Central Aortopulmonary Shunt

Anatomy and Physiology

The patient’s age, size, anatomy, physiology, and overall physical condition are key factors in determining the most appropriate systemic-to-pulmonary artery shunt. The mBTT shunt is commonly used due to its minimal dissection requirements in a chest that will likely require reoperation, its relative technical ease, and the ability to regulate blood flow based on graft size. However, a key limitation of the mBTT shunt is its unequal distribution of blood flow to the branch pulmonary arteries due to its anastomosis to either the right or left pulmonary artery (opposite the side of the ductus).

The primary objective of CAPS is to ensure adequate pulmonary blood flow to promote the development and growth of the hypoplastic pulmonary artery bed, alleviate hypoxia and cyanosis, and support overall growth. For CAPS to effectively promote pulmonary artery growth, the patient must have confluent branch pulmonary arteries.

Advantages and Disadvantages of Central ShuntsCAPS offer several benefits and challenges when used in small children with congenital heart disease:

Advantages

• Suitable for small children with small peripheral vessels
• Avoids distortion of the pulmonary branch arteries
• Provides equal blood flow to both branch pulmonary arteries
• Lower occlusion rate compared to BTT shunts
• Avoids subclavian artery steal syndrome

Disadvantages:

• Risk of developing pulmonary arterial hypertension
• Difficulty of takedown
• High risk of graft injury upon reoperation

Preoperative imaging is essential to guide the decision-making process

• A transthoracic echocardiogram is crucial for evaluating the underlying congenital heart pathophysiology.
• A computed tomography angiogram provides detailed anatomical information, including aortic arch sidedness, brachiocephalic branching pattern, patent ductus arteriosus sidedness, pulmonary artery morphology, and coronary artery anatomy.
• An angiogram is valuable, particularly in patients with diminutive pulmonary arteries and significant aortopulmonary collateral arteries.

Hemodynamic Considerations:

• CAPS function similarly to a patent ductus arteriosus, allowing continuous blood flow during diastole.
• Careful management is necessary to balance pulmonary and systemic circulation (Qp:Qs ratio), with an ideal ratio of 1:1.
• Excessive pulmonary blood flow can result in systemic steal, leading to secondary organ dysfunction.
• Additionally, continuous diastolic flow in the pulmonary arteries can cause coronary steal, increasing the risk of myocardial ischemia.

Postoperative care is critical to optimize patient outcomes and prevent complications:

• Ventilation management: Avoid respiratory alkalosis to prevent unwanted changes in pulmonary vascular resistance.
• Hemodynamic management: Monitor systemic and diastolic blood pressure closely, using milrinone, vasopressin, epinephrine, and norepinephrine as needed.
• Oxygen delivery optimization: Maintain hemoglobin levels between 12 to 14 g/dL and ensure the patient remains euvolemic.
• Shunt patency: Initiate an unfractionated heparin infusion in the early postoperative period and transition to antiplatelet therapy for long-term management.

By carefully considering patient-specific factors and implementing meticulous perioperative management, CAPS can serve as an effective bridge to definitive surgical repair in children with congenital heart disease.

Indications

Central aortopulmonary shunts are commonly used in significant pulmonary artery hypoplasia cases, such as pulmonary atresia with ventricular septal defects and major aortopulmonary collateral arteries.[13] These shunts involve an anastomosis between the main pulmonary artery and the aorta, theoretically providing equal blood flow to both branch PAs, promoting balanced growth and development. Palliation through a central aortopulmonary shunt is often necessary as an interim solution to facilitate the growth of hypoplastic but confluent central pulmonary arteries while carrying a lower risk of shunt thrombosis and overflow than other shunt types.[11][14]

Pulmonary Artery Growth Assessment: The Nakata IndexThe Nakata index, also known as the pulmonary artery index, is a key metric for assessing pulmonary artery size; this is calculated by summing the cross-sectional areas of the right and left pulmonary arteries and dividing them by the patient’s body surface area. One-stage surgical correction is typically considered in patients with a Nakata index above 150 mm²/m². For patients with a lower Nakata index, palliation with an aortopulmonary shunt is often recommended to promote pulmonary artery growth before definitive repair.[15]

