Introduction and History
A surgically created connection between a systemic artery and the pulmonary artery (PA) is termed a systemic-to-pulmonary artery shunt or, more commonly, a BT shunt (after Blalock and Taussig, the first physicians to describe the connection). Most would agree that the description of this procedure in the Journal of the American Medical Association (JAMA) in 1945 has revolutionized the field of pediatric cardiology. 
Helen Taussig, a pediatric cardiologist at Johns Hopkins at the time, had several patients with complex cyanotic heart disease with right-sided heart obstruction in the form of either pulmonary stenosis or pulmonary atresia. In that seminal paper, she noted that their cyanosis increased or became more apparent after closure of the ductus arteriosus. She approached Robert Gross, a surgeon in Boston (who, in 1938, was the first to close a patent ductus arteriosus), and asked if he would surgically create a ductuslike connection in these patients. After he refused, she posed the same question to Alfred Blalock, the Chief of Surgery at Johns Hopkins.
Blalock, a few years prior to her request, had attempted to create an animal model for pulmonary hypertension through an anastomosis of the divided subclavian artery to the pulmonary artery.  He was also attempting to treat coarctation of the aorta by turning the subclavian artery down and anastomosing it to the aorta below the area of the coarctation.  A young African American surgical technician named Vivien Thomas was performing this work in an experimental animal laboratory.
After Taussig met with Blalock and Thomas, they decided to use the subclavian artery–to–pulmonary artery connection as a way to augment the pulmonary blood flow. Thomas confirmed this in the laboratory, and Blalock finally performed the procedure on a 15-month-old child with severe cyanosis.  Thomas alone had done all of the previous animal work (in >200 animals), and Blalock had practiced the procedure only once in the animal laboratory.
During the surgery on the child, Blalock had Thomas stand behind him on a stool so that he could observe and instruct him accordingly. This team went on to perform this operation on 2 more children and published their results in JAMA in 1945 to worldwide acclaim. However, the paper listed only the names of Blalock and Taussig, and not until decades later were Thomas's many and important contributions officially recognized. Johns Hopkins bestowed upon him an honorary Doctor of Laws degree in 1976. At the time of Blalock's retirement in 1964, more than 2000 shunt operations had been performed at Hopkins.
Various surgeons have since described many other modifications to this original operation. Since its initial use in tetralogy of Fallot (TOF) and other cyanotic cardiac conditions associated with pulmonary stenosis or pulmonary atresia, the systemic-to-pulmonary artery shunt has been applied to other cardiac defects. For example, systemic-to-pulmonary artery shunting is usually the first of several operations for the treatment of different forms of a single ventricle.
Palliative Surgery and Indications for a Shunt
Palliative surgery increases pulmonary blood flow, temporarily stabilizing the patient and allowing him or her to undergo a future corrective or palliative procedure. Thus, the purpose of the surgically created systemic-to-pulmonary artery shunt is to increase pulmonary blood flow in the interim prior to definitive (ie, anatomic) reconstruction. In their paper, Blalock and Taussig clearly outline the different factors important in the production of cyanosis. Building on the original monograph by Lundsgaard and Van Slyke (1923),  these authors delineated the following 4 factors:
The height of the hemoglobin
The volume of the venous blood shunted into the systemic circulation
The rate that peripheral tissues use oxygen
The extent of aeration of the lungs
They pointed out the importance of a fifth factor (ie, the circulation of the blood flow through the lungs). They maintained that this may be most important because a certain minimal blood flow to the lungs is required to sustain life; the larger the pulmonary blood flow, the higher the systemic oxygen saturation. Even in the presence of intracardiac mixing, increasing the amount of pulmonary blood flow can increase the oxygen saturation of the aortic blood, hence, decreasing cyanosis).
They demonstrated this by delivering blood to the lungs via a surgically created systemic-to-pulmonary artery shunt. The surgeons recognized that, although this did not cure the problem, the procedure alleviated cyanosis and improved exercise tolerance and quality of life in these patients. Indeed, they noted that this was done at the expense of an increased volume load on the left side of the heart because some of this blood had recently returned from the lungs.
