PULMONARY ATRESIA WITH VENTRICULAR SEPTAL DEFECT

Pulmonary atresia with ventricular septal defect (PAVSD) is a congenital cardiac malformation characterized by discontinuity of blood flow from the right ventricle to the pulmonary arteries, a ventricular septal defect (VSD) resulting from anterior deviation of the infundibular (conal) septum, and an overriding aorta. Because it shares many attributes of tetralogy of Fallot (TOF), it is also referred to as TOF with pulmonary atresia. The incidence of PAVSD is estimated to be 1/10,000 live births.¹ A right aortic arch may be seen in up to 45% of patients.² PAVSD is also associated with major aortopulmonary collaterals (MAPCAs) that, in some cases, are the sole supply of pulmonary blood flow. The morphology of the pulmonary circulation, which varies significantly among patients, determines the management and prognosis of this malformation. PAVSD may also be associated with other intracardiac defects such as tricuspid atresia or stenosis, complete atrioventricular (AV) canal, complete or corrected transposition of the great arteries, left superior vena cava, anomalies of the coronary sinus, dextrocardia, and asplenia or polysplenia syndrome. These more complex forms of PAVSD are not discussed in this chapter. The most common associated genetic defect is a 22q11 microdeletion, found in up to 34% of patients with PAVSD, and up to 65% of patients with MAPCAs are found to have this abnormality.³ Other phenotypic abnormalities with the 22q11 microdeletion include submucosal cleft palate, abnormal facies, delayed development, and mental retardation. Neonates with trisomy 13 or 18 may have PAVSD as well, and the prognosis is extremely poor for these children.

Anatomy and Pathophysiology

Like TOF, PAVSD is associated with anterior deviation of the infundibular septum, a conoventricular VSD, and the resulting aortic override (Fig. 120-1). The spectrum of pulmonary atresia varies from purely valvular or subvalvular atresia (with intact main and branch pulmonary arteries) to complete absence of central pulmonary arteries. Pulmonary blood flow depends on the presence of an aortopulmonary communication, which may exist in the form of a patent ductus arteriosus (PDA), other major nonductal collaterals arising from the descending aorta that connect to the central native pulmonary arteries, or MAPCAs that directly enter the hilum and join the segmental pulmonary arteries. In utero, the pulmonary parenchyma is perfused by branches from the dorsal aorta as well as by the sixth aortic arch, which forms the basis for the central pulmonary arteries. Abnormal development of the central pulmonary arteries leads to persistence of the dorsal aortic branches, which ultimately develop into MAPCAs.

Figure 120–1 Anatomic specimen and schematic demonstrating the features of pulmonary atresia with ventricular septal defect. Ao, aortic; AoV, aortic valve; PA, pulmonary artery; RV, right ventricle; RVOT, right ventricular outflow tract; TV, tricuspid valve; VSD, ventricular septal defect.

The lungs may be supplied by the native pulmonary arteries, by MAPCAs, or by both. If confluent central pulmonary arteries are present, the blood supply to the lung may arise from a PDA, a central “ductlike” collateral, or a MAPCA. The behaviors of these three sources of pulmonary blood flow differ in their predisposition to constrict over time, which has implications for the stability of pulmonary blood flow. Whereas MAPCAs or ductlike collaterals might remain patent beyond the neonatal period, the PDA is likely to constrict postnatally as a result of the presence of prostaglandin-responsive ductal tissue. Although the distinction may be difficult to establish by preoperative imaging studies, the location on the aorta from which the collateral originates is suggestive. A solitary collateral that arises just distal to the left subclavian artery (off the left aortic arch) and travels to a central pulmonary confluence is likely to be a PDA, whereas a collateral that arises from any other location off the aorta and supplies a central confluence is termed a ductlike collateral. A vessel that exits the aorta and travels directly to the pulmonary parenchyma is likely to be a MAPCA. There may be heterogeneity in the sources of pulmonary blood flow in a given patient, so one segment of lung may be supplied by the native pulmonary artery but another segment by a MAPCA. Some segments may have a dual supply (Fig. 120-2). Patients with confluent central pulmonary arteries that are supplied by a PDA are less likely to have significant MAPCAs, and those with nonconfluent central pulmonary arteries depend on MAPCAs for pulmonary blood flow.