Indications for Aortopulmonary Shunt

Aortopulmonary shunts are indicated as a palliative intervention in patients with cyanotic congenital heart lesions with extremely low weight or hypoplastic pulmonary vasculature, including:

• Tetralogy of Fallot with hypoplastic pulmonary arteries
• Tricuspid atresia with pulmonary stenosis
• Pulmonary atresia with an intact ventricular septum and right ventricular-dependent coronary circulation
• Pulmonary atresia with ventricular septal defects and diminutive, discontinuous pulmonary arteries
• Ebstein anomaly
• Single ventricle defects with pulmonary or aortic atresia

By carefully selecting candidates for central shunts, surgeons can optimize pulmonary artery growth, improve oxygenation, and pave the way for future definitive repair in patients with complex congenital heart disease.

Contraindications

The following are perceived contraindications to the establishment of CAPS:

• Irreversible pulmonary hypertension (Eisenmenger physiology): If pulmonary vascular resistance is at or above systemic levels, a central aortopulmonary shunt will not increase pulmonary flow effectively and instead can cause immediate right-to-left shunting with severe hypoxemia.
• Severely depressed ventricular function: All central shunts increase pulmonary venous return to a uni- or biventricular heart, which can precipitate heart failure if the systemic ventricle cannot accommodate the extra volume.
• Lack of a suitable pulmonary artery target: An absolute anatomical contraindication is the absence or inaccessible location of the target pulmonary artery for a given shunt. For instance, a Potts shunt cannot be performed if the patient has no left pulmonary artery or if the left PA is extremely hypoplastic and unable to receive flow. Similarly, a Waterston shunt is not feasible if the right pulmonary artery is absent or severely small. In patients with discontinuous pulmonary arteries (eg, pulmonary atresia with no central pulmonary artery connecting the two lungs), a central shunt to one side alone would leave the other lung unfused. This scenario requires alternative strategies (eg, unifocalization of collaterals) rather than a classic Potts/Waterston.
• Concurrent viable definitive repair option: In modern practice, early primary repair is preferred for many conditions (like tetralogy of Fallot) to avoid shunt-related complications. Thus, a central shunt is contraindicated when proceeding with definitive surgery is possible and safe.

Equipment

The CAPS should be conducted in a specialized congenital cardiac surgery operating room equipped with continuous hemodynamic monitoring, appropriate instruments, and a team trained in neonatal cardiac surgery. The equipment typically required for the CAPS includes but is not limited to:

• Cardiopulmonary bypass pump, circuit, and cannulas (appropriate to the patient's age)
• Cardioplegia solution
• Echocardiography with a transesophageal and epicardial probe
• Sterile drapes, gowns, supplies
• Surgical instruments, including a sternotomy saw, thoracotomy sets, blades, clamps, and fine sutures
• Conduit material for planned shunt type (Teflon, GoreTex)

Occasionally, patching materials are required, preferably auto-pericardial—especially during takedown.

Personnel

The personnel typically required to perform the CAPS includes but is not limited to:

• Congenital cardiac surgeon
• Pediatric cardiac anesthesiologist
• First surgical assistant
• Surgical technician or operating room nurse
• Circulating or operating room nurse
• Pediatric perfusionist
• Pediatric cardiologist with or without an echocardiographer

Perioperative care should ideally be performed in a specialized congenital cardiac intensive care unit where pediatric intensivists and nurses are familiar with the unique postoperative care required for these neonates.

Preparation

When preparing for a CAPS procedure, all relevant clinical imaging, including a transthoracic echocardiogram, should be reviewed. Particular attention should be paid to the size of the ascending/descending aorta, the Nakata or McGoon index, ventricular function, the level of atrioventricular valve regurgitation, the size and level of restriction of the ventricular septum, and the anatomy of the aortic arch and isthmus.

