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Management of Single Ventricle and Cavopulmonary Connections
Single-ventricle circulation is a generic term that covers a wide range of structural cardiac abnormalities. It is widely accepted that the definitive surgical palliation for these hearts is the Fontan circulation, whereby the pulmonary and systemic blood flows are in series with the single ventricle connected to the systemic circulation. To optimize clinical outcomes with the Fontan, many patients need prior interventions to adjust the pulmonary or systemic circulation. Because of the changing physiology in the early years of life, a series of operations is often necessary.
Terminology and Anatomy
In this chapter, the term single ventricle refers to congenital cardiac malformations that lack two completely well-developed ventricles in which functionally there is only a single ventricular chamber that supports both the pulmonary and systemic circulations. A truly morphologic univentricular heart is rare; more often there is an additional rudimentary chamber. Over the years, numerous classifications and terms for these hearts have emerged. The Congenital Heart Surgery Nomenclature and Database Project minimum data set categorized single ventricles into seven broad categories : hearts with common inlet atrioventricular connection (double-inlet right ventricle and double-inlet left ventricle), hearts with absence of one atrioventricular connection (tricuspid atresia and mitral atresia), hearts with common atrioventricular valve and only one well-developed ventricle (unbalanced common atrioventricular canal defect), hearts with only one fully developed ventricle and heterotaxia syndrome (single-ventricle heterotaxia syndrome), and other rare forms of univentricular hearts that do not fit in one of these categories. A more comprehensive classification of the Congenital Heart Surgery Nomenclature and Database Project subclassifies these categories of single ventricle into four hierarchies spanning more than three printed pages. Tricuspid atresia is the most common type of single ventricle, with an incidence of 1% to 3% of all congenital heart lesions.
In addition, there are some cardiac abnormalities in which, in the presence of two well-developed ventricles, the anatomy precludes biventricular repair. Examples include hearts with major straddling of the atrioventricular valve and double-outlet right ventricle with remote ventricular septal defect. Treatment strategies for these nonseptatable hearts are the same as those applied to univentricular hearts.
The management of hypoplastic left heart syndrome, a common form of univentricular heart with a dominant right ventricle and rudimentary left ventricle is not discussed in this chapter. Chapter 128 is dedicated to this topic.
Natural History
Patients born with a single functional ventricle generally have a dismal long-term prognosis without eventual surgical intervention. Current results of the Fontan procedure as the final palliative surgical procedure for these patients are generally good, but it must be recognized that management of these patients begins at birth if they are to be satisfactory candidates for the Fontan procedure. The natural history of single-ventricle circulations is greatly influenced by the degree of pulmonary blood flow and associated lesions, such as coarctation of the aorta, systemic outflow tract obstruction, and anomalies of pulmonary or systemic venous return. In addition, noncardiac abnormalities may be present.
Severe obstruction to pulmonary blood flow at birth is an important determinant for early death. Patients with unobstructed pulmonary flow can develop congestive heart failure in infancy or later, and, if unoperated, can develop pulmonary vascular disease. The clinical course may be worsened by the presence of left-sided obstructive lesions such as coarctation of the aorta. In a small subset of patients, the pulmonary and systemic circulations are well balanced because of unrestricted systemic blood flow and sufficient pulmonary obstruction to control pulmonary blood flow. These patients have a more favorable life expectancy. The aim of surgical intervention is to improve the natural history by balancing blood flow between the pulmonary and systemic circulations and ultimately separating these circulations. In addition, other significant hemodynamic abnormalities should be corrected.
Clinical Presentation and Preoperative Evaluation
Clinical presentation is determined by the amount of pulmonary blood flow and the associated cardiac lesions. Patients with restricted pulmonary blood flow will exhibit cyanosis. Some may have a duct-dependent pulmonary circulation and will become rapidly cyanosed as the ductus arteriosus closes after birth. In the case of a large left-to-right shunt, the patient will present with congestive heart failure. Symptoms will deteriorate with falling pulmonary resistance in the first weeks after birth. Although in most single ventricles there is mixing of circulations, streaming may occur, particularly in complex hearts, resulting in differential saturations in the great arteries.
A precise anatomic diagnosis is essential to allow proper planning of the surgical procedures. In particular, information on the size and course of the pulmonary arteries, degree of pulmonary blood flow, and presence of accessory lesions is required, as well as an assessment of cardiac function. History, clinical presentation, chest x-ray, and electrocardiogram offer important but nonspecific information. Echocardiography provides detailed information on the structure and function of the heart and has the advantage that it is a noninvasive investigation that can be performed at the bedside. In particular, in infants who have excellent echocardiographic windows, enough information often can be gathered to proceed to operation. Cardiac catheterization is uncommonly needed to evaluate pulmonary vascular resistance or accessory sources of pulmonary blood flow. Interventional procedures, such as balloon atrial septostomy for restrictive atrial communication in hearts without two well-formed atrioventricular valves, can be carried out to supplement, or sometimes replace, surgical treatment.
