The terms univentricular and single have proven to be amongst the most controversial adjectives used to describe some of the most complex congenital malformations of the heart. Since the mid-1990s, light has begun to emerge at the end of the tunnel for those who have attempted to provide a logical framework for the description of these complex conditions. This has stemmed from the realisation by clinicians that the decision underscoring the eventual presence or absence of a functionally univentricular arrangement is whether treatment of the individual patient is to proceed towards creation of biventricular circulations, or rather to establish the Fontan circuit, which of necessity produces circulatory patterns supported by only one ventricle, even if two chambers are unequivocally present within the ventricular mass. 1 For example, in a patient with critical aortic stenosis, the clinician will seek to determine whether the best option for the patient is to seek to achieve biventricular repair or opt for univentricular palliation, recognising that such a patient possesses both morphologically right and left ventricles. There are many patients, therefore, including those with hypoplastic left heart syndrome, those having pulmonary atresia with intact ventricular septum, as well as some with straddling of the atrioventricular valves or double outlet ventricle, for whom the clinician may choose a mode of treatment culminating in production of a functionally univentricular heart. All these conditions have either already been considered in other chapters—hypoplasia of the left heart in Chapter 29 and pulmonary atresia with intact septum in Chapter 30 , or will be considered in chapters to follow, specifically straddling atrioventricular valves in Chapter 40 and double outlet ventricles in Chapter 33 . In this chapter, we are concerned with describing the underlying morphology, and clinical diagnosis, of the lesions that produced most controversy previously in the definition of single ventricles or univentricular hearts, namely, double inlet ventricle and atrioventricular valvar atresia. In the next chapter, having established the principles for analysis of these problematic lesions, we will discuss the long-term options for all those patients having the functionally univentricular arrangements.
PHILOSOPHICAL CONSIDERATIONS
When considered from the stance of cardiac morphology, hearts having a solitary chamber within the ventricular mass, and hence truly being univentricular, are extremely rare. They do exist, and when found, they most usually exhibit double inlet to a chamber having indeterminate apical trabeculations, and in which the only septal structure present is the muscular outlet septum ( Fig. 31-1 ). The essence of such hearts is that the atrial chambers are joined to the solitary ventricle. It is possible, of course, that such an arrangement, with the atrial chambers connected exclusively to a solitary ventricle of indeterminate morphology, can be found when either the right- or left-sided atrioventricular connection is absent, but such hearts are exceedingly rare. It was recognition of the significance of such a univentricular arrangement of the atrioventricular junctions, nonetheless, than clarified the polemics underscoring the previous disagreements. 2,3 Thus, subsequent to the important study that had appeared in 1964, 4 it had become accepted 5 that the criterion for definition of a single ventricle was the presence of double inlet. Those accepting this concept, however, ignored that fact that, when the morphology of the small ventricle in patients with tricuspid atresia was compared to that of the same chamber in those with double inlet left ventricle, the comparison being made in the setting of lesions with the same ventriculo-arterial connections, then the small ventricles were essentially indistinguishable 6 (compare Figs. 31-2 and 31-3 ). This is because, in the setting of either the commonest form of tricuspid atresia or in those with double inlet left ventricle, the big ventricle is of left ventricular morphology, while the small ventricle is an incomplete right ventricle, lacking totally its inlet component ( Figs. 31-4 and 31-5 ). Recognition of this feature led to the realisation that, in both these anomalies, it was the atrioventricular connection, rather than the ventricular mass, which was univentricular. 2,3 This, in turn, led to the realisation that all patients with congenitally malformed hearts, with one small exception, could be divided into those with biventricular atrioventricular connections, this group made up of those with concordant, discordant, and ambiguous and mixed atrioventricular connections, and those unified because the atriums are joined to only one ventricle, either because both atriums are committed to the same ventricle, in other words those with double inlet ventricle, or because one of the connections between the atriums and the ventricular mass is completely absent, producing the commonest form of atrioventricular valvar atresia. The very small third group of patients is made up of those with absent atrioventricular connection and straddling and overriding of the solitary atrioventricular valve, 7 this combination producing a uniatrial but biventricular connection ( Fig. 31-6 ). In all those with univentricular atrioventricular connections ( Fig. 31-7 ), apart from the very small minority having solitary and indeterminate ventricles, there is a second ventricle within the ventricular mass that, of necessity, is incomplete and rudimentary. Among these patients, three additional groups stand out as requiring separate description, namely, those with double inlet, those with absence of the right atrioventricular connection, and those with absence of the left atrioventricular connection. It is patients with hearts of this type we describe in this chapter, along with their close cousins with imperforate atrioventricular valves, the latter also producing unequivocal atrioventricular valvar atresia.