Given these patients' high pulmonary vascular resistance, achieving balanced systemic (Qs) and pulmonary (Qp) blood flow is crucial; a Qp:Qs ratio approximating 1 is desirable. Oxygen saturations should ideally range between 75% and 85%. Supplemental oxygen and invasive respiratory support should be limited, as too much blood flow into the pulmonary circulation will decrease systemic cardiac output. The patent arterial duct should not be manipulated until after the establishment of the shunt.

Technique or Treatment

The Waterston-Cooley shunt and the Potts shunt are rarely used today, so central shunts using PTFE and the MEE (or Melbourne) shunt will be described. Central shunts using PTFE can be performed side-to-side or end-to-side, with good results in shunt patency and growth of the pulmonary arteries.[9][16][17][18][19]

Potts Shunt

This procedure is performed via a left thoracotomy, involving dissection of the descending aorta and the left pulmonary artery. The distal left pulmonary artery is secured using snares. At the same time, a curved vascular clamp is applied to occlude a section of the descending aorta and the proximal left pulmonary artery, thus maintaining systemic circulation and decreasing the risk of spinal ischemia and paraplegia. The anastomosis is taken through this clamped portion using continuous 8-0 polypropylene, after deciding the size of the anastomosis. The Potts shunt is more straightforward than the classical BTT, and cardiopulmonary bypass is optional. A novel valved, unidirectional Potts shunt has been proposed for treating idiopathic pulmonary arterial hypertension in adults. Dissimilar to the classic Potts shunt, this new procedure allows for right-to-left shunting, delaying the need for a lung transplant in these patients.[20]

Waterston-Cooley Shunt

Unlike the Potts shunt, this shunt is performed through a right thoracotomy. The rest of the procedure is similar to the Potts shunt, with the crucial difference of clamping the right pulmonary artery and the ascending aorta instead of the left pulmonary artery and the descending aorta. The Cooley technique involves an anterior intrapericardial approach to the superior vena cava. The Waterston shunt is typically taken down during definitive intracardiac repair via median sternotomy. Before cardiopulmonary bypass (CPB) initiation, careful dissection and occlusion of the pulmonary arteries are performed to prevent pulmonary runoff and ensure adequate systemic perfusion. During CPB, with the aorta cross-clamped distal to the shunt, the aorta and right pulmonary artery are separated along the original anastomotic line. The aortic opening is usually closed primarily with a running suture, while a PTFE or pericardial patch is used to repair the right pulmonary artery, addressing any stenosis caused by the shunt. An alternative technique, first proposed by Cooley, involves anteriorly opening the aorta after cross-clamping and closing the opening from within or completely transecting the aorta to improve the exposure of the right pulmonary artery; this is mainly due to this complicated takedown and the need for right pulmonary artery repair that the Waterston shunt fell out of favor.

PTFE Central Shunt

This shunt is sometimes called the modified Davidson shunt in recognition of the work of Dr James Davidson, who first described the procedure in 1955. Through a median sternotomy, the main pulmonary artery and its branches are thoroughly dissected. The vessels are looped and snared. Cardiopulmonary bypass can be used at the surgeon’s discretion. Heparin is administered before clamping the vessels. Gore-Tex conduit is chosen and prepared. The size and length of the graft determine the amount of pulmonary blood flow and should be tailored based on the patient's size. Usually, grafts are 3 to 6 mm, depending on the patient’s body weight and the main pulmonary artery size.

