatal series, the ratios of right pulmonary artery diameter and pulmonary valve annulus diameter to descending aortic diameter increased substantially after modified BTT shunt placement, suggesting that effective shunt flow can promote pulmonary artery growth when the graft is patent and well positioned [1]. Conversely, shunt-related pulmonary artery distortion or distal branch pulmonary artery stenosis may complicate later staged palliation or definitive repair [4].

3. Choice and Preparation of the PTFE Graft

The graft is typically a small-caliber PTFE vascular conduit. In neonates and small infants, commonly used graft sizes include 3.0 mm, 3.5 mm, and 4.0 mm. Selection must account for body weight, pulmonary artery size, systemic oxygen requirement, ventricular physiology, pulmonary vascular resistance, and the risk of pulmonary overcirculation.

Graft sizing is one of the most important determinants of outcome. A larger graft may improve oxygen saturation but increases the risk of excessive pulmonary blood flow, diastolic runoff, systemic hypoperfusion, pulmonary edema, and ventricular volume loading. A smaller graft may reduce pulmonary overcirculation but increases the risk of inadequate pulmonary blood flow and shunt thrombosis. In neonates with single-ventricle physiology, increased shunt size-to-weight ratio has been identified as an independent risk factor for mortality [5]. In small infants intended for biventricular repair, a 3.0-mm graft provided adequate pulmonary blood flow without hospital death or shunt occlusion, whereas a larger graft group included a case of cardiogenic shock from excessive pulmonary blood flow [6].

The graft must be cut to an appropriate length. The ideal graft is:

1. Short enough to avoid redundancy and kinking
2. Long enough to avoid tension on either anastomosis
3. Oriented in a smooth, gentle course
4. Free from compression by the sternum, thymus, lung, or surrounding mediastinal structures
5. Positioned to avoid distortion of the subclavian/innominate artery and pulmonary artery

Excessive graft length predisposes to kinking, thrombosis, and unstable pulmonary blood flow. Excessively short length may place tension on the anastomoses and cause bleeding, distortion, or narrowing.

4. Proximal Anastomosis to the Systemic Artery

The systemic artery is carefully mobilized to obtain sufficient exposure while avoiding unnecessary circumferential devascularization. Partial occlusion or temporary control is applied depending on the operative strategy and patient stability.

The graft is anastomosed end-to-side to the systemic artery using fine polypropylene suture. In reported neonatal techniques, 8-0 polypropylene has been used for small-caliber PTFE graft anastomosis [7]. The arteriotomy should be appropriately sized and matched to the graft diameter. A narrow arteriotomy may create inflow obstruction; an excessively large arteriotomy may increase the risk of bleeding or technical distortion.

Key technical principles include:

1. Maintain a clean, non-traumatized arterial edge
2. Create an arteriotomy proportional to the graft diameter
3. Avoid narrowing the native systemic artery
4. Ensure accurate heel and toe alignment
5. Prevent purse-string narrowing of the graft orifice
6. Confirm that the graft exits in the intended direction without torsion

The proximal anastomosis determines the reliability of systemic inflow. Even mild stenosis at this site can significantly reduce shunt flow because of the small graft diameter and high dependence on laminar, unobstructed flow.

5. Distal Anastomosis to the Branch Pulmonary Artery

The distal anastomosis is constructed end-to-side between the PTFE graft and the branch pulmonary artery. The pulmonary arteriotomy should be wide enough to provide a non-stenotic outflow but not so long that it distorts the pulmonary artery.

Technical priorities include:

1. Select a pulmonary artery site that allows a straight graft course
2. Avoid ductal tissue when possible
3. Prevent narrowing of the branch pulmonary artery lumen
4. Ensure that the graft opening is not hooded, twisted, or partially obstructed
5. Avoid excessive traction on the pulmonary artery
6. Maintain precise suture spacing and depth to reduce bleeding and stenosis

The distal anastomosis is particularly important because branch pulmonary artery distortion may compromise future pulmonary artery growth and future staged palliation or biventricular repair. Reintervention to maintain pulmonary blood flow remains a significant clinical issue after shunt palliation, and shunted patients may develop distal branch pulmonary artery stenosis requiring later catheter-based or surgical management [4].

6. Graft Geometry: The Central Technical Determinant

The function of a modified BTT shunt is highly dependent on graft geometry. Because the conduit is small, even minor kinking, twisting, compression, or anastomotic narrowing can markedly alter flow.

An ideal graft course is:

1. Straight
2. Short
3. Non-kinked
4. Non-compressed
5. Free of torsion
6. Positioned without tension
7. Oriented in a physiologic direction from systemic inflow to pulmonary outflow

A technically patent shunt may still function poorly if the graft is too long, sharply angulated, or compressed after chest closure. Therefore, the graft should be inspected after positioning, after de-airing, after unclamping, and again before chest closure.

One-year patency rates of approximately 90–91% have been reported in historical and neonatal series, but patency is influenced by patient age, graft size, surgical technique, anticoagulation strategy, and postoperative hemodynamics [1,8]. In contemporary cohorts, shunt thrombosis or stenosis remains frequent enough to require systematic surveillance and rapid intervention when suspected [3].