Most commonly, the shunt is from a systemic artery (eg, the subclavian or the aorta) to the pulmonary artery (main, right, or left). Another type of palliative shunt is from a systemic vein (superior vena cava [SVC]) to the pulmonary artery. This is termed a Glenn shunt. Because this directs only deoxygenated blood to the pulmonary artery prior to its entry into the heart, it reduces the volume load on the heart. However, because the pressure head here is the SVC pressure, the pulmonary artery pressures must be low for this to be successful. This is typically the case after a baby is older than 2-3 months of age.
Palliative shunts have become less common treatments for most cyanotic heart malformations since the advent of initial corrective procedures during infancy. The lower frequency is primarily due to improved extracorporeal circulation methods, modern perioperative supportive care, and refinements in operative technique. Nevertheless, certain subsets of patients require initial palliative surgery because their cardiac disorders are not generally amenable to initial corrective surgery, owing to either anatomic or physiologic causes.
An example of the former includes patients with TOF with pulmonary atresia or small pulmonary arteries. An example of physiology dictating the need for initial palliation with a shunt is tricuspid atresia or another single ventricle situation. In this circumstance, surgeons can perform the ultimate operations of the bidirectional Glenn and Fontan shunts only after a decline in neonatal pulmonary vascular resistance, a process that can take months.
The indications for a systemic-to-pulmonary artery shunt include the following:
Tetralogy of Fallot
Although TOF was the diagnosis of the first child to undergo a shunt, this operation is no longer routinely performed for this condition. The timing of complete repair remains controversial; some centers advocate early (often neonatal) repair, citing concerns about potential attrition from palliative procedures, effects of chronic cyanosis, and better development of the pulmonary arteries with early repair. [6, 7, 8, 9]
Other centers point out that, although repair in early infancy is possible with comparable mortality rates, it is associated with higher morbidity and greater physiologic stress. [10, 11] The latter group advocates repair at age 3-12 months. Thus, although the push continues toward single reparative operations at younger ages, a palliative shunt may initially be performed in some situations.
In cases of TOF with hypoplastic arteries, palliative shunt may be initially performed.
In cases of TOF in which an anomalous coronary artery, usually the left anterior descending coronary artery, crosses the right ventricular (RV) outflow tract, a palliative shunt may be initially performed. The incidence of a major coronary artery crossing the RV outflow tract in TOF has been reported as approximately 5%. [12, 13, 14]
In such a situation, repair in a larger child should theoretically allow the surgeon more room to relieve the RV obstruction and avoid a conduit.
In cases of TOF with associated complex cardiac lesions, a palliative shunt may initially be performed.
Most patients with tricuspid atresia have pulmonic stenosis in the presence of normally related great arteries (type 1b).  Depending on the degree of the stenosis, these patients may require an initial palliative systemic-to-pulmonary artery shunt prior to a Glenn or a Fontan procedure.
Pulmonary atresia with intact ventricular septum
The systemic-to-pulmonary artery shunt is the method of choice to establish ductal independent pulmonary blood flow. In patients who do not have coronary cameral fistulae, in addition to a shunt, placement of a transannular patch across the pulmonic valve helps to decompress the right ventricle and promote its growth. In patients with RV-dependent coronary circulation, this is contraindicated, and only a shunt may be performed. These patients may be treated as though they have a single ventricle and undergo a Fontan procedure at a later time, or they may be listed for transplantation.
Pulmonary atresia and ventricular septal defect
Early unifocalization of the pulmonary blood flow is the procedure of choice;  however, in the presence of diminutive, discontinuous pulmonary arteries, a systemic-to-pulmonary artery shunt has a limited role. The stated objective is to stimulate growth of the pulmonary arteries prior to placement of a conduit from the right ventricle to the pulmonary arteries.
