Modified Blalock-Taussig-Thomas Shunt

Anatomy and Physiology

In the United States, CHD affects 1% of all deliveries, with about 25% of these cases classified as cyanotic congenital heart lesions.[11][12] 

Common cyanotic CHD anatomic lesions include, but are not limited to:

• Tetralogy of Fallot (TOF)
• Ebstein anomaly
• Hypoplastic left heart syndrome
• Total anomalous pulmonary venous return
• Transposition of great arteries
• Tricuspid atresia and other univentricular conditions
• Truncus arteriosus 

Alternative physiological classification systems of cyanotic congenital heart lesions include:

• Right-to-left shunting
• Inadequate pulmonary blood flow
• Common mixing lesions

The mBTT shunt offers temporary pulmonary arterial flow for cyanotic infants who are not yet candidates for definitive cardiac repair. Other aortopulmonary shunts, such as the Potts and Waterston shunts, are now rarely used due to high complication rates and challenges with removal during final repair (see Image. Systemic-to-Pulmonary Shunts). The Sano shunt, a right ventricle-to-pulmonary artery conduit, is used during stage I palliation for hypoplastic left heart syndrome and was first introduced by Norwood and colleagues in 1981.[13]

Indications

Palliative shunt procedures are less common today due to advancements in extracorporeal circulatory support, neonatal intensive care units, and improved surgical techniques that enable early corrective interventions. However, certain neonates with specific cyanotic congenital heart defects still require initial palliative surgery, as many of these conditions are not immediately treatable with corrective interventions. These defects often cannot be fully repaired initially, necessitating augmented pulmonary blood flow to support pulmonary artery development.

Additionally, neonates with intracerebral bleeding or other contraindications to cardiopulmonary bypass may be suitable candidates for a palliative mBTT shunt. Severely premature neonates with cyanotic heart lesions may also receive an mBTT shunt until they are mature enough for definitive cardiac reconstructive surgery.

Examples of cardiac anomalies that prevent complete repair but may be treated with the mBTT shunt include:

• Hypoplastic left heart syndrome: An mBTT shunt is placed as part of the Norwood procedure during stage 1 of the repair.[14] 
• Congenitally corrected transposition of the great arteries: An mBTT shunt with pulmonary artery banding is the principal palliative treatment when this anomaly cannot be corrected.
• Coronary artery anomalies: Approximately 5% of neonates with TOF demonstrate the left anterior descending coronary artery crossing the right ventricular outflow tract.[15][16][17]

The mBTT shunt is utilized in various congenital cardiac lesions, particularly univentricular conditions where bidirectional Glenn and Fontan shunts cannot be established until neonatal pulmonary vascular resistance decreases.[18] Other cardiac anomalies that necessitate augmented pulmonary blood flow and promote pulmonary artery development include:

• Pulmonary atresia.
• Ebstein anomaly with functional pulmonary atresia.
• Inadequate pulmonary arteries.
• TOF, with palliative shunts for symptomatic, low-birth-weight neonates who cannot thrive on medical management.
• Ventricular septal defect with hypoplastic major aortopulmonary collateral arteries, where an mBTT shunt may be an option alongside unifocalization.
• Right ventricular outflow tract obstruction, including tricuspid stenosis or atresia, which was the primary indication for the BT shunt as described in the 1945 seminal article.[19]

The modern approach to surgical decision-making is grounded in experience and requires balancing multiple, often conflicting, performance objectives. For example, while the conduit size must be adequate to ensure sufficient blood supply to the pulmonary vascular bed for proper oxygenation, an oversized conduit can cause excessive pulmonary blood flow, leading to complications like pulmonary edema and heart failure. Moreover, the hemodynamics of the shunt may result in additional complications, including shunt stenosis or thrombosis, distortion of the pulmonary arteries, compromised coronary perfusion, and uneven growth of the pulmonary arteries.

Contraindications

Pulmonary hypertension with elevated pulmonary vascular resistance is a relative contraindication for establishing an mBTT shunt.

Equipment

Cardiopulmonary bypass must be available if cardioplegia is required during mBTT shunt placement, although not all cases will necessitate its use. If the surgical approach involves a median sternotomy, an electric sternotomy saw and stainless steel closure wires are essential. In contrast, a minimally invasive thoracotomy requires a Finochietto or Tuffier rib spreader, along with long-shafted pediatric instruments and standard cardiac surgery tools. PTFE grafts with predetermined diameters are the preferred conduits, as they ensure a precise fit for efficient anastomosis and optimal blood flow.