7. De-airing and Unclamping

Before establishing flow through the shunt, the graft must be carefully de-aired. Air embolism into the pulmonary circulation may be better tolerated than systemic embolism, but meticulous de-airing remains essential, particularly in patients with intracardiac mixing or right-to-left shunting.

Unclamping should be gradual and controlled. Abrupt opening of the shunt may produce sudden changes in systemic and pulmonary flow.

After unclamping, the surgeon and anesthesiologist should assess:

1. Immediate shunt filling and pulsatility
2. Bleeding from both anastomoses
3. Graft orientation and absence of kinking
4. Systemic arterial pressure
5. Diastolic pressure
6. Oxygen saturation
7. Ventricular function
8. Evidence of pulmonary overcirculation or inadequate pulmonary blood flow

The early postoperative circulation should be actively managed to maintain adequate systemic pressure and shunt flow. Early heparin initiation and careful hemodynamic management have been associated with reduced shunt blockage in neonatal series [7]. Careful perioperative coagulation monitoring is also emphasized because shunt thrombosis and occlusion can be sudden and fatal [9].

8. Hemodynamic Consequences After Shunt Opening

The modified BTT shunt creates a continuous systemic-to-pulmonary runoff. Therefore, its physiologic effect is not limited to oxygen saturation. It also changes systemic vascular loading conditions and diastolic perfusion pressure.

Expected findings after successful shunt opening include:

1. Improved systemic oxygen saturation
2. Continuous or systolic-diastolic shunt flow
3. Increased pulmonary blood flow
4. Reduced cyanosis
5. A decrease in systemic diastolic pressure due to pulmonary runoff
6. Potential increase in pulmonary venous return and ventricular volume load

The key balance is between adequate pulmonary blood flow and preservation of systemic perfusion. In one series, postoperative oxygen saturation increased significantly after modified BTT shunt placement, with no graft blockage during follow-up; however, survival remained influenced by underlying anatomy and concomitant procedures [10]. Thus, improved oxygenation confirms physiologic effect but does not by itself prove optimal systemic oxygen delivery.

9. Pulmonary Overcirculation

Pulmonary overcirculation occurs when the shunt provides excessive pulmonary blood flow relative to systemic blood flow. This may occur when the graft is too large, pulmonary vascular resistance falls rapidly, or systemic vascular resistance is high enough to drive excessive runoff into the pulmonary circuit.

Clinical and hemodynamic signs include:

1. Excessively high oxygen saturation for the intended physiology
2. Low diastolic blood pressure
3. Wide pulse pressure
4. Metabolic acidosis despite acceptable oxygen saturation
5. Poor systemic perfusion
6. Pulmonary edema
7. Ventricular volume loading
8. Difficulty separating from mechanical ventilation in severe cases

Pulmonary overcirculation is a common and clinically important complication. In a contemporary single-center cohort, pulmonary overcirculation occurred in 30% of patients after Blalock-Taussig shunt placement [3]. Higher graft index was associated with mortality, supporting the principle that graft size must be interpreted relative to body size rather than as an isolated diameter [3]. In small infants, excessive pulmonary blood flow has also been reported with larger grafts, whereas a 3.0-mm graft may provide a safer balance in selected low-weight patients planned for biventricular repair [6].

In the operating room, improved saturation must therefore be interpreted together with diastolic pressure, lactate trend, urine output, mixed venous saturation when available, and overall systemic perfusion.

A high oxygen saturation is not always a sign of an optimal shunt.

10. Shunt Thrombosis

Shunt thrombosis is one of the most catastrophic early complications of a modified BTT shunt. Because the conduit is small and supplies critical pulmonary blood flow, acute obstruction may rapidly cause severe hypoxemia, acidosis, cardiovascular collapse, and cardiac arrest.

Risk factors include:

1. Small graft diameter
2. Low cardiac output
3. Hypotension
4. Hemoconcentration
5. Technical narrowing at either anastomosis
6. Graft kinking or twisting
7. Inadequate anticoagulation or antiplatelet therapy
8. Competitive or unstable pulmonary blood flow
9. Excessive manipulation or endothelial injury at the anastomosis

Reported rates of shunt-related obstruction vary by era, population, and definition. Early shunt occlusion of 3.3% was reported in a neonatal modified BTT shunt series [1]. Other neonatal series reported shunt block requiring reoperation in 3 of 46 patients [2], and sudden fatal shunt occlusion or failure has been described during early follow-up [9]. In a contemporary cohort, thrombosis or stenosis occurred in 23% of patients, emphasizing that shunt obstruction remains a major source of morbidity despite improvements in perioperative care [3].

Early recognition is essential. Sudden desaturation, loss of shunt murmur, reduced pulmonary blood flow on echocardiography, or abrupt hemodynamic collapse should prompt immediate evaluation for shunt obstruction.

11. Immediate Intraoperative Assessment

After the shunt is opened and hemodynamics stabilize, the operative team should confirm both anatomic and physiologic adequacy.

