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Ann Thorac Surg 1998;66:644-648
© 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
Presented at the Workshop on "One and One-Half Ventricle Repairs," Gubbio, Italy, Dec 6–7, 1996.
Abstract
Background. In hearts having the atriums connected only to a dominant left ventricle, typified by double-inlet left ventricle but seen also in lesions such as tricuspid atresia, subaortic obstruction, when it exists, is usually found at the level of the ventricular septal defect when the aorta is supported by the rudimentary right ventricle.
Methods. Heart specimens were examined to determine the nature and position of the ventricular septal defect existing between dominant left and rudimentary right ventricles when the ventriculoarterial connections are discordant.
Results. Most commonly, the ventricular septal defect is positioned between the muscular apical trabecular septum and the muscular outlet septum. This type of defect is found not only in double-inlet left ventricle, but also in hearts with absence of either the right or left atrioventricular connection when the other atrium is connected to a dominant left ventricle, irrespective of the position of the rudimentary and incomplete right ventricle. Obstructive lesions within the aortic arch are commonly associated with restriction at the site of the ventricular septal defect. The atrioventricular conduction bundle takes a constant course relative to the margin of the septal defect.
Conclusions. Because subaortic obstruction is almost always caused by a restrictive ventricular septal defect, relief of the obstruction can be achieved by surgical enlargement of the septal defect, bearing in mind the course of the atrioventricular conduction system.
The potential for subaortic obstruction, when seen, exists most frequently at the level of the ventricular septal defect in hearts having the atriums connected only to a dominant left ventricle when the aorta is supported by the rudimentary and incomplete right ventricle. These hearts are typified by having double-inlet left ventricle, but the arrangement is found also in hearts with absence of the right or left atrioventricular connection where the other atrium is connected to a dominant left ventricle. This review concentrates on the morphology of the restrictive ventricular septal defect existing between dominant left and rudimentary right ventricles (the so-called bulboventricular foramen) when the ventriculoarterial connections are discordant.
Problems in terminology
The surgical problems in dealing with obstruction at the level of the ventricular septum in hearts with a dominant left and a rudimentary right ventricle have long been recognized [1]. Describing the optimal solution to these problems has been bedeviled by lack of agreement on the most appropriate name for the interventricular communication [2]. We ourselves have contributed to this confusion in no small way because, at various times we have described this hole between the ventricles as the "bulboventricular foramen" [3] or the "outlet foramen" [4]. At the time, these seemed suitable terms, and some continue to use them. It seems to us, nonetheless, that one of the major advantages of recognizing as ventricles all those chambers within the ventricular mass that possess an apical trabecular component has been the ability to describe at the same time the septum separating them as the ventricular septum. There can be little doubt that this structure is the ventricular septum because it carries the ventricular conduction tissues on its crest. It possesses the fine left ventricular trabeculations on one of its surfaces, and the coarse right ventricular trabeculations on the other surface (Fig 1). When this structure is described in this way as the ventricular septum, it then becomes clear that the hole between the ventricles can be described simply and logically as the ventricular septal defect.
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Almost always, in hearts having univentricular atrioventricular connection to a dominant left ventricle, the ventricular septal defect, which forms the only inflow to the rudimentary right ventricle, is positioned between the muscular apical trabecular septum and the muscular outlet septum (Fig 2). Very rarely, the muscular septum can be totally lacking so that the leaflets of the aortic and pulmonary valves are in fibrous continuity in the roof of the defect. Other than in this circumstance, the muscular outlet septum and the apical trabecular septum are themselves continuous with the musculature of the ventricular mass such that the margins of the defect, when viewed from the rudimentary right ventricle, are made up exclusively of muscle (see Fig 2). The crest of the apical septum can be overridden by the aortic valve when there is malalignment between the apical trabecular and outlet septums. This does not change the basically muscular nature of the rims of the septal defect. We have never observed a perimembranous defect in the setting of univentricular connection to a dominant left ventricle in the sense that there is fibrous continuity between the leaflets of the arterial valve arising from the rudimentary ventricle and the leaflets of one of the atrioventricular valves. We have found hearts in which the ventricular septal defect, rather than being between the apical and outlet components of the septum, is embedded within the apical muscular septum. Such defects nonetheless still possess exclusively muscular rims (Fig 3).
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Any heart in which the atriums are connected exclusively to the dominant left ventricle will have a ventricular septal defect providing the inflow to the rudimentary and incomplete right ventricle. Thus, this type of ventricular septal defect will be found in all hearts with double-inlet left ventricle, irrespective of whether there are two atrioventricular valves or a common valve guarding the atrioventricular junction. This type of defect will also be found in hearts with absence of either the right or the left atrioventricular connection in which the other atrium is connected to a dominant left ventricle (Fig 4). The particular morphology of the ventricular septal defect will be found irrespective of whether the rudimentary and incomplete right ventricle is right-sided or left-sided. Some authorities describe the position of the rudimentary right ventricle according to concepts of ventricular topology, and then infer the nature of the remaining atrioventricular valve when one atrioventricular connection is absent according to embryologic assumptions [5]. Sometimes, however, the rudimentary right ventricle is positioned neither to right nor left, but is located directly anterior within the ventricular mass. Furthermore, when the atrioventricular valves are connected within the dominant left ventricle, then both tend to resemble the morphologically mitral valve [6]. When one connection is absent, therefore, the remaining valve will resemble morphologically the mitral valve irrespective of whether the valve itself, or the rudimentary an incomplete right ventricle, is right-sided or left-sided. For all these reasons, we prefer to describe the atrioventricular valves in hearts having a univentricular atrioventricular connection to a dominant left ventricle simply as the right-sided and left-sided valves rather than trying to distinguish them as "mitral" or "tricuspid" [7]. Similarly, we describe absence of the right-sided or left-sided atrioventricular connection rather than trying to arbitrate between "mitral" and "tricuspid" atresia [8]. This may be appreciably more long-winded, but it is far more explicit in situations where, for example, there is the usual atrial arrangement and absence of the left-sided atrioventricular connection within the right atrium connected to a dominant left ventricle. When the rudimentary right ventricle is directly anterior, or to the right, in this circumstance it may be impossible to adjudicate whether the heart itself was trying to produce "mitral" or "tricuspid" atresia. The descriptive approach is much simpler.
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In the setting described, subaortic obstruction is produced when the aortic valve is supported exclusively by the rudimentary and incomplete right ventricle, and when the ventricular septal defect itself becomes restrictive. Very rarely, this situation can exist when the rudimentary right ventricle supports both arterial trunks (double-outlet right ventricle). Such a situation is unlikely to be compatible with prolonged life. In most instances, the defect becomes restrictive when the ventriculoarterial connections are discordant (see Fig 1). In this circumstance, the major flow of blood from the ventricular mass is through the pulmonary trunk supported by the dominant left ventricle. There then tends to be a discrepancy in size between the arterial trunks, with most blood reaching the descending aorta through the arterial duct in the setting of either isthmal hypoplasia, severe tubular hypoplasia of the aortic arch, or interruption of the arch at the isthmus. At all events, the finding of arch obstruction in the setting of a dominant left ventricle should always focus attention on the ventricular septal defect, whereas the converse is equally true. It is most unlikely for the ventricular septal defect