MORPHOLOGY
As we have already explained in Chapter 1 , there are three possible junctional arrangements producing anatomical univentricular atrioventricular connections (see Fig. 32-7 ). The first is when the cavities of right- and left-sided atrial chambers are connected directly to the same ventricle. This is called double inlet atrioventricular connection, irrespective of whether the atrioventricular junctions are guarded by two atrioventricular valves or a common valve. The other two arrangements exist when one atrioventricular connection is absent, giving absence of the right-sided and left-sided atrioventricular connections, respectively. All of these patterns at the atrioventricular junction can be found in patients having usually arranged, mirror-imaged or isomeric atrial appendages. Each type of atrioventricular connection can also be found with the atriums connected to a dominant right ventricle, a dominant left ventricle, or rarely, to a solitary and morphologically indeterminate ventricle (see Fig. 32-1 ).
Atrioventricular Connections
The feature underpinning the anatomy of absent right atrioventricular connection is emphasised when patients having an imperforate tricuspid valve ( Fig. 31-8 ) are compared with those having complete absence of the right atrioventricular connection ( Fig. 31-9 ). Although these two anatomical entities produce the same physiological effect, their morphology is totally different. An imperforate valve can only be formed in the setting of a discrete atrioventricular junction. With this arrangement, the parietal myocardium of the atrium is continuous with the parietal ventricular wall. Hence, the cavities of the atrium and the underlying ventricle are in potential communication. When the atrioventricular connection is absent, this is not the case. The parietal walls of the right atrium and of the ventricle have no direct continuity, but instead they meet at the central fibrous body, with the fibro-fatty tissues of the atrioventricular groove filling the space between the adjacent layers of atrial and ventricular myocardium. The atrium has a complete muscular floor, which is the lateral wall of the appendage. A muscular dimple is frequently seen antero-superior to the orifice of the coronary sinus. This is often, incorrectly, presumed to represent the site of the right atrioventricular orifice. Sectioning through the dimple shows that it overlies the atrioventricular membranous component of the central fibrous body, and points to the outflow tract of the morphologically left ventricle ( Fig. 31-10 ). A similar distinction needs to be made between absence of the left atrioventricular connection and biventricular atrioventricular connections with an imperforate mitral valve. When the left connection is absent, the fibrofatty tissue of the left atrioventricular groove separates completely the muscular floor of the left atrium from the underlying ventricular myocardium ( Fig. 31-11 ).
When analyzing the morphology of the atrioventricular junctions in hearts with double inlet ventricle, it should be noted that double inlet exists because both atrial vestibules connect to the same ventricle, irrespective of whether the right- and left-sided atrioventricular junctions are guarded by two separate atrioventricular valves or a common valve ( Figs. 31-12 and 31-13 ). And, as we have already shown in Figure 31-7 , double inlet ventricle can occur with any atrial arrangement. The usual arrangement is the most common, but isomerism of the right atrial appendages is by no means rare, particularly when there is double inlet right ventricle. Common atrioventricular valves are also particularly frequent when there is right isomerism.
Abnormalities of the atrioventricular valves are frequent, in particular in hearts with double inlet left ventricle. In this setting, when there are separate right and left atrioventricular valves, both usually resemble mitral valves, lacking any tethering to the antero-superiorly located ventricular septum. They are also usually supported by separate papillary muscles, but often separated on the inferior ventricular wall by a prominent ridge. The two valves can also share papillary muscles, with no plane of cleavage between them, this feature ruling out the potential for ventricular septation. Where one or the other arterial valve is connected to the left ventricle, both atrioventricular valves are usually in continuity with it, although either, or rarely both, may be separated from it by persistence of the ventriculo-infundibular fold. Either the right or the left atrioventricular valve may be imperforate or stenotic, or may straddle or override the ventricular septum. In the setting of overriding and straddling atrioventricular valves, as we will explain in greater detail in Chapter 33 , providing more than half of the vestibule of the overriding junction is connected to the dominant ventricle, we continue to categorise the connection as double inlet. In patients with absence of either the right or left atrioventricular connection, the remaining atrioventricular valve may straddle, producing the rare uniatrial and biventricular connection (see Fig. 31-6 ).