Surgical Preparation for the Fontan Circulation
Because the ultimate success of the Fontan operation depends on a suitably low pulmonary vascular resistance and adequate pulmonary artery architecture, it is critical to commence the preparation for a Fontan procedure in the newborn period by appropriately regulating pulmonary blood flow. There is a wide variation in the distribution of pulmonary and systemic blood flow in patients with single-ventricle circulation. Some patients have restricted pulmonary blood flow, others have unrestricted pulmonary flow, and a few have a naturally balanced circulation. Most patients, therefore, require palliative procedures leading up to the Fontan circulation, either to restrict or to augment pulmonary blood flow. The choice of procedure is guided by the underlying anatomy and pulmonary vascular resistance, both of which are subject to change over time. It should always be borne in mind that improper palliative procedures can result in the loss of Fontan candidacy. The ultimate goal of surgical procedures leading up to the Fontan circulation are to (1) improve clinical symptoms, (2) provide optimal pulmonary artery architecture and low pulmonary vascular resistance, (3) preserve systolic and diastolic ventricular function, (4) preserve atrioventricular valve function, (5) relieve systemic ventricular outflow tract obstruction, and (6) provide anatomic setup for a definitive Fontan repair.
Inadequate Pulmonary Blood Flow
In the neonatal and early infantile period, when pulmonary vascular resistance is still high, if the child is ductal dependent or has inadequate systemic oxygen saturations, a systemic to pulmonary artery shunt, usually a modified Blalock-Taussig shunt, is performed. Although this was classically performed through a thoracotomy approach, a sternotomy is now commonly used because it allows for concomitant ductal ligation and correction of stenosis of the proximal left pulmonary artery if present. Despite decades of experience with the modified Blalock-Taussig shunt, there remains a persistent early and intermediate morbidity and mortality with this procedure.
Excessive Pulmonary Blood Flow
In the case of excessive pulmonary blood flow, it is imperative to limit the pulmonary blood flow to protect the patient from developing pulmonary vascular disease and ventricular dysfunction as a result of chronic volume overload. Adequate tightness of a pulmonary artery band can be difficult to achieve, and a band that is too loose initially can result in unprotected pulmonary arteries until the child grows into the band. The resultant pulmonary vascular disease may affect the ultimate suitability of the child for the Fontan procedure. Other complications of pulmonary artery banding include distortion of the pulmonary arteries, especially if the band migrates, and erosion of the band. Because of the fixed diameter of the pulmonary band, as the child grows, pulmonary blood flow will eventually be inadequate and cyanosis will result, necessitating further surgical intervention. Often in infants with unobstructed pulmonary blood flow and single-ventricle anatomy, a concomitant coarctation will need to be repaired at the time of pulmonary artery banding.
To avoid the complications of pulmonary artery banding in the neonate or infant with excessive pulmonary blood flow and single-ventricle physiology, Bradley and colleagues suggested the strategy of pulmonary artery division with placement of a systemic to pulmonary artery shunt; this strategy has yielded excellent clinical results.
Systemic Outflow Obstruction (Subaortic Stenosis)
Some children with single ventricles can present in infancy with systemic ventricular outflow tract obstruction (subaortic stenosis), which may also develop later. The resultant myocardial hypertrophy can adversely affect the eventual suitability for a Fontan procedure. Patients at risk are those in whom the aorta arises above a small outlet chamber, such as in tricuspid atresia or double-inlet left ventricle with a rudimentary right ventricle and transposition of the great arteries, particularly if the ventricular septal defect (sometimes referred to as a bulboventricular foramen) is small or if there is coexisting aortic arch obstruction. A modified Damus-Kaye-Stansel procedure can be performed to establish unobstructed systemic arterial outflow. This involves transection of both great arteries, anastomosis of the facing aortic and pulmonary walls, and connection of the distal aorta to the perimeter of the reconstructed proximal great artery. Depending on the pulmonary vascular status, blood flow to the central pulmonary arteries can be reestablished via a systemic to pulmonary shunt, a bidirectional cavopulmonary anastomosis, or the Fontan operation. The Damus-Kaye-Stansel can be performed as a primary procedure or after a previous pulmonary artery band. Alternatively, a Norwood strategy can be followed in these patients (see Chapter 128 ). Some have advocated direct relief of the systemic outflow obstruction by surgical enlargement of the ventricular septal defect (bulboventricular foramen) or resection of subaortic stenosis.