Personnel

The personnel required for the placement of an mBTT shunt typically include:

• Pediatric cardiothoracic surgeon
• Surgical first assistant
• Cardiac anesthesiologist
• Surgical technician or operating room nurse
• Circulating or operating room nurse
• Perfusionist, if cardiopulmonary bypass is utilized

Each healthcare team member plays a vital role in ensuring the procedure's success and safety.

Preparation

Preoperative evaluation of a cyanotic neonate involves a thorough prenatal and postnatal medical history, along with a complete physical examination. Congenital cardiac defects may occur in isolation or as part of a syndrome, making it essential to identify all associated anomalies. A transthoracic echocardiogram, interpreted by an experienced pediatric cardiologist, should include a detailed evaluation of the ascending aorta, aortic arch, and subclavian vessels. In some cases, coronary angiography or computed tomography angiography may be necessary to complete the assessment.

When evaluating a cyanotic neonate, close attention should be given to any history of recurrent respiratory infections, which may signal abnormal pulmonary blood flow. Performing mBTT shunt placement on a neonate or infant with an active respiratory infection increases the risk of perioperative complications. Premature and term infants with a body weight of less than 3 kg are at heightened risk of perioperative complications and may require intraoperative and postoperative cardiopulmonary support.

The diameter of the PTFE interposition graft should be determined preoperatively using the Hagen–Poiseuille equation:

Q = Δpπd⁴/8Lμ

where,

• Q = Volumetric flow rate
• Δp = Pressure difference
• π (pi) = 3.14159...
• d = Diameter of the graft
• L = Length of the graft
• μ = Blood viscosity

A small change in shunt diameter can exponentially affect blood flow, whereas shunt length has a lesser impact. Neonates with a body weight of 2.5 to 4.0 kg typically receive adequate pulmonary blood flow with a PTFE conduit measuring 3.0 to 3.5 mm in diameter. For neonates with a body weight of less than 2.5 kg, a conduit smaller than 3.0 mm is recommended, while those with a body weight of more than 4.0 kg may benefit from a shunt with a 4.0 mm diameter.[20]

Technique or Treatment

Surgical Technique for Modified Blalock-Taussig-Thomas Shunt Placement

The mBTT shunt procedure is typically performed without cardiopulmonary bypass in most centers. While the classical BT shunt was performed via a thoracotomy on the side of the aortic arch, the mBTT shunt is now routinely placed through a median sternotomy. This surgical approach offers several advantages, including the flexibility to place the shunt on either side of the chest, enhanced surgical exposure, and easier access to urgent cannulation and cardiopulmonary support if needed. Moreover, median sternotomy reduces the risk of intraoperative injury to the recurrent laryngeal nerve and lowers the likelihood of postoperative Horner syndrome.

A median sternotomy also allows for the repair of concomitant congenital lesions, such as a patent ductus arteriosus. However, early use of this approach can hinder the development of collateral flow from the internal thoracic arteries, which may complicate and pose challenges for future cardiac interventional procedures.[21]

Regardless of the surgical approach, many surgeons ligate the azygos vein early during the repair of complex lesions to simplify future shunt takedown procedures. mBTT shunt placement also necessitates a subtotal thyroidectomy. Additionally, a laryngoscopic examination during intubation is performed before making the surgical incision.

A standard median sternotomy is performed with a skin incision slightly shorter than the length of the sternum. Mosquito forceps are used to detach the diaphragmatic attachments from the xiphoid process and the inferior sternum. After dividing the sternum, a subtotal thyroidectomy is performed on the side of the shunt placement, taking special care to avoid injury to the recurrent laryngeal nerve. The cervical extension of the thyroid may be preserved on the opposite side. An extensive thymectomy is conducted using electrocautery to excise thymic tissue from its capsule, particularly on the side planned for the shunt.

The pericardium is incised longitudinally at the center, and traction sutures are placed to facilitate access to the right atrium. Early in the procedure, the right atrium is exposed, and a pursestring suture is secured in preparation for emergency cannulation or the insertion of an Intracath^® to monitor right atrial pressure.

The innominate artery is identified and dissected up to the subclavian-to-carotid artery bifurcation. The decision to place the proximal anastomosis on either the innominate or subclavian artery depends on the surgeon's preference and anatomical feasibility, ensuring that no distortion occurs. Retracting the aorta facilitates access to the right pulmonary artery, which should be dissected from its attachment to the main pulmonary artery on one end to the level of the lung hilum on the other.