This condition is often associated with functional pulmonary atresia due to inadequate RV forward flow in the presence of elevated pulmonary vascular resistance. This situation improves after the natural fall in resistance during the first weeks of life. Occasionally, the forward flow may be inadequate, which may necessitate a systemic-to-pulmonary artery shunt. This is also true in the presence of anatomic pulmonary atresia.
Single ventricle situation with pulmonary or aortic atresia
Hypoplastic left heart syndrome in an example of this condition and is an indication for a systemic-to-pulmonary artery shunt, which is part of the Norwood operation, the initial surgery for this condition.
Types of Shunts
Several types of palliative shunts are recognized. An ideal shunt is expected to have the following attributes:
Be rapid and technically simple to perform
Provide adequate but not excessive pulmonary blood flow, hence minimizing the risk of congestive cardiac failure and pulmonary hypertension
Provide good long-term patency
Be technically easy to close when repair is completed
Result in no residual cardiopulmonary abnormalities after closure
Although several modifications of a systemic-to-pulmonary artery shunt are known and are briefly discussed below, the modified Blalock-Taussig shunt is the most common type currently used.
Classic Blalock-Taussig shunt
The classic Blalock-Taussig shunt (CBTS) was the original operation described by Alfred Blalock and Helen Taussig and involves a direct anastomosis between the transected subclavian artery (or the innominate artery) and the pulmonary artery. As they described it, the operation was performed on the side opposite the aortic arch to minimize kinking of the subclavian artery as it crosses over the aortic prominence. In addition, the longer innominate artery reduces kinking of the pulmonary artery.
The CBTS is an end-to-side anastomosis between the subclavian (or the innominate) and pulmonary arteries. It is performed in an extrapericardial fashion. One of the unique advantages of this shunt is the predictability of blood flow, because the subclavian artery diameter prevents excessive blood flow and, hence, congestive cardiac failure. For the same reason, this shunt is unlikely to cause pulmonary vascular disease. Other advantages include ease of closure during corrective surgery and potential adaptive growth of the anastomosis.
One of the major disadvantages included thrombosis of the shunt due to its smallness. In addition, the mutilation of the subclavian or the innominate artery is a drawback of this procedure, although it did not appear to have a clearly significant clinical effect. Significant arm ischemia from subclavian artery division was uncommon, although the pulse in the ipsilateral arm was often not felt for days after the operation.
Neurologic disadvantages are rare and include risk of recurrent laryngeal nerve injury, phrenic nerve injury, and Horner syndrome.
A connection between the descending aorta and left PA, the Potts shunt was initially suggested as an alternate to the CBTS in neonates. It is easier to perform because it does not involve vessels of small caliber, such as the subclavian artery, thus resulting in a lesser incidence of shunt thrombosis and occlusion. It fell from favor because of the high incidence of subsequent pulmonary hypertension. Other reasons this shunt was abandoned include the preferential blood flow to one lung with kinking and distortion of the pulmonary artery and technical difficulties with takedown.
This connection between the ascending aorta and right pulmonary artery is also no longer used because it has similar disadvantages to the Potts shunt (excessive pulmonary blood flow, risk of pulmonary hypertension). Congestive cardiac failure was reported in as many as 20% of patients with either of these shunts. 
This is an intrapericardial anastomosis from the ascending aorta to the right pulmonary artery. The proposed advantages of this technique included using a right anterolateral thoracotomy incision for all approaches, avoiding mediastinal dissection (which could precipitate extrapleural bleeding from dilated collaterals), and using the same incision for other necessary surgical procedures (eg, complete repair). Furthermore, the anastomosis was created within the future operative field of total repair. This allowed closure of the palliative anastomosis without additional dissection during final repair.
This shunt is no longer commonly used because technical aspects of this procedure were considered challenging, and construction of an improperly sized anastomosis could lead to heart failure and pulmonary congestion. In addition, because the pericardium is invaded, the patient is at higher risk for adhesions in the region of future cardiac repair.