Figure 120–2 Variations in aortopulmonary collateral morphology in pulmonary atresia with ventricular septal defect. The aortopulmonary collateral artery is shown connecting to the central pulmonary artery (A), connecting to the lobar pulmonary artery (B), and without connection to a pulmonary artery (C).

Classification

There is no standard classification system for PAVSD, but several have been proposed. Most classification schemes focus on the patterns of pulmonary blood flow. The classification system adopted by the Congenital Heart Surgery Nomenclature Project broadly divides pulmonary artery anatomy into the following categories: central pulmonary arteries without MAPCAs; a dual flow, with confluent central pulmonary arteries and MAPCAs; and MAPCAs, with nonconfluent or absent central pulmonary arteries.⁴

Another classification scheme divides patients further according to the specific patterns of pulmonary artery anatomy that are most commonly encountered (Fig. 120-3):

I. Valvar pulmonary atresia with a decent-sized main pulmonary artery and confluent central pulmonary arteries. This variant is associated with a PDA and therefore not associated with MAPCAs.

II. Absent main pulmonary artery with confluent, decent-sized or mildly hypoplastic central pulmonary arteries supplied by a PDA, without MAPCAs

III. Confluent central pulmonary arteries with MAPCAs but no PDA

IIIa. Central pulmonary artery Z-score > −2.5

IIIb. Central pulmonary artery Z-score < −2.5

IV. Nonconfluent or absent central pulmonary arteries with MAPCAs

V. Nonconfluent central pulmonary artery with PDA and MAPCAs

Figure 120–3 Variations in pulmonary supply from native pulmonary arteries (hatched grey) and major aortopulmonary collaterals (black) encountered in patients with pulmonary atresia with ventricular septal defect.

Diagnostic Studies

The ultimate goal of treatment for PAVSD is to establish right ventricle–to–pulmonary artery continuity and eliminate left-to-right shunts yet maintain low right ventricular pressures. The treatment strategy thus hinges on the morphology of the pulmonary circulation, which must be delineated by diagnostic studies. The source of blood flow, the size of the vessels, and the degree of flow restriction must be characterized preoperatively, as these parameters determine the timing and nature of subsequent treatment.

Chest Radiography and Electrocardiography

On chest radiographs, lung fields may be hypovascular or hypervascular, depending on the degree of blood flow restriction. A right-sided aortic arch may be demonstrated by the position of the aortic knob, and the diminutive pulmonary artery results in an exaggerated sabot shape of the cardiac silhouette. With electrocardiography, rightward deviated QRS axis and evidence of right atrial and ventricular hypertrophy are frequently seen.

Echocardiogram

Echocardiography is the primary modality for diagnosis of this defect, and it provides a significant portion of the anatomic information needed for initial decision making in neonates and infants. Echocardiography is poor at delineating pulmonary anatomy in adolescents and adults, however. The components that are best evaluated by echocardiography include location, size, and direction of flow in the ventricular septal defect, size and confluence of the central pulmonary arteries, and presence of a PDA or other collaterals. The echocardiogram is less reliable at delineating MAPCAs. Retrograde flow in the descending aorta is suggestive, but not diagnostic of, significant collaterals. After repair or palliation, echocardiogram is used to detect and characterize residual right ventricular outflow tract obstruction, conduit regurgitation, and the presence and direction of flow across any residual VSD, and to assess right ventricular size and pressures.

Cardiac Catheterization

Cardiac catheterization is preferred for patients with PAVSD who have nonconfluent or hypoplastic central pulmonary arteries. These patients have the highest likelihood of having MAPCAs, which are best delineated by catheterization (Fig. 120-4). The goal of catheterization is to assist with surgical planning by evaluating the anatomy, size, degree of stenosis, and the presence of dual supply from native pulmonary arteries or collaterals. Vascular supply to all segments must be clearly determined. Native pulmonary arteries may require visualization by pulmonary venous wedge injection if there are no collaterals to the native pulmonary arteries. In older children and adults, measurements of individual pulmonary artery pressures are obtained to determine the risk of development of pulmonary hypertension in high-pressure collaterals. The pulmonary-to-systemic blood flow ratio (Qp/Qs) can be estimated by the Fick method if all pulmonary flow arises from collaterals (i.e., there is no previous right ventricle–to–pulmonary artery [RV-to-PA] conduit). Interventions that may be performed at catheterization include balloon dilation of native pulmonary vessels, stenting of highly restrictive MAPCAs, or coiling of redundant dual-supply collaterals.