The surrounding lymph nodes and lymphatics should be resected to ensure the PTFE conduit lies comfortably, eliminating any residual anastomotic tension at the end of the operation. The PTFE graft is beveled on the side intended for anastomosis with the subclavian artery, while the other end is directed toward the right pulmonary artery. Although the PTFE graft has proven to be a superior option, there has been renewed interest in using homografts from the femoral artery or saphenous vein.[22] 

The proximal anastomosis is performed using an atraumatic C-shaped vascular clamp applied to the innominate or subclavian artery. A longitudinal incision is made, and the beveled end of the PTFE graft is sutured to the arteriotomy with monofilament nonabsorbable sutures, such as 6-0, 7-0, or 8-0 polypropylene, in a continuous manner. After completing the proximal anastomosis, heparinization is administered to minimize clot formation within the graft. The graft is cross-clamped, and the C-clamp is released to assess the proximal anastomosis for leaks and to ensure optimal graft position and length. Once confirmed, the distal anastomosis is performed on the right pulmonary artery.

The pulmonary artery is clamped with an atraumatic C-clamp, and an arteriotomy is made along the clamped area to prepare for anastomosis with the distal end of the PTFE conduit. Many surgeons prefer to test the patient's oxygen saturation and hemodynamics by temporarily clamping the pulmonary artery before creating the arteriotomy. The distal anastomosis is performed using the same materials and technique as the proximal connection to the innominate artery. Careful attention is required to ensure proper orientation of the distal anastomosis when the clamps are released. The mBTT shunt is then inspected at both ends for leakage and checked for fibrinous clots within the graft.

In patients who are duct-dependent, the patent ductus arteriosus should be ligated only after confirming the patency of the newly created systemic-to-pulmonary shunt. Shunt patency and flow can be evaluated using echocardiography or manual testing. Manual testing of the shunt involves clamping and releasing it while palpating for a thrill in the distal pulmonary artery and monitoring hemodynamic changes. A spontaneous rise in systemic oxygen saturation, detected via photoplethysmography, serves as a reliable indicator of a functional shunt.

Reversal of heparinization is not essential. Standard practices include routine chest closure with mediastinal drains, placement of atrioventricular pacing leads, and sternal approximation using stainless steel wires. Most patients tolerate the operation well, with minimal need for inotropic support. Upon completion of the procedure, patients require intensive care for at least 1 week, with some potentially needing postoperative extracorporeal membrane oxygenation.

Surgical Technique for Modified Blalock-Taussig-Thomas Shunt Takedown

The takedown of an mBTT shunt is an integral part of any intracardiac repair. Shunt takedown halts the mixing of systemic and pulmonary blood, preventing the superior tenting of the pulmonary artery as the child grows. Tenting has been recognized as a significant contributor to unbalanced pulmonary blood flow between the lungs.

Previous operative notes must be reviewed before the takedown procedure. Preoperative evaluation should include echocardiography, with angiography performed if needed. Following a median sternotomy, comprehensive dissection is often required to identify and mobilize the right pulmonary artery, which may be encased in a thick fibrous peel.

The cardiopulmonary bypass should not be established until the surgeon has full control of the systemic-to-pulmonary shunt, especially in patients with duct-dependent physiology. Once the cardiopulmonary bypass flow is initiated, the shunt may be ligated proximally and distally using hemostatic clips. The surgeon must ensure the PTFE conduit is completely disconnected to prevent arterial tenting and imbalanced pulmonary blood flow. 

Transcatheter occlusion of the mBTT shunt has been reported in hybrid repair settings and isolated cases. However, the risk of embolization from a clot or device is higher with the transcatheter approach than with surgical takedown.[23]

Complications

Despite advancements in surgical techniques, enhanced intensive care services, and cutting-edge surgical materials, systemic-to-pulmonary artery shunt operations continue to exhibit high in-hospital mortality rates, ranging from 2.3% to 16%.[24] However, only 2 factors—body weight less than 4.25 kg and the timing of surgery (elective versus emergency)—have been found to significantly affect mortality following a BT shunt procedure. Specifically, a body weight of less than 4.25 kg increases the mortality risk by 20.8 times, while emergency surgery raises the risk by 3.5 times. Notably, no studies have specifically investigated the correlation between the timing of surgery and mortality risk.

The primary disadvantage of using PTFE as the conduit for the mBTT shunt is the sporadic occurrence of serous leakage through the synthetic polymer fabric. This leakage can lead to localized seroma formation and an extended requirement for tube thoracostomy.

The most significant complication of the mBTT shunt procedure is pulmonary overcirculation in the early postoperative period. This condition may arise from high systemic vascular resistance, low pulmonary vascular resistance, or an inappropriately large systemic-to-pulmonary shunting. Pulmonary overcirculation can result in systemic hypotension, ventricular overload, pulmonary edema, and acidosis. If left uncorrected, it may lead to cardiac arrest and death. The imbalance in blood shunting from the systemic to pulmonary circulation heightens the risk of heart failure and compromises systemic perfusion of vital organs, potentially leading to intestinal ischemia and necrotizing enterocolitis. 