The Ventricles
In almost all hearts in which there is a univentricular atrioventricular connection, there are complementary ventricles present of left and right ventricular morphology, separated by the apical muscular ventricular septum (see Figs. 31-4 and 31-12 ). In the majority of patients with absent right atrioventricular connection, this being the arrangement producing classical tricuspid atresia, the left atrium is connected to a dominant left ventricle (see Fig. 31-9 ). In these hearts, and indeed in all those having a dominant left ventricle, the incomplete right ventricle is carried on the antero-superior shoulder of the ventricular mass, almost always with its trabecular component to the right. In hearts with double inlet left ventricle, when the incomplete right ventricle is again found in an antero-superior position, the apical trabecular component can be either right- or left-sided, or directly anterior. In all situations in which the left ventricle is dominant, unlike in hearts with biventricular atrioventricular connections, the hypoplastic apical muscular ventricular septum does not extend to the crux.
When it is the right ventricle which is dominant, the incomplete left ventricle is located in postero-inferior position, usually to the left of the dominant ventricle, but rarely to the right. In both of these settings, the hypoplastic ventricular septum does extend to the crux. Absence of the left atrioventricular connection with the right atrium connected to a dominant right ventricle produces one of the frequent variants of hypoplastic left heart syndrome (see Fig. 31-11 ). Absence of the right atrioventricular connection can also rarely be found when the left atrium is connected to a dominant right ventricle, and again the incomplete left ventricle will usually be located postero-inferiorly, this arrangement being found most frequently in association with straddling and overriding of the left atrioventricular valve ( Fig. 31-14 ).
Ventriculo-arterial Connections
Any type of ventriculo-arterial connection must be anticipated to exist in the setting of the univentricular atrioventricular connection, as long as they are anatomically feasible. Should there be a solitary ventricle of indeterminate morphology, however, the only possibilities are double outlet from the solitary ventricle (see Fig. 31-1 ), or one of the variants of single outlet, usually pulmonary or aortic atresia rather than common arterial trunk. When the left ventricle is dominant, then concordant ventriculo-arterial connections are the rule in the setting of tricuspid atresia, and discordant connections are typically found when there is double inlet. When it is the left atrioventricular connection which is absent, then it is most usual to find aortic atresia with origin of the pulmonary trunk from the right ventricle, this being one of the frequent variants of hypoplastic left heart syndrome (see Fig. 31-11 and Chapter 29 ). As emphasised, however, any connection must be anticipated to exist, and concordant ventriculo-arterial connections are by no means rare when there is double inlet left ventricle, this combination producing the so-called Holmes heart (see Fig. 31-2 ). Should there be double inlet to, and double outlet from, the dominant left ventricle, then the incomplete right ventricle is represented only by its apical trabecular component, which continues to be positioned antero-superiorly, albeit either to the right or the left. When it is the right ventricle that is dominant, then most usually there is also double outlet from the right ventricle. The incomplete left ventricle is then represented only by its apical trabecular component, positioned postero-inferiorly, and usually to the left. Concordant ventriculo-arterial connections can be found, most usually when there is a common atrioventricular valve, but other connections are rare.
Interventricular Communication
In patients with a dominant left ventricle, irrespective of the precise atrioventricular connection, the hole between the ventricles is almost always completely surrounded by muscle, being located between the apical and outlet components of the ventricular septum. This hole has been described in various ways over the years, being called a bulboventricular foramen, an outlet foramen, or a ventricular septal defect. It is an integral part of one of the circulations when the ventriculo-arterial connections are concordant or discordant. It has a tendency to become obstructive with time (see Figs. 31-2 and 31-3 ). In the setting of discordant ventriculo-arterial connections, such a reduction in the size produces subaortic obstruction ( Fig. 31-15 ), but results in subpulmonary obstruction when it is the pulmonary trunk which arises from the incomplete right ventricle. It is well established that the defect can decrease markedly in size, albeit that a purported causal association with banding of the pulmonary trunk 8 remains to be proved.