Figure 120–4 Angiograms demonstrating major aortopulmonary collateral to left lung (A) and redundant collateral to left lung also supplied by hypoplastic native pulmonary arteries (B).

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a useful adjunct to cardiac catheterization in patients with MAPCAs. The advantage of MRI is its ability to image native pulmonary arteries that may not be accessible by cardiac catheterization. It is superior to other imaging modalities at delineating the relationship of MAPCAs to other mediastinal structures (airway, esophagus).⁶ MRI is used to estimate RV volumes and the conduit regurgitation fraction in completely repaired patients, and it can be used to guide decision making regarding conduit replacement.

Radionuclide Lung Perfusion Scanning

In patients with stenosis of native pulmonary arteries after unifocalization, decreased blood flow to specific segments of lung may result in cyanosis and increased right ventricular afterload. A useful screening tool to detect segmental malperfusion is lung perfusion scanning. It can be used to screen patients for the presence of significant perfusion defects that may indicate stenosis of the native pulmonary arteries or MAPCAs.⁷

Treatment

Medical Treatment

Prostaglandins are infused at 0.01 to 0.1 μg/kg/min to maintain ductal patency in neonates with PDA-dependent pulmonary blood flow. Preoperative mechanical ventilation is not necessary and may even be detrimental if it lowers the PVR and increases the left-to-right shunt. Diuretics may be used for patients with heart failure symptoms.

Surgical Considerations

The ultimate goal of the surgical repair strategy is creation of biventricular circulation with unobstructed RV-to-PA continuity and elimination of intracardiac or extracardiac shunts with low resultant right ventricular pressure. To attain this goal, a combination of the following interventions may be used. Commonly employed strategies apply specific components either at a single operation (complete primary repair) or at multiple stages (staged repair).

Unifocalization

Unifocalization is a technique in which MAPCAs are disconnected from the descending aorta and anastomosed to a common confluence that can receive blood supply from a stable source—usually the RV or a systemic shunt. Unifocalization facilitates eventual establishment of RV-to-PA continuity by providing a centrally located confluence to which a conduit may be sewn. Ideally, this confluence provides low resistance to right ventricular output and supplies all segments of the lung. To perform a successful unifocalization requires careful review of preoperative studies (catheterization and MRI) to delineate the anatomy and size of vessels, and a dual supply by MAPCAs and native pulmonary arteries. If central native pulmonary arteries are present, collaterals may be anastomosed to them. Unifocalization procedures may be performed via either a sternotomy or a thoracotomy approach. Unifocalized vessels may be supplied by a shunt from the systemic circulation or by a right ventricular conduit. If high vascular resistance is found by catheterization in segments supplied by an unrestrictive MAPCA (common in the older child or adult), unifocalization to a systemic shunt (rather than to the RV-to-PA conduit) may be preferable to provide the higher perfusion pressures capable of maintaining adequate blood flow and oxygen saturations.

A typical MAPCA consists of a proximal muscular segment that resembles systemic arterial vessel and a distal thin-walled segment that resembles a pulmonary artery. During the natural history of these aortopulmonary collaterals, it is often the proximal muscular segment that is susceptible to stenosis. Thus, during the process of unifocalization, it is preferable to perform all anastomoses to the distal pulmonary artery–like segment to avoid the risk of restenosis.