Obstructed Pulmonary Venous Return
A particularly unfavorable subset of patients with single ventricles is those with total anomalous pulmonary venous connection. Even with successful correction of the pulmonary venous return, these patients can have persistent pulmonary artery hypertension, probably related to fetal pulmonary vein and artery abnormalities, which precludes the application of the Fontan procedure. To a lesser extent, children with a restrictive atrial communication and an abnormal left atrioventricular valve (as in mitral atresia) are at risk for the development of pulmonary artery hypertension. Opening of the atrial septum by percutaneous atrial septostomy or stent placement in the catheterization laboratory or surgical atrial septectomy should be performed once this condition has been recognized.
Bidirectional Cavopulmonary Anastomosis
The original cavopulmonary shunt was the Glenn procedure (anastomosis of the divided right pulmonary artery to the superior vena cava with ligation of the proximal superior vena cava). The advantages of a venous over an arterial shunt in the palliation of cyanotic heart disease are twofold. First, the venous blood that enters the pulmonary artery is much more desaturated, and therefore a higher take-up of oxygen per milliliter of blood is possible. Second, systemic venous return is diverted to the lungs, thus reducing the volume load on the single ventricle. The original Glenn anastomosis fell into disfavor; it could not be used in neonates because of elevated pulmonary vascular resistance. There were also reports of the development of ipsilateral pulmonary arteriovenous malformations. However, it was noted that patients with a previous classical Glenn anastomosis fared better with a subsequent Fontan procedure. For patients considered high risk for the Fontan procedure, a bidirectional cavopulmonary anastomosis (the divided superior vena cava anastomosed to the pulmonary arteries supplying both lungs) could be used successfully as an alternative to the Fontan or as a staging procedure for the Fontan ( Fig. 129-1 ). The hemi-Fontan modification of the cavopulmonary shunt involves patch augmentation of the central pulmonary arteries and a connection between the right atrial–superior vena cava junction and pulmonary arteries. Norwood and Jacobs reported improved survival with the Fontan operation in children with hypoplastic left heart syndrome who had undergone an intermediate hemi-Fontan procedure. Subsequent studies showed significantly improved outcomes in patients with a Fontan procedure who were staged with a bidirectional cavopulmonary anastomosis compared with those who were not staged. The bidirectional cavopulmonary anastomosis can be performed safely in children younger than 6 months and is now routinely performed electively between 4 and 6 months of age, even in children without prior surgical palliation.
Additional Sources of Pulmonary Blood Flow
Whether an additional source of pulmonary blood flow (such as a patent ductus arteriosus, patent right ventricular outflow tract or a tight pulmonary artery band, or systemic to pulmonary artery shunt) should be eliminated at the time of the cavopulmonary anastomosis remains open to discussion. Comparisons of patients with and without sources of additional pulmonary blood flow after a bidirectional cavopulmonary anastomosis have shown no deleterious effect at the time of the eventual Fontan procedure. Evidence shows that leaving an additional source of pulmonary blood flow after the bidirectional cavopulmonary anastomosis can enhance pulmonary arterial growth. Some patients do not tolerate this increased pulmonary blood flow, which manifests as pleural effusions and superior vena cava syndrome. These patients can be managed by catheter-based techniques to occlude the sources of additional pulmonary blood flow.
Some have argued that a bidirectional Glenn anastomosis with an additional source of pulmonary blood flow can serve as definitive palliation without a subsequent Fontan procedure, especially in high-risk cases. A multi-institutional paper from Italy reviewed 246 patients with a mean age 4.7 ± 6.2 years (range, 12 months to 30 years) who underwent a bidirectional cavopulmonary anastomosis leaving intact antegrade accessory pulmonary blood flow. On intermediate follow-up of 4.2 ± 2.8 years, 173 patients (70.3%) did not require a completion Fontan procedure and had a mean resting arterial oxygen saturation of 87% ± 4%. The actuarial freedom from a Fontan procedure at 7 years was 70.2% for the entire cohort. These results suggest that this strategy provided sustained palliation for selected patients with a single-ventricle circulation and may indicate that the Fontan route is not necessarily the universal palliation for all single-ventricle circulations.
Bilateral Bidirectional Cavopulmonary Anastomoses
Approximately 15% of children with single ventricles undergoing a bidirectional cavopulmonary anastomosis will have a persistent left superior vena cava draining to the coronary sinus. Although an earlier report from the Hospital for Sick Children in Toronto suggested that the presence of a second superior vena cava was a risk factor for thrombosis and failure to progress to the Fontan procedure, a report from our institution did not identify any adverse clinical outcomes in patients undergoing bilateral bidirectional cavopulmonary anastomoses compared with those undergoing a unilateral bidirectional cavopulmonary anastomosis.
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