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Ann Thorac Surg 1998;66:616-620
© 1998 The Society of Thoracic Surgeons
a Department of Paediatrics, Imperial College School of Medicine at National Heart and Lung Institute, London, England, United Kingdom
Address reprint requests to Prof Anderson, Department of Paediatrics, Imperial College School of Medicine at National Heart and Lung Institute, Dovehouse St, London SW3 6LY, UK
Presented at the Workshop on "One and One-Half Ventricle Repairs," Gubbio, Italy, Dec 6–7, 1996.
Abstract
Background. The concept of "one and a half" ventricular repair is to use "half" a ventricle to support the pulmonary circulation. The component makeup of any ventricle needs clarification for us to understand the nature of the so-called half ventricle.
Methods. The components of normal and abnormal ventricles are reviewed.
Results. Normal ventricles possess an inlet, an apical trabecular component, and an outlet. This tripartite approach is also logical in the description of congenitally malformed ventricles. Rudimentary and incomplete ventricles lack one or more of its component parts, and are usually hypoplastic. The location and morphology of the rudimentary ventricles correlate with the disposition of the atrioventricular conduction system.
Conclusions. Recognition of the ventricular components permits determination of ventricular morphology and guidelines for the location of the atrioventricular conduction axis.
The major expansion in the use of the Fontan procedure and its derivatives such as the cavopulmonary connection has focused surgical attention on ventricular morphology. Nowadays, the surgeon is increasingly exploring the option of incorporating "half" a ventricle in the pulmonary circulation as part of the so-called one and a half ventricle repair. This, of course, begs the questions of what is a ventricle, and can we have half a ventricle? This is more so because from the introduction of the procedure, surgeons have tended to describe the hearts submitted to repair by the Fontan option as "univentricular." The logician would surely question the possibility of manufacturing one and a half ventricles from a so-called univentricular heart! In reality, most of the hearts repaired in this fashion possess one big and one small ventricle [1], so the real question devolves on the nature of the small ventricle in hearts with double-inlet left ventricle, tricuspid atresia, or pulmonary atresia with an intact ventricular septum. That will be the focus of this review.
How do we analyze normal ventricles?
Any definition of chambers within the ventricular mass must start with a consideration of the ventricles of the normal heart, because congenitally malformed hearts are derived from the building blocks of normality. Traditionally, the normal ventricles were considered to possess a sinus and a conus [2, 3], yet it is difficult to discern anatomic evidence supporting the presence of two such components (Fig 1). In contrast, as pointed out by Goor and Lillehei [4], it is an easy matter to perceive the normal ventricles as possessing an inlet, an apical trabecular component, and an outlet. As we will see, this tripartite approach to ventricular analysis also permits logical description of congenitally malformed ventricles, something that is much more difficult to achieve whenusing the concept of sinus and conus [5]. Although the ventricles themselves can readily be categorized in terms of three components, however, the tripartite approach does not lend itself as readily to description of the normal ventricular septum. This is because, in the normal heart, as a consequence of the deeply wedged location of the subaortic outflow tract, much of the inlet of the right ventricle is separated by the muscular ventricular septum from the left ventricular outlet. And, because of the extensive free-standing muscular subpulmonary infundibulum, very little of the normal muscular septum separates the outflow tracts in a fashion that justifies the description of a normal muscular outlet septum. In the normal heart, therefore, it is preferable only to recognize the overall muscular septum, and to distinguish it from the relatively small membranous septum. The atrioventricular conduction axis is then sandwiched between these two normal ventricular septal components, with the axis itself branching on the crest of the muscular ventricular septum.
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Already we have described the relationships of the right and left ventricles of the normal heart. But how do we distinguish between them? And what permits us to recognize their rudiments when the ventricular mass is abnormally constituted? In answering these questions, we are guided by an important philosophical principle established by Van Praagh and colleagues [6]. The stimulus for the concept was provided by problems that used to exist in defining "univentricular hearts." It was enunciated when we, with our colleagues, had suggested that hearts with double inlet to a right ventricle could be considered "univentricular" [7]. We had derived this suggestion, illogically as it happens, from another premise put forward by Van Praagh and colleagues [8], namely, that the criterion for distinction of a single ventricle was the presence of double-inlet atrioventricular connection. In those hearts that we described as being univentricular examples of a right ventricle [7], there was unequivocally another chamber present within the ventricular mass (Fig 2). Van Praagh and colleagues [6] pointed out, quite rightly, that this second chamber could readily be identified as a rudimentary and incomplete left ventricle because of the structure of its apical component, which they described as the ventricular sinus. Because of this, they argued that malformed ventricles should be recognized morphologically on the basis of their most constant component, and they dubbed this principle the "morphologic method." For the hearts in question, namely, those with double-inlet right ventricle, it is clear that the apical trabecular component is the most constant part of the left ventricle, because most examples of double-inlet right ventricle also possess double outlet from this dominant ventricle (see Fig 2). The principle of the morphologic method is equally applicable, nonetheless, to hearts that possess double inlet to a dominant left ventricle, with its fine apical trabeculations, because the rudimentary and incomplete chamber can be recognized as a right ventricle on the basis of the coarse trabecular pattern of its apical component (Fig 3). Thus, by using this principle of defining ventricles according to the pattern of their apical part, we can recognize abnormal ventricles as being morphologically left or morphologically right. Concentration on apical trabeculations also permits recognition of the rarest ventricular pattern, that of a solitary and indeterminate ventricle. In this latter malformation, which truly produces a univentricular arrangement, the apical component is uniformly coarse, much coarser than a dominant right ventricle (compare Figs 2a and 4). Thick muscle bundles percolate throughout the ventricle, permitting distinction from those hearts with a huge ventricular septal defect. In the latter malformations, a remnant of the apical muscular septum separates the apical trabecular components of the morphologically right and left ventricles.