Modified Blalock-Taussig shunt
The modified Blalock-Taussig shunt (MBTS) is currently the modification most commonly used. In 1962, Klinner originally described the procedure, which others subsequently detailed.  The authors proposed this operation to prevent the mutilating effects of the CBTS. The latter group also indicated that the MBTS is useful when CBTS cannot easily be performed, such as on the same side as the aortic arch. They also maintained that pulmonary artery distortion is less likely than with CBTS.
Closure of these shunts is also technically easy. An interposition polytetrafluoroethylene (PTFE, or Gore-Tex) graft between the subclavian artery and the pulmonary artery is used to prevent sacrificing the subclavian artery. PTFE was found to be superior to Dacron because it has a smaller pore size that limits the tissue ingrowth but allows for fibroblastic incorporation to bind it to surrounding structures. 
Excellent patency rates of 90% at age 2 years have been reported.  They advocate using shunts of at least 5 mm, even in small infants, arguing that the orifice of the subclavian artery serves to regulate blood flow through the shunt.
Rare reported complications include leakage of serous fluid through the PTFE in the chest and pseudoaneurysm formation, which can cause massive fatal hemoptysis.
A central shunt is an anastomosis between the ascending aorta and the main pulmonary artery made of PTFE. A central shunt uses a short PTFE connection between the ascending aorta and the main pulmonary artery. The pericardium is incised via a median sternotomy, and a 4-mm straight PTFE graft with 2 beveled ends is used for the anastomosis. The central shunt may be indicated in neonates and children younger than 3 months. It can be performed only in infants with a patent ductus arteriosus or some other source of pulmonary blood flow. A central shunt may be especially useful with bilateral small branch pulmonary arteries, a concomitant procedure requiring a median sternotomy, or both.
The advantages of this technique are as follows:
Applicability to small children with small peripheral vessels
Prevention of distortion of pulmonary arteries
Provision of equal pulmonary blood flow to both lungs
Lower occlusion rate (compared with the CBTS or MBTS techniques)
Avoidance of subclavian artery steal
Ease of closure during corrective repair
The primary disadvantages are entry into the pericardium and inapplicability for patients without a patent ductus arteriosus or other source of pulmonary blood flow.
The operation is generally performed via a median sternotomy. The main technical aspects of the operation involve using a short graft, ensuring proper sizing of the aortotomy (usually with an aortic punch device), and avoiding temporary shunt clamping. As a result, the central shunt has become more widely used in recent years. Immediately postoperatively, patients may be anticoagulated with heparin for 24-72 hours. Patients may be placed on long-term aspirin therapy for shunt patency maintenance.
Watterson et al described a special type of central shunt for patients with pulmonary atresia, ventricular septal defect, and hypoplastic pulmonary arteries.  The shunt is described as an end-to-side connection between a transected main pulmonary artery and the side of the ascending aorta. In their hands, the direct central end-to-side shunt has proven satisfactory in attaining pulmonary artery growth in patients with small central pulmonary arteries. This shunt has also been termed the Mee shunt after the senior surgeon involved in its description.
Gates et al described a modification of the central shunt. A distal Gore-Tex end-to-side pulmonary artery anastomosis is created. The proximal anastomosis is created side to side between the anterior ascending aorta and the side of proximal portion of the Gore-Tex graft. The proximal end is either clipped or oversewn. This shunt has the advantages described above, including technical simplicity, a low rate of shunt thrombosis, and if connected to the main pulmonary artery, will allow growth of both branch pulmonary arteries without distortion. 