The interventricular communication in hearts with dominant right ventricles is usually perimembranous, since the hypoplastic septum extends to the crux in this setting, and the defect is directly related to the atrioventricular valve or valves. In most instances when the right ventricle is dominant, both arterial trunks also arise from the dominant right ventricle, and the left ventricle is represented only by its apical trabecular component, albeit often in association with minimal straddling of the left atrioventricular valve ( Fig. 31-16 ). In this situation, the interventricular communication is not an integral part of the circulation to one or other outflow tract, as it usually is when the left ventricle is dominant, and hence the morphology of the interventricular communication is of less clinical significance.
Associated Malformations
The interventricular communication is part and parcel of the morphology of hearts having dominant right or left ventricles, and does not exist in those with solitary and indeterminate ventricles. In those with dominant left ventricles, it is the size of the interventricular communication that is the major determinant of clinical presentation, recognising that this feature is, of course, also dependent on the ventriculo-arterial connections. The presence of other associated malformations can then further modify the clinical presentation. Any lesion that can feasibly exist must be anticipated to be present in some patient at some time. Some lesions, nonetheless, are particularly frequent, such as deficiencies of the floor of the oval foramen, producing an interatrial communication. An intact atrial septum can also produce problems in the setting of absence of the left atrioventricular connection. Persistent patency of the arterial duct is frequent, this being an important part of the circulation when there is pulmonary stenosis or atresia, or aortic coarctation or interruption. It is the obstructive lesions in one or the other of the outflow tracts, nonetheless, which are probably the most important associated lesions. In the setting of a dominant left ventricle, deviation of the muscular outlet septum can produce either subpulmonary or subaortic obstruction, depending on whether the ventriculo-arterial connections are concordant or discordant. A deviated outlet septum can also produce either subpulmonary or subaortic obstruction in the setting of double outlet ventricle. Subarterial obstruction can also be produced by tissue tags, or by anomalous attachment of the tension apparatus of the atrioventricular valves. Other lesions of the pulmonary pathways must be anticipated, such as discontinuous pulmonary arteries, whilst anomalies of venous connection are by no means infrequent, particularly when there is isomerism of the atrial appendages. And the atrial appendages themselves can be juxtaposed. In short, a multitude of associated malformations can be found in patients having either double inlet ventricle or absence of one atrioventricular connection.
Conduction Axis
An understanding of the structure of hearts with univentricular atrioventricular connection, as described above, provides the information needed to determine the location of the atrioventricular conduction axis. This is because the ventricular components of the conduction axis are carried on the crest of the apical muscular ventricular septum, and the atrioventricular node, or in some instances nodes, is formed at the point, or points, of union of the apical septum with the atrioventricular junctions.
When the left ventricle is dominant, the atrioventricular conduction axis is grossly abnormal. 9,10 The pattern is dictated by the lack of any ventricular septum at the crux. Because of this, the regular atrioventricular node in the atrial septum is unable to make contact with the atrioventricular conduction tissues positioned astride the apical ventricular septum. Instead, an anomalous atrioventricular node is found in the superior quadrant of the right atrioventricular orifice ( Fig. 31-17 ). From this node, the conduction axis penetrates the atrioventricular fibrous plane, the precise course of the non-branching bundle then depending on the position of the incomplete right ventricle. When the right ventricle is right sided, the bundle is able to descend directly onto the septum, and is unrelated to the pulmonary outflow tract. When the incomplete ventricle is left sided, the bundle extends antero-superiorly around the pulmonary valvar orifice to reach the septum. Irrespective of the position of the right ventricle, the relation of conduction tissue and the interventricular communication is the same when viewed from the right ventricular aspect ( Fig. 31-18 ). The conduction axis branches on the left ventricular aspect of the septum, and is well below the septal crest. Only the right bundle branch extends upwards, piercing the septum to ramify in the apical component of the incomplete right ventricle. The basic disposition of the axis, therefore, is always the same, being dictated by the orientation of the ventricular septum. The septum joins the atrioventricular junction more parietally when there is straddling and overriding of the right atrioventricular valve. Because of this, the node and penetrating bundle are formed postero-laterally in the right atrioventricular orifice (see Chapter 33 ). The position of the conduction axis relative to the interventricular communication is the same be there absence of the right or left atrioventricular connection. One important variation is found, however, should there be an apical muscular interventricular communication in addition to the usual defect between the outlet and apical components of the muscular septum. In the presence of two defects, the conduction axis courses within the muscular bar separating the holes ( Fig. 31-19 ). A further variation is found in the setting of tricuspid atresia due to absence of the right atrioventricular connection. Because there is no right atrioventricular valvar orifice, the atrioventricular node is found adjacent to the dimple, with the apex of the triangle of Koch now serving as a guide to the location of the node ( Fig. 31-20 ).
