Definition

Many terms are used interchangeably to discuss this group of cardiac lesions, including single ventricle, functional single ventricle, univentricular heart, and functionally univentricular heart. Before an in‐depth discussion can ensue, a proper definition of a functionally univentricular heart must be established. The definition provided by Jacobs and Anderson is the most appropriate: “the entire ventricular mass was functionally univentricular whenever one or other ventricle was incapable, for whatever reason, of supporting either the systemic or the pulmonary circulation” [1].

Classification

The segmental approach of Van Praagh [2] and the sequential segmental approach of Anderson [3] account for the way in which the components of the heart are joined together. This still requires an account of the lesions to be found within the heart. In this regard, controversies continued as to how best to describe the chambers within the ventricular mass. In an attempt to resolve these issues, the European Association for Cardiothoracic Surgery (EACTS) and the Society of Thoracic Surgeons (STS), along with societies representing pediatric cardiologists, joined to form the International Society for Nomenclature of Pediatric and Congenital Heart Disease (ISNPCHD). This Society, in turn, then created the International Pediatric and Congenital Cardiac Code (IPCCC) [4, 5]. Based on a consensus of nomenclature from this group [1], the IPCCC includes the following as functionally univentricular hearts listed as “single ventricle (functionally)” [6]: double‐inlet left ventricle (DILV), double‐inlet right ventricle (DIRV), mitral atresia, tricuspid atresia, unbalanced atrioventricular septal defect (AVSD), single ventricle and heterotaxy syndrome, single ventricle other, and single ventricle and total anomalous pulmonary venous connection (TAPVC). Although several other lesions are not listed under “single ventricle” in the IPCCC code, they produce similar pathophysiology and ultimately share a common treatment algorithm. They also satisfy the definition offered by Jacobs and Anderson [1]. These lesions are hypoplastic left heart syndrome (HLHS), pulmonary atresia with intact ventricular septum (PA/IVS), biventricular hearts with straddling atrioventricular valves, and complex forms of double‐outlet right ventricle (DORV).

Double‐Inlet Ventricles

A double‐inlet ventricle is a rare cardiac lesion in which both atrial chambers connect to a dominant ventricle, either through two separate atrioventricular valves or a common atrioventricular valve. Overriding of one of the atrioventricular valves must be greater than 50% to be classified as double‐inlet ventricle. The hypoplastic chamber, which is incomplete in that it lacks an inlet component, is of the opposite morphology. When the dominant ventricle is of left morphology, the connection between the two chambers used to be termed a “bulboventricular foramen.” In fact, it is a ventricular septal defect. In the setting of DILV, it is the size of this channel and the location of the aorta that determine the degree of systemic outflow obstruction [7].

DILV is the commonest variant, and its own commonest pattern is when the ventriculoarterial connections are discordant and the incomplete right ventricle is left‐sided. The right‐sided dominant left ventricle then gives rise to the pulmonary trunk, with the left‐sided incomplete morphologically right ventricle giving rise to the aorta [7]. A less common variant occurs when the ventriculoarterial connections are concordant. In this setting, the incomplete morphologically right ventricle is right‐sided, as it can also be when the ventriculoarterial connections are discordant. In some circumstances, there can be double outlet from either the dominant or the incomplete ventricles. These are rare, as are the other forms of double inlet, which can be to a dominant right ventricle, or a solitary and indeterminate ventricle (DIIV). These latter variants are more often associated with isomerism of the atrial appendages, and typically have double outlet with or without pulmonary atresia [7].

Mitral Atresia

Mitral atresia is defined as atresia of the left atrioventricular connection. When found with aortic stenosis or atresia, the patient is considered to have HLHS. If the atrial septum is restrictive or intact, the prognosis is worse given the usual association of pulmonary venous hypertension. Mitral atresia is usually associated with concordant ventriculoarterial connections or DORV. It is rarely associated with transposition [8].