Contrarily, shunt distortion or thrombosis can result in pulmonary hypoperfusion, hypoxia, and cyanosis. To prevent thrombosis, prophylactic heparinization is initiated in the immediate postoperative period at a rate of 10 units/kg/h, with adjustments made based on subsequent coagulation profile results. Additionally, aspirin is started at a dosage of 3 to 5 mg/kg once daily, not exceeding a maximum of 75 mg/d, and is continued for the patient's lifetime.

Although technically demanding and associated with a risk of significant complications, transcatheter intervention can serve as a viable rescue approach for reestablishing patency in the event of thrombotic blockage in an mBTT shunt.[25] Continuous, vigilant monitoring of hemodynamics during early postoperative management is crucial, accompanied by a low threshold for surgical reintervention when necessary.

The functionality of an mBTT shunt can be clinically assessed through palpation and auscultation. A palpable thrill on the side of the shunt that radiates to the infraclavicular region, which was not present preoperatively, is a reassuring sign of shunt functionality and can be confirmed by auscultation over the area. A continuous murmur, best heard in the same subclavicular region, indicates that the systemic-to-pulmonary shunt functions effectively. Transthoracic echocardiography may be used to quantify shunted blood flow, while computed tomography angiography can provide a conclusive evaluation of shunt blood flow.

Clinical Significance

In a large retrospective study spanning 6 decades of research at the Johns Hopkins Hospital, Williams et al reported an overall decline in the use of the mBTT shunt.[18] However, However, they observed a relative increase in its application for treating univentricular heart lesions. Notably, over 75% of the shunts analyzed in this study were classical BT shunts, which limits the applicability of the findings to the mBTT shunt. The mBTT shunt is also considered safer than the Potts or Waterston shunt.[26]

The Single Ventricle Reconstruction trial revealed that patients who underwent the Norwood procedure with right ventricle-to-pulmonary artery shunts had higher survival rates at 12 months without the need for cardiac transplantation compared to those receiving mBTT shunt placement. However, long-term survival rates for both groups were similar, and patients in the Norwood procedure group experienced a significant decline in overall right ventricular function.[27] At 6 years, there were no significant differences between the 2 groups in the risks of death, transplant, or catheter-based interventions.[28][29] Another study revealed that patients with the mBTT shunt experienced a greater decline in weight-for-age z score compared to those with a Sano shunt.[30]

When comparing the mBTT shunt with newer, less invasive, and more cost-effective palliative techniques, such as ductal stenting, McMullan et al reported a higher percentage of procedure-related complications and distal branch pulmonary artery stenosis in the mBTT group, while freedom from intervention was similar in both groups.[31] Right ventricular outflow tract stenting is another less-invasive palliative option that has demonstrated greater safety while being equally effective as the mBTT shunt in neonates with complex TOF lesions.[32] 

The classical BT shunt procedure, known as "the blue baby operation" in the 1940s, gained international fame and has saved the lives of millions of children worldwide. However, palliative procedures in congenital cardiac surgery are increasingly falling out of favor in contemporary clinical practice. The focus has shifted from palliating cardiac lesions to prioritizing the avoidance of complications associated with these interventions.

Enhancing Healthcare Team Outcomes

Modern prenatal care enables the in-utero diagnosis of many congenital cardiac lesions. Neonates requiring surgical repair for complex cardiac conditions benefit from delivery at a tertiary cardiac center when possible. Effective communication and collaborative planning among healthcare specialists, including obstetricians, maternal-fetal medicine specialists, neonatal intensive care teams, pediatric cardiologists, and pediatric cardiothoracic surgeons, during the antenatal period ensures well-coordinated care while the patient is still a fetus. These prearranged plans can then be implemented at delivery, ideally in a controlled environment, thereby optimizing outcomes.

All congenital abnormalities can be evaluated once the neonate is stabilized. Neonatal intensive care clinical staff and pharmacists have integral roles in caring for neonates with cyanosis from complex cardiac lesions. Additionally, family counselors and palliative care teams within many neonatal intensive care units provide necessary support to families navigating the complexities of the care system.

The decision to perform a palliative cardiac procedure requiring intraoperative and postoperative cardiopulmonary support involves the expertise of multiple interprofessional team members, including surgeons, anesthesiologists, perfusionists, cardiologists, pediatricians, echocardiographers, dietitians, nurses, and pharmacists. Care coordinators play a vital role in facilitating communication among these professionals to optimize outcomes for patients undergoing mBTT shunt placement.