When the right ventricle is dominant, the disposition of the conduction axis is determined in part by the septal orientation, and in part by ventricular topology. Hearts with left-sided incomplete left ventricles have a right-handed pattern of ventricular topology. Then, because the septum extends to the crux, the connecting node is in its anticipated regular position. The atrioventricular bundle penetrates postero-inferiorly, and branches astride the apical muscular septum. 11 When the left ventricle is right sided, the ventricular topology is left-handed. This is then the dominant feature in determining the location of the conduction tissue, over and above the fact that the septum reaches to the crux. In the only heart of this type we studied histologically, there was a sling of conduction tissue between anterior and regular nodes. The ventricular conduction tissue descended on a trabeculation within the right ventricle. 12 We anticipate that the same rules will apply when either the right or left atrioventricular connection is absent in association with univentricular connection to a dominant right ventricle.
When there is a solitary and indeterminate ventricle, then because there is no apical trabecular septum, there cannot be a normal conduction system. Most frequently, in our experience, 13 there has been an anterior node with the bundle descending onto a free-standing muscle bar. The bundle can also descend in the parietal ventricular wall or else drop from a regular node. Slings of conduction tissue have been found in the postero-inferior ventricular wall in patients with right isomerism. 14
Huge Ventricular Septal Defects
Hearts also exist in which most of the ventricular septum is absent, but an apical muscular rim persists, dividing the ventricular mass into right ventricular and left ventricular components ( Fig. 31-21 ). Because of the presence of the apical remnant of the ventricular septum, the ventricular inlets in this setting are committed to recognisably separate apical ventricular components. More importantly, in most cases but not all, it is usual to find a rim of septum extending from the apical septum to the crux. In our experience, this rim has always carried the non-branching bundle from a regularly positioned node. Morphologically, therefore, the hearts are readily distinguished from those with double inlet atrioventricular connection. We categorise them as huge ventricular septal defects, diagnosing the atrioventricular connections according to the union of the atrial chambers with the apical ventricular components.
Morphogenesis
The problems arising from morphogenesis relate as much to terminology of the embryonic heart tube as they do to positive disagreements. When considering the occurrence of double inlet connection, therefore, we should remember that, initially, the orifice of the atrioventricular canal was supported almost exclusively above the inlet part of the primary heart tube, with the presumptive arterial pedicles supported by the outlet component ( Fig. 31-22 ). As described in Chapter 3 , the apical component of the developing left ventricle balloons from the inlet component of the ventricular loop, while that of the developing right ventricle balloons from the outlet part. The ballooning of the two apical ventricular components occurs concomitant with development of the apical muscular septum. If the two pouches did not grow separately, but rather there was formation of a general apical component from the primary heart tube, then the end result would be a solitary ventricle of indeterminate morphology (see Fig. 31-22 , left hand panel). Whether the atrioventricular junctions are guarded by two valves, or a common atrioventricular valve, depends on the partitioning of the atrioventricular junction. Should the pouches form in normal fashion, but the atrioventricular junction remain connected only to the inlet part of the ventricular loop, then the end result would be double inlet left ventricle. The hypoplastic trabecular component derived from the outlet part of the loop would form the basis of the incomplete right ventricle (see Fig. 31-22 , lower right hand panel). Again, the arrangement of the atrioventricular valves would depend upon the mode of development of the atrioventricular junction. The ventriculo-arterial connections present would depend on the development of the outlet portions. The position of the incomplete right ventricle would probably be determined by the initial looping of the primary tube, but equally it could be influenced by rotation of the entire heart.