Tricuspid Atresia

Tricuspid atresia has been the traditional symbol of functionally univentricular hearts, as it was the first condition to be treated with the Fontan procedure. Tricuspid atresia is defined as atresia of the right atrioventricular connection and is most commonly due to absence of the right atrioventricular connection. In this setting, the fibro‐adipose tissue of the right atrioventricular junction separates the floor of the right atrium from the incomplete right ventricle. In rare cases, nonetheless, there may be an imperforate tricuspid valve, which can show evidence of Ebstein malformation [7]. Consistent anatomy includes an atrial septal defect as the sole egress of the right atrium, a patent left atrioventricular connection, a hypoplastic right ventricle, and a dominant left ventricle. Variable anatomy includes the presence and size of a ventricular septal defect, variation in the ventriculoarterial connections, and the degree of pulmonary stenosis. A time‐honored system for classification was based on the ventriculoarterial connections. So‐called type I has concordant ventriculoarterial connections with spiraling and normally related great arteries. It is the commonest variant. In type II, the ventriculoarterial connections are discordant, while in type III, which is exceedingly rare, there is a common arterial trunk. Further subclassification depends on pulmonary blood flow, and is applicable to the variants with concordant ventriculoarterial connections. In type a, there is pulmonary atresia; in type b, there is subvalvar or valvar pulmonary stenosis; while type c, which is rare, has no pulmonary stenosis [9]. The remainder of the cardiac anatomy is typically characterized by normal systemic venous connections, usual atrial arrangement, right‐handed ventricular topology, usual coronary arteries, and a normal conduction system [7].

Unbalanced Atrioventricular Septal Defect

An unbalanced AVSD occurs when the left and right components of the common atrioventricular valve are not equally distributed over the ventricles. This causes unequal blood flow from the atrial chambers to each ventricle. For example, when the valve is displaced rightward, there is increased flow to the right ventricle, resulting in a dominant right ventricle with a hypoplastic left ventricle. In this case, the adequacy of the hypoplastic left ventricle for use in a biventricular repair is, in part, determined by the size of the left atrioventricular valvar component, the left mural leaflet, and the distance between the papillary muscles.

“Single Ventricle with Heterotaxy Syndrome”

This term, as defined by the Nomenclature Committee, is synonymous with the presence of isomeric atrial appendages, as opposed to the usual or mirror‐imaged atrial arrangement. In most instances associated with a functionally univentricular heart, there is right isomerism. As such, there are multiple anomalies, both cardiac and noncardiac [10]. The noncardiac anomalies can involve the lungs, which also show evidence of isomerism, and most of the abdominal organs, which are jumbled up. Abnormal ciliary function can lead to recurrent pulmonary infections and bronchiectasis. There can be absence of the spleen, or multiple spleens, both of which can cause abnormal splenic function and susceptibility to encapsulated bacteria. The liver can be midline, and the stomach is often on the right. The small and large intestines can have abnormal attachments predisposing to malrotation, volvulus, and obstruction [10, 11].

The cardiac anomalies often produce functionally univentricular physiology. Some of these patients, nonetheless, can be managed by biventricular repair. The presentation, based on the degree of systemic and pulmonary blood flow, will depend on the type and severity of anomalies. Abnormal systemic venous connections may consist of bilateral superior or inferior caval veins, interruption of the inferior caval vein with azygous or hemiazygous continuation, variable drainage of the hepatic veins, and aberrant portal veins. There is usually a common atrium due to a poorly developed atrial septum. Common atrioventricular septal defects, often unbalanced, abnormalities of the outflow tracts, and totally anomalous pulmonary venous connection are frequent. There may be pulmonary stenosis or atresia as well as pulmonary arterial abnormalities. Abnormal rhythms, notably atrial arrhythmias and complete heart block, are common [10–12].

The constellation and frequency of anomalies are related to the sidedness. Absence of the spleen is usually associated with bilateral right‐sidedness, or right isomerism, and multiple spleens with bilateral left‐sidedness, or left isomerism [10, 11]. Right isomerism is associated with a worse prognosis than left isomerism [13].

Overall, the so‐called heterotaxy syndrome is made up of a complex group of patients. The diagnosis of a functionally univentricular heart in this setting carries a worse prognosis compared to other types of functionally univentricular hearts. In large series of similar time periods from the Mayo Clinic, the early mortality following a modified Fontan procedure was 9.5% for “heterotaxy syndrome,” compared to 3% for DILV and 2% for tricuspid atresia [14–16].