Bradley et al proposed the right ventricle-to-pulmonary artery (RV-PA) shunt as an alternative palliation technique in low birth weight infants. A 5- or 6-mm Gore-Tex graft is placed from the right ventricle to the pulmonary artery bifurcation in patients with TOF and PA, TOF and severe pulmonary stenosis in whom the right ventricular outflow tract was felt to be unusable for future repair, and in TOF and AVSD and transposition with pulmonary atresia requiring palliation before subsequent biventricular repair. The RV-PA shunt avoids issues with proximal shunt location in patients with abnormal aortic arch branching, preservation of diastolic coronary perfusion pressure as in the Sano modification for the palliation for hypoplastic left heart syndrome, and allows for placement of a larger diameter shunt which may decrease the risk of shunt thrombosis. This shunt may be clipped and subsequently dilated in the cardiac catheterization lab if required. However, this technique requires cardiopulmonary bypass, aortic cross clamping for the proximal shunt anastomosis in order to prevent air emboli, and a right ventriculotomy incision.  The proximal anastomosis to the RV could be simplified using the so-called “dunk” technique for the Sano modification. 
This is a connection between a systemic vein, the SVC, and the right pulmonary artery. As initially described, this involved an end-to-side anastomosis of the SVC to the end of the right pulmonary artery, which provides unidirectional pulmonary blood flow. This is now termed the classic Glenn shunt. The modification in current use is termed the bidirectional Glenn because it involves the connection of the SVC to the right pulmonary artery, which is still connected to the main pulmonary artery; hence, blood from the SVC can enter both the pulmonary arteries.
One of its most significant advantages is that, unlike the other systemic to pulmonary shunts, it does not increase the volume load on the ventricle.
Disadvantages of this shunt relate to its specific anatomic and physiologic limitations, the most important of which is that the pulmonary vascular resistance must be low. As a result, children younger than 3-6 months are generally excluded, as are patients with any degree of elevated pulmonary vascular resistance.
Despite improved early outcomes after stage 1 palliation for hypoplastic left heart syndrome, a 10-15% incidence of late death continues to be seen before stage 2 palliation (ie, while the patient has a modified Blalock-Taussig shunt but before the creation of the bidirectional cavopulmonary connection, such as a Glenn operation.  Although several factors have been implicated, such as restrictive atrial septal defect, neoaortic arch obstruction, pulmonary artery distortion, coronary insufficiency, and atrioventricular valve insufficiency, the role of the systemic-to-pulmonary artery shunts have received special attention.
Infants may suffer the catastrophic results of a stenosed or obstructed shunt because of their propensity to develop dehydration in the presence of an anatomically small shunt. In addition, a shunt provides a run-off from the systemic circulation in favor of the pulmonic circulation. A consequence of this run-off is the low diastolic pressures in the aorta that contribute to coronary insufficiency. Sano et al proposed a right ventricle–to–pulmonary artery shunt in an attempt to overcome the obstacles noted with a systemic-to-pulmonary artery shunt.  They described the use of a 4-mm nonvalved PTFE tube in patients who weighed less than 2.5 kg and a 5-mm nonvalved PTFE tube in infants who weighed more than 2.5 kg. In their initial report, they described an improvement after stage 1 mortality from 62% to 89%. Several centers have confirmed this effect. [27, 28]
Although this shunt has clear advantages over the MBTS in the setting of a Norwood operation for single ventricle, some disadvantages are recognized. As noted by Sano et al, these shunts become obstructive over time, usually in approximately 3 months.  This occurs sooner than that noted with MBTS. In addition, the effects of a ventriculotomy on a systemic ventricle are of concern. Interestingly, Norwood was the first to attempt a valved conduit from the right ventricle to the pulmonary artery in this situation but abandoned the procedure because of its high failure rate.
Operative Details of a Modified Blalock-Taussig Shunt
The operation may be performed on either side through a lateral thoracotomy in the fourth intercostal space. The pulmonary artery is dissected out, and a side-biting clamp is placed on the origin of the dissected subclavian artery. Before applying the vascular clamp, the vagus nerve and its recurrent laryngeal branch must be identified. The obliquely trimmed prosthesis is anastomosed end to side on the subclavian artery with continuous Prolene sutures.
Heparin is administered intravenously before the Gore-Tex conduit is anastomosed to a transverse arteriotomy made on the anterior aspect of the pulmonary artery near its upper edge. The inner surface of the shunt is flushed with saline solution. The length of the graft must be adjusted so that it lies straight without kinking. de Leval et al (1981) stress the importance of maintaining adequate systemic arterial pressure throughout the operation to prevent early thrombosis of the conduit. 