Closure of Ventricular Septal Defect

The VSD may be closed with Dacron, polytetrafluoroethylene (PTFE), or pericardial patch material. The decision to close the VSC depends on the surgeon’s assessment of the pulmonary runoff. Considerations include adequacy of the pulmonary artery and right ventricular outflow tract (RVOT) reconstruction, caliber of the central pulmonary arteries, and degree of intrapulmonary vascular disease. If unobstructed right ventricular output is anticipated, closure of the defect is warranted. It is important to accurately judge the adequacy of RVOT reconstruction, as this guides management of the VSD. Failure to close the VSD in the setting of unobstructed right ventricular outflow and low PVR leads to significant left-to-right shunting and congestive heart failure, which may be poorly tolerated and require reoperation. In contrast, closure of the VSD in the presence of an inadequate pulmonary artery bed results in right ventricular hypertension and failure.

Preoperative imaging and tools are available to help guide management of the VSD. On the basis of angiography and MRI, a surgeon must determine if the number and caliber of existing pulmonary vascular segments are sufficient to support a full cardiac output. Recruitment of pulmonary vasculature supplying greater than 12 lung segments is a prerequisite for attaining low pulmonary resistance. Several indices of pulmonary artery caliber have been used to predict postoperative right ventricular pressures and successful VSD closure. The McGoon index is the ratio of the sum of the diameters of the proximal extrapericardial right and left pulmonary arteries to the diameter of the descending aorta at the level of the diaphragm. A McGoon index of 1 predicts an RV/LV pressure ratio of 0.7. The Nakata index is the normalized sum of the cross-sectional areas of the right and left pulmonary arteries. The Birmingham formula estimates the RV/LV pressure ratio as 0.484 × (RPA/Ao + LPA/Ao) + 0.2007, where RPA is the diameter of right pulmonary artery at the hilum, Ao is the diameter of the descending aorta, and LPA is the diameter of the left pulmonary artery at the hilum. Such indices have been variably successful in guiding intraoperative management, but they are useful tools to assist the surgeon with preoperative planning. A tool to assist with intraoperative decision making regarding VSD closure is functional assessment of vascular resistance after single-stage unifocalization. Reddy and Hanley have championed determination of static vascular resistance by measuring the pressure generated by perfusing the unifocalized PAs with constant flow perfusate, and this method has been shown to predict postoperative right ventricular pressures.⁸

An alternative to complete VSD closure is placement of a fenestration within the patch if only a moderate degree of RVOT obstruction is anticipated. If the pulmonary reconstruction might be inadequate, performing a fenestrated VSD closure with PTFE or pericardium carries low risk and avoids deleterious consequences of suprasystemic right ventricular pressures that results from inappropriate complete closure. Fenestration within a VSD patch may spontaneously close over time, or it can be closed in the catheterization laboratory. Similarly, an inadequate fenestration in a PTFE or pericardial patch material can be balloon dilated in the catheterization laboratory if, postoperatively, suprasystemic right ventricular pressures are encountered.

Right Ventricle–to–Pulmonary Artery Conduit

An RV-to-PA conduit can be sewn to confluent central native pulmonary arteries or to unifocalized pulmonary vascular segments. The type of conduit chosen depends on the size of the child, caliber of the distal pulmonary vascular bed, status of the VSD, and compliance of the RV. The distal anastomosis of a conduit to unifocalized pulmonary arteries may require a patch augmentation or interposition graft (Dacron or PTFE). If the VSD is left open and the pulmonary arteries are of decent size, the RV-to-PA conduit may need to be downsized to avoid excessive pulmonary blood flow (see Choice of Conduit, later). Currently available conduits include aortic or pulmonary homografts, bovine jugular vein grafts, or prosthetic tube grafts (PTFE or Dacron) with or without pulmonary valve. Conduits placed in small children generally require catheter intervention or surgical reconstruction as the child grows.