The graft length can vary depending on the patient's size, but approximately should be between 0.5 cm. The distal central PA is dissected and incised. The graft is anastomosed end-to-side to the main pulmonary artery with an 8-0 polypropylene running suture. The anterior aspect of the ascending aorta is partially clamped with a side-biting clamp. A punch hole of the same diameter as the shunt is performed. Ensuring the proper size of the aortotomy is a key feature of the operation (usually, a 4 mm aortic punch is used). For an end-to-side shunt, the graft is beveled and anastomosed to the aorta around the punch using a 7-0 or an 8-0 polypropylene running suture. For a side-to-side shunt, another incision is made on the posterior aspect of the graft, and the graft is anastomosed to the ascending aorta in a side-to-side manner with a 7-0 or 8-0 polypropylene running suture. The clamps and snares are released. The shunt is deaired. The proximal end of the shunt is closed with 2 hemostatic clips.[16][21][22]

Mee (Melbourne) Shunt

The "Melbourne shunt" originates from the Royal Children's Hospital in Melbourne, Australia, where this technique was extensively applied and further advanced, while the alternative nomenclature "Mee shunt" honors Dr Robert Mee, a distinguished cardiac surgeon who contributed significantly to the development and refinement of this surgical approach. The Mee shunt results in the reimplantation of the main PA to the ascending aorta. This can be performed through a central sternotomy or left thoracotomy. Cardiopulmonary bypass can be used at the surgeon's discretion after Heparin administration.

The ascending aorta and the diminutive main and branch PAs are mobilized, and silastic snares are placed around the left and right PAs. The left side of the ascending aorta is side-clamped as posteriorly as possible. A button of the aortic wall is excised, and the mobilized main pulmonary artery is anastomosed end to side to the aorta using continuous 7-0 polypropylene. Although the intention behind the inception of this shunt was to develop the hypoplastic pulmonary arteries, the results discovered 2 decades later were counterproductive. Results showed that the large diameter of the aorta stretched the right PA, leading to its stenosis in half of the subjects.[23][24]

Complications

Post-operative care is critical in the first 24 hours. Short-term risks include overcirculation, persistent cyanosis, or shunt thrombosis. Intensive care unit care is required to balance the blood flow to the lungs and systemic circulation. Medical management includes careful attention to vent management, fluid administration, vasoactive agents, and optimization of hematocrit. Patients should be anticoagulated 24 to 72 hours post-procedure to decrease the risk of shunt thrombosis.[10] 

The Society of Thoracic Surgeons Congenital Heart Surgery Database revealed that 33% of all shunt deaths occurred in the first 24 hours.[12] Aspirin (10 mg/kg) should be given when the shunt is created and until the shunt is taken down.[4][25] Patency of the shunt ranges from 1 to 4 years.[10] Risk factors for late mortality (90 days) in patients undergoing systemic to pulmonary shunts include lower body weight, preoperative ventilator support, right atrial isomerism and coexistence of major aortopulmonary collateral arteries, and an unbalanced atrioventricular septal defect.[26]

Clinical Significance

CAPS assists in the treatment of cyanotic congenital heart defects that will ultimately result in either univentricular or biventricular cardiac repairs. Central shunts allow for satisfactory PA growth in 72% of patients identified by increased Nakata index and improved oxygenation.[9][11] Recent studies and meta analysis results have compared patent ductus arteriosus (PDA) stenting with traditional surgical aortopulmonary shunts for initial palliation of ductal-dependent pulmonary circulations. The evidence indicates no significant difference in mortality between PDA stenting and surgical shunt. The advantages of patent ductus arteriosus (PDA) stenting encompass reduced durations of mechanical ventilation, abbreviated hospital stays, and a lower incidence of complications. Additionally, patient profiles differ, with a greater proportion of those undergoing PDA stenting having an intact ventricular septum.[27]

Enhancing Healthcare Team Outcomes

Patients with congenital heart disease undergoing CAPS will require lifelong multidisciplinary medical care. The care of these complex cyanotic congenital heart defects occurs in neonates, children, adolescents, and adults. This requires coordinated efforts from a team of caregivers, including ancillary staff, respiratory therapists, neonatal and pediatric nurses, pediatric and adult specialists such as pediatric cardiologists and intensivists, adult congenital heart disease specialists, and congenital cardiac surgeons.