“Single Ventricle with Total Anomalous Pulmonary Venous Connection” and Other Lesions

Totally anomalous pulmonary venous connection is discussed in Chapter 35. It is always present in the setting of right isomerism, even if the pulmonary veins return to the heart. When present in the setting of a functionally univentricular heart, the prognosis is poor. The remaining category of other lesions encompasses very rare cardiac defects in which the morphology of the dominant ventricle is unclear.

First‐Stage Palliation

Second‐Stage Palliation

The second stage creates a superior cavopulmonary anastomosis as the source of pulmonary blood flow while volume unloading the ventricle. Other sources of pulmonary blood flow are typically eliminated at this stage. If the pulmonary trunk is divided, the pulmonary valvar leaflets should be resected or incorporated in the suture line to prevent a blind‐ending pouch distal to the valve, which can be a source for systemic thromboembolism [55, 56].

This stage is typically performed between 4 and 6 months of age. A preoperative cardiac catheterization is traditionally performed to assess surgical candidacy, although selective use of magnetic resonance imaging in low‐risk patients is gaining favor at many experienced centers. Major factors considered include patient growth; systemic oxygen saturation; pulmonary arterial size, pressure, and resistance; the state of the shunt if present; atrioventricular valvar regurgitation; systemic ventricular function and pressure; systemic outflow obstruction; and need for additional intervention [47, 57]. Once deemed an appropriate candidate, the options at this stage include HFP, BDG, or the Kawashima procedure.

Hemi‐Fontan Procedure

The hemi‐Fontan procedure is the procedure of choice at the University of Michigan. Despite being technically more complex than a BDG, it is favored for several reasons (Figure 27.3) [47]. Routine augmentation of the right and left pulmonary arteries ensures optimal pulmonary artery anatomy, and the more complex operation when performed at this stage simplifies the lateral tunnel Fontan when the postoperative hemodynamics are more demanding. At the Fontan stage, the entire cardiac output, with the exception of a fenestration, passes through the lungs. Long anesthetic and cardiopulmonary bypass times, as well as the need for multiple blood products, can negatively impact the lungs, which more seriously affects the early postoperative course following the Fontan. In addition, longer cross‐clamp and bypass times at the Fontan have been associated with increased early and late mortality [58]. Lastly, based on mathematical modeling, the hemi‐Fontan procedure with lateral tunnel Fontan circulation has more favorable flow patterns, with less energy loss and more equal distribution of inferior caval venous blood flow, as compared to the BDG with extracardiac conduit Fontan circulation (Figure 27.4) [59].

The current technique used at the University of Michigan differs from the original description by Norwood because of two modifications. First, the right atriotomy does not cross the cavoatrial junction to avoid potential injury to the artery supplying the sinus node, and subsequent sinus nodal dysfunction. Second, a separate PTFE patch, rather than the allograft patch augmentation of the pulmonary arteries, is used to partition the right atrium, facilitating the lateral tunnel Fontan [47].

This technique has been well described in the literature [47, 60]. Cannulae are placed in the ascending aorta, superior caval or innominate vein, and inferior caval vein. Cardiopulmonary bypass is initiated at 28–34 °C, depending on surgeon preference. If present, native pulmonary blood flow or a systemic to pulmonary artery shunt is occluded. The azygous vein is ligated. The right and left pulmonary arteries are mobilized to the lobar branches and controlled. A longitudinal central pulmonary arteriotomy is made and extended leftward beyond the insertion of the ductal stump and rightward to a point directly posterior to the superior caval vein. An aortic cross‐clamp is applied and antegrade cardioplegia is administered. A right atriotomy is performed from the right atrial appendage toward, but not across, the cavoatrial junction. The apex of the right pulmonary arteriotomy is then sutured to the posterior wall of the superior caval vein, such that its external surface serves as part of the augmentation of the right pulmonary artery. This suture line is carried medially where the inferior edge of the central pulmonary arteriotomy is connected to the posterior edge of the right atriotomy. A PTFE patch is sutured at the level of the border of the oval fossa posteriorly, and then incorporated into the eventual closure between the right atriotomy and patch augmentation of the pulmonary arteries anteriorly. This partitions the hemi‐Fontan pathway from the common atrium. The heart is de‐aired. The aortic cross‐clamp is removed. Allograft patch augmentation of the central pulmonary arteries is then performed, thereby completing the connection as the roof of the pathway. Control of the pulmonary arteries is released. The remainder of the operation is conducted in the usual fashion, including direct pressure measurements of the hemi‐Fontan pathway pressure and common atrial pressure to assess the transpulmonary gradient. Intracardiac pressure monitoring lines are typically not left in the hemi‐Fontan pathway to avoid thrombosis (Figure 27.5).