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As we have discussed, the traditional definition of a single ventricle, or a univentricular heart, was the presence of double-inlet atrioventricular connection. This apparently stems from the precedent of Taussig [9], who named such hearts "single ventricle with rudimentary chamber," and implicitly denied the second chamber its ventricular status. This approach was subsequently embraced by Van Praagh and colleagues [8], but these investigators, without specifying any reason, denied the univentricular accolade for hearts with atrioventricular valvar atresia. The concept subsequently received endorsement from Edwards [10], but again no reason was given why hearts with atrioventricular valvar atresia were excluded from the univentricular category. This was the situation when our colleagues and ourselves studied the structure of the most frequent prototype of tricuspid atresia [11]. Apart from the obvious morphologic similarities between the rudimentary ventricles in hearts having double-inlet left ventricle and those with tricuspid atresia (Fig 5), morphometric analysis showed that what differences existed between the morphology of the small ventricles depended on the ventriculoarterial rather than the atrioventricular connections [12].
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Do all hearts with atrioventricular valvar atresia have univentricular connections?
The analysis that shows that hearts with double-inlet ventricle have a univentricular atrioventricular connection shows equally well that some examples of atrioventricular valvar atresia must also fall within this category, but not all. Hearts can have tricuspid atresia because the morphologically tricuspid valve is present but imperforate, and can show this arrangement even when the tricuspid valve is deformed by Ebstein’s malformation. In this setting, the distal component of the right ventricle can be remarkably similar to the small ventricle seen in "classic" tricuspid atresia. This is because the imperforate valvar membrane is formed at the distal extent of the inlet component, sequestering the apical trabecular and outlet components as the so-called functional right ventricle. The right ventricle, therefore, can be small even when it possesses all its component parts. Such an arrangement with ventricular hypoplasia is seen in those examples of tricuspid atresia in which the imperforate valvar membrane is formed at the atrioventricular junction (Fig 6).
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How, then, can we define a ventricle?
On the basis that normal ventricles possess three parts, and that the apical trabecular component is the most constant of these parts, a good working definition of a ventricle is any chamber within the ventricular mass that has an apical component. Such ventricles will be normal when they possess an apical component together with an inlet and an outlet. Normal ventricles are of right or left morphology, depending on the trabecular pattern of the apical component, and such right and left ventricles always coexist within the same ventricular mass. Normal ventricles can be hypoplastic, however, particularly in the setting of severe stenosis or atresia of their outlet component. Abnormal ventricles can possess more or less of their anticipated three components, and are found in hearts with abnormal segmental connections. The solitary ventricle of indeterminate morphology is the only true univentricular heart, and it usually coexists with double inlet and double outlet from the solitary chamber (see Fig 4). It can be found, nonetheless, with absence of one or other of the inlets or outlets. The other abnormal ventricles will be either of right and left morphology, but will coexist within the same ventricular mass. One of the ventricles will be dominant, and almost always will be the one attached to the atriums. The small ventricle will be incomplete and rudimentary because it lacks one or more of its component parts, and it usually will also be hypoplastic.
Rudimentary ventricles will be of right or left morphology depending on the nature of their apical trabeculations, and can themselves be right-sided or left-sided in relation to the dominant ventricle. Rudimentary and incomplete right ventricles are always located anterosuperiorly relative to their dominant partner, whereas rudimentary and incomplete left ventricles are located posteroinferiorly within the ventricular mass. Because of this, the apical trabecular septum is always malaligned relative to the atrial septum in hearts with univentricular atrioventricular connection to a dominant left ventricle, so the atrioventricular node is an anterior structure. In this setting, the ventricular septal defect can always be enlarged, irrespective of the atrioventricular or ventriculoarterial connections, and irrespective of the right-sided or left-sided location of the rudimentary right ventricle, by resecting a wedge of apical septum closest to the obtuse margin of the ventricular mass. The outlet septum can also be resected if of suitable size, because it never carries the conduction axis.
In hearts with dominant right ventricles, the atrioventricular conduction axis arises from the regular atrioventricular node when the rudimentary and incomplete left ventricle is left-sided, but there will be an anterior atrioventricular node, or a sling of conduction tissue [14], when the rudimentary left ventricle is right-sided, because the ventricular mass will then show left-hand topology. The atrioventricular conduction axis will be bizarrely located in solitary ventricles of indeterminate morphology [15], because such hearts lack completely the apical trabecular septum, which usually carries the ventricular bundle branches.
On occasion, hearts will be found in which a subarterial infundibulum is sequestered as a separate component within the ventricular mass. Within our proposed working definition, such an infundibular chamber will not be considered a ventricle because it lacks an apical component. Sometimes an infundibulum can be sequestered together with part of the apical trabecular component, as in so-called double-chambered right ventricle, but this anomaly represents division of a normally structured right ventricle rather than formation of an abnormal ventricle in its own right [16].
Conclusions
Definition of a ventricle, be it normal or abnormal, on the basis of its apical trabecular component permits logical rules to be established for determination of its morphology. This sets the scene for subsequent determination of the atrioventricular and ventriculoarterial connections, as well as establishing guidelines with which to predict the location of the atrioventricular conduction axis.
Acknowledgments
This work was supported by the British Heart Foundation.
References
This article has been cited by other articles:
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  R. H. Anderson and S. Y. Ho Which hearts are unsuitable for biventricular correction? Ann. Thorac. Surg., August 1, 1998; 66(2): 621 - 626. [Abstract] [Full Text] [PDF]
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