The role of low-dose aspirin in preventing shunt thrombosis has not been systematically studied. A small study from Germany found that aspirin effectively reduced the rate of shunt occlusion;  however, another study showed no difference in patency rates. 
A systemic-to-pulmonary artery shunt presents a volume load to the heart, a low diastolic blood pressure with the potential for coronary steal, and a conduit that is a stimulus for the coagulation system and a potential nidus for infection. These shunt-related physiologic factors are important to recognize as more reports surface of interim mortality in patients with shunts.
In a study of 80 patients who underwent a systemic to pulmonary artery shunt below a weight of 3 kg, the discharge survival was 96%. The incidence of shunt reintervention was higher (37% vs 6%) in those that received 3-mm shunts versus those with 3.5-mm or 4-mm shunts. 
Fenton et al reported a significant incidence of sudden death among infants who had undergone shunting.  This was observed in children with a single ventricle and in those with double ventricle anatomy. Autopsy-proven shunt thrombosis was the leading cause (although several cases appear unexplained), and the mortality rate of patients discharged and given aspirin was almost identical to that of patients discharged with no anticoagulation medication.
Death may be preceded by unexplained irritability. Fenton et al recommends assiduously avoiding dehydration to minimize the chance of shunt thrombosis and paying careful attention to any infant with a shunt who has an apparently mild illness or irritability. A child with a systemic-to-pulmonary artery shunt and clinical symptoms (ie, irritability, emesis, poor appetite) should be promptly evaluated with echocardiography or cardiac catheterization. The infant must be closely observed, and an earlier second operation must be considered.
Shunt thrombosis is a devastating complication of systemic-to-pulmonary artery shunts and can occur both early and late postoperatively. One study has shown that 21% of modified Blalock-Taussig shunts have greater than 50% obstruction at the time of takedown and shunt size less than 4mm is a risk factor for high grade stenosis. The obstruction appears to be due to myofibroblastic proliferation and endothelial cell ingrowth associated with organic thrombus.  A multicenter prospective observational study has shown that aspirin lowers the risk of death and shunt thrombosis.  In a multicenter, double-blind, event-driven trial, the addition of clopidogrel has not shown a reduction in mortality from any cause or shunt-related morbidity in infants with cyanotic congenital heart disease palliated with a systemic-to-pulmonary artery shunt, most of whom received concomitant aspirin therapy.  However some groups may wish to add clopidogrel in addition to aspirin in infants who have a 3mm shunt.  Aspirin resistance has recently been identified as a potential cause of shunt thrombosis. Aspirin responsiveness can be determined by monitoring the aspirin reaction units and adjusting the dose of aspirin.  Heparin-coated PTFE grafts may hold the promise of decreasing the incidence of shunt thrombosis, especially in smaller diameters. 
A recent study from the Society of Thoracic Surgeons Congenital Heart Surgery Database has shown a discharge mortality of 7.2% and composite morbidity endpoint consisting of ECMO, low cardiac output or unplanned reoperation of 13.1% for neonates undergoing a modified Blalock-Taussig shunt with or without cardiopumonary bypass and with or without ligation of a patent ductus arteriosus.  The mortality rate for systemic-to-pulmonary artery shunts remains high and is increasing, unlike results of other operations for complex congenital heart disease, due to the shift in the use of shunts in more complex cases.  PA/IVS is one of the highest risk groups.
The patient's age, size, anatomy, physiology, severity of cyanosis, and overall physical condition are factors that help to determine the appropriate shunt in a particular case. The following are general guidelines:
An MBTS is usually performed in newborns and older children with pulmonary stenosis, including TOF, who have only mild-to-moderate pulmonary hypoplasia.
An aortopulmonary shunt (ie, central) is usually favored if the pulmonary arteries are significantly hypoplastic.
In selected instances, a modified Glenn shunt may be more appropriate for patients older than 3-6 months.