Bidirectional Glenn Procedure

The BDG procedure is currently the more commonly performed second‐stage palliation at most centers. Advocates favor this approach for the technical ease and the ability to perform it without cardiac arrest or even without cardiopulmonary bypass [42, 61]. In addition, despite our institution’s preference for the hemi‐Fontan procedure, in some patients that is technically not possible and a BDG procedure is used instead. Most commonly, this is in the setting of totally or partially anomalous pulmonary venous connection to the superior caval vein, where the vein must be divided above the anomalous connection to preserve pulmonary venous return to the atrium. Other examples include a single left superior caval vein in the setting of the usual atrial arrangement, in which the vein does not connect to the morphologically right atrium, and the usual atrial arrangement with right‐sided heart, or mirror‐imaged arrangement with left‐sided heart, where accessing the morphologically right atrium is difficult due to the overlying ventricular mass. Lastly, various forms of isomerism, where the inferior caval and hepatic veins are on the opposite side to the superior caval vein, can make the procedure impossible. If bilateral superior caval veins are present, we elect to perform a right modified hemi‐Fontan procedure with a left BDG procedure.

Cannulae are placed in the ascending aorta, superior caval or innominate vein, and right atrium or inferior caval vein. Cardiopulmonary bypass is initiated at 32–34 °C, depending on surgeon preference and without aortic cross‐clamp. Caval venous tourniquets are engaged. If present, native pulmonary blood flow, or a systemic to pulmonary artery shunt, is occluded. The azygous vein is divided. The superior caval vein is clamped above the cavoatrial junction and divided. The stump on the atrial side is oversewn in two layers. A longitudinal arteriotomy is made on the superior aspect of the right pulmonary artery adjacent to the free end of the superior caval vein. Patch augmentation of the pulmonary arteries can be performed if necessary, and the superior caval vein can be spatulated to enlarge the orifice. An end‐to‐side anastomosis is then performed between the superior caval vein and undivided pulmonary artery on that side, with specific details based on surgeon preference. The remainder of the operation is conducted in the usual fashion, as described above for the modified hemi‐Fontan procedure. Modifications of this technique have been described without cardiopulmonary bypass by using a shunt to decompress the superior caval vein [42, 61].

Variable	HFP/LT	BDG/ECC	BDG/IECC
Applicability	Most patients	All patients	All patients
Age at completion Fontan	Usually younger	Usually older	Usually older
Cardiopulmonary bypass required	Yes/Yes	No/No	No/Yes
Aortic cross‐clamp required	Yes/Yes	No/No	No/Yes
Second stage	Technically more complex	Technically easier	Technically easier
Third stage	Technically easiest	Technically easier	Technically easier
Fenestration	More feasible and durable	Less feasible and durable	More feasible and durable
PA status	Routine augmentation	Selective augmentation	Selective augmentation
Efficiency of blood flow	Optimal	Less optimal	Less optimal
Distribution of IVC flow to each lung	Equal	Can be unequal if caval offset	Can be unequal if caval offset
Growth potential	Yes	No	No
Thrombogenicity	Potentially less	Potentially more	Potentially more
Atrial suture lines	More	Less	Intermediate
Atrial exposure to Fontan pressure	More	Less	Less
Arrhythmias	Potentially more	Potentially less	Potentially less
Percutaneous access to common atrium	Maintained	Not maintained	Maintained
Future transplantation	Requires more PA reconstruction	Requires less PA reconstruction	Requires less PA reconstruction

Note that these advantages are believed to exist by those who advocate each approach, although in most cases there is no well‐supported data in the literature. Therefore, there is no clear consensus on the optimal type and practice patterns vary by surgeon and institution [42, 47, 52, 57–61, 65–72, 74, 77, 78].

BDG/ECC, bidirectional Glenn/extracardiac conduit; BDG/IECC, bidirectional Glenn/intra‐extracardiac conduit; HFP/LT, hemi‐Fontan procedure/lateral tunnel; IVC, inferior caval vein; PA, pulmonary artery.