Transanular Patch

In patients with pulmonary valvular atresia, the presence of an imperforate membrane at the valve level is the primary abnormality. Infundibular muscular stenosis may also be present, and the main pulmonary artery is variably hypoplastic. This anatomy is amenable to augmentation of the RVOT with a transanular patch that extends from the infundibulum to the bifurcation of the main pulmonary artery. This approach reduces the need for RVOT reoperation, but it creates pulmonary regurgitation. Use of a monocusp transannular patch may prevent early pulmonary regurgitation with this approach.^9,10

Maintenance of Patent Foramen Ovale

Generally, following neonatal repair of PAVSD, a low compliance RV leads to elevated right atrial pressures. Postoperative right ventricular dysfunction and hypertension may further elevate the right atrial pressures. Without an atrial septal defect or a patent foramen ovale (PFO), elevated right atrial pressures lead to hepatic congestion, anasarca, and low cardiac output. A small atrial-level defect (between 3 and 4 mm) allows right-to-left shunting when right ventricular dysfunction, hypertension, or poor compliance is expected. In this setting, the right-to-left shunting allows maintenance of cardiac output at the expense of mild desaturation, which is well tolerated in neonates and infants.

Systemic-to-Pulmonary Artery Shunt

Systemic-to-pulmonary artery shunts may be employed under certain circumstances. Some centers advocate this as the initial palliative procedure in all patients with TOF and PAVSD with confluent central pulmonary arteries. As a part of this procedure, the PDA or other major collateral may be ligated or left open. The advantages of this approach when compared with complete repair include avoidance of the upfront mortality of a neonatal repair while promoting growth of the pulmonary arteries.¹¹ The major disadvantages include distortion of the pulmonary arteries and the risk of shunt thrombosis and fatal hypoxic event until a stable source of pulmonary blood flow is established.^12,13

At our institution, neonatal systemic shunts in patients with PAVSD with confluent pulmonary arteries are infrequently used and are reserved for neonates with significant noncardiac morbidities in whom the risk of complete repair is deemed to be high. Examples of such shunts include the Waterston shunt (ascending aorta–to–pulmonary artery graft), Melbourne shunt (direct end-to-side anastomosis of pulmonary artery to ascending aorta), and modified Blalock-Taussig shunt (subclavian artery to pulmonary artery graft).

Subclavian-to-pulmonary artery shunts may also be employed as part of the staged unifocalization procedure. These shunts are constructed with PTFE, pericardial, or allograft tube grafts, and they promote growth of pulmonary artery segments prior to eventual complete repair. This procedure, generally performed through a thoracotomy incision, involves unifocalization of all pulmonary vascular segments to an allograft, one end of which is placed anterior to the airway for subsequent access through a sternotomy. A shunt between the subclavian artery and the allograft is created to provide blood supply to the pulmonary vasculature. This shunt is divided when continuity is subsequently established between the RV and the unifocalized confluence. Pulmonary segments with elevated PVR may require higher driving pressure to maintain adequate blood flow and patency, and a systemic shunt may be superior to a low-pressure RV-to-PA conduit in such segments.

Transplantation

Lung or heart–lung transplantation may be applied selectively for certain patients, generally adolescents or adults, with irreversible pulmonary hypertension. For patients with normal ventricular function, lung transplantation, with or without cardiac repair (VSD closure or conduit change), may be performed. Heart–lung transplantation must be considered for patients with myocardial dysfunction or complex intracardiac defects.¹⁴

Cardiopulmonary Bypass Strategy

Cardiopulmonary bypass is required for procedures involving VSD closure or RV-to-PA conduit placement. Thus, single-stage unifocalization with an RV-to-PA conduit via a sternotomy approach requires cardiopulmonary bypass, whereas unifocalization to a systemic shunt through a thoracotomy does not. The bypass strategy must be tailored to patients with MAPCAs, because runoff into the pulmonary circulation may result in systemic hypoperfusion as well as poor visualization during intracardiac repair or pulmonary artery reconstruction as a result of flooding of the field with collateral blood flow. Control of all accessible collaterals after initiation of bypass is recommended, and higher flows (cardiac index of up to 4.0 L/min/m², if necessary) are used to maintain systemic perfusion. Hypothermia reduces metabolic demand and provides protection of end organs when there is reduced systemic perfusion. It allows the surgeon to reduce bypass flows or enter brief periods of circulatory arrest to improve visualization of the surgical field during critical portions of the operation. When appropriate, maintenance of cardiac contractions and pulsatile flow (by not completely emptying the heart) during bypass may provide superior systemic perfusion in the presence of collateral runoff. Cardioplegic arrest is required for repair of VSD, but it may also be used during placement of the RV-to-PA conduit.

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