11.1 Surgical Assessment

The surgeon should inspect:

1. Proximal anastomosis
2. Distal anastomosis
3. Graft course
4. Graft length
5. Absence of bleeding
6. Absence of kinking or compression
7. Relationship to the sternum and surrounding structures
8. Branch pulmonary artery geometry

11.2 Physiologic Assessment

The anesthesiology, perfusion, cardiology, and surgical teams should evaluate:

1. Oxygen saturation
2. Arterial blood pressure
3. Diastolic pressure
4. Lactate and acid-base status
5. Ventricular function
6. Pulmonary venous return and atrial filling
7. Need for vasoactive support
8. Adequacy of ventilation and pulmonary vascular resistance control

The goal is not simply to create a patent shunt, but to create a controlled source of pulmonary blood flow that supports systemic oxygen delivery without stealing systemic perfusion. This is especially important in single-ventricle patients and low-weight neonates, in whom the margin between pulmonary overcirculation and inadequate shunt flow is narrow [5].

12. Common Technical Pitfalls

Important technical pitfalls include:

1. Graft too long → redundancy, kinking, thrombosis
2. Graft too short → tension, bleeding, distortion
3. Proximal anastomosis too small → inflow obstruction
4. Distal anastomosis too small → outflow obstruction
5. Graft twist → functional obstruction despite apparent patency
6. Pulmonary artery distortion → branch PA stenosis or uneven pulmonary blood flow
7. Excessive graft size → pulmonary overcirculation and low diastolic pressure
8. Inadequate de-airing → embolic risk
9. Failure to reassess after chest approximation → compression or altered graft geometry
10. Inadequate anticoagulation or coagulation monitoring → early shunt thrombosis
11. Excessive systemic-to-pulmonary runoff → systemic hypoperfusion despite improved saturation

These pitfalls explain why the modified BTT shunt is technically and physiologically demanding despite its apparent simplicity.

13. Clinical Principle

The modified BTT shunt is not merely a connection between two vessels. It is a calibrated surgical resistor placed between the systemic and pulmonary circulations.

Its success depends on three simultaneous goals:

1. Anatomically unobstructed systemic-to-pulmonary flow
2. Hemodynamically appropriate pulmonary blood flow
3. Preservation of systemic perfusion, especially diastolic pressure

A technically elegant shunt is short, straight, non-kinked, and constructed with wide end-to-side anastomoses. A physiologically successful shunt improves oxygenation without producing pulmonary overcirculation or systemic hypoperfusion.

The central operative judgment is therefore not simply whether the graft is open. The surgeon must determine whether the graft is the correct size, in the correct geometry, delivering the correct amount of pulmonary blood flow for the patient’s weight, anatomy, ventricular physiology, and perioperative vascular resistance.

References

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[2] Rao M, Bhan A, Talwar S, Sharma R, Choudhary S, Airan B, Saxena A, Kothari S, Juneja R, Venugopal P. Modified Blalock-Taussig shunt in neonates: determinants of immediate outcome. Asian Cardiovasc Thorac Ann. 2000;8(4):339-343.

[3] Sasikumar N, Hermuzi A, Fan C, Lee K, Chaturvedi R, Hickey E, Honjo O, van Arsdell GV, Caldarone C, Agarwal A, Benson L. Outcomes of Blalock-Taussig shunts in current era: A single center experience. Congenit Heart Dis. 2017;12(6):808-814.

[4] McMullan DM, Permut LC, Jones TK, Johnston TA, Rubio AE. Modified Blalock-Taussig shunt versus ductal stenting for palliation of cardiac lesions with inadequate pulmonary blood flow. J Thorac Cardiovasc Surg. 2014;147(1):397-403.

[5] Alsoufi B, Gillespie S, Kogon B, Schlosser B, Sachdeva R, Kim DW, Clabby M, Kanter K. Results of palliation with an initial modified Blalock-Taussig shunt in neonates with single ventricle anomalies associated with restrictive pulmonary blood flow. Ann Thorac Surg. 2015;99(5):1639-1647.

[6] 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.

[7] Swain S, Dharmapuram A, Reddy P, Ramdoss N, Raghavan S, Kona SM. Neonatal Blalock-Taussig shunt: technical aspects and postoperative management. Asian Cardiovasc Thorac Ann. 2008;16(1):7-10.

[8] Tsai KT, Chang CH, Lin PJ. Modified Blalock-Taussig shunt: statistical analysis of potential factors influencing shunt outcome. J Cardiovasc Surg (Torino). 1996;37(2):149-152.

[9] Ahmad U, Fatimi S, Naqvi I, Atiq M, Moizuddin S, Sheikh KB, Shahbuddin S, Naseem T, Javed M. Modified Blalock-Taussig shunt: immediate and short-term follow-up results in neonates. Heart Lung Circ. 2008;17(1):54-58.

[10] Li D, Wang Y, Lin K, An Q. Modified Blalock-Taussig shunt: a single-center experience and follow-up. Heart Surg Forum. 2020;23(5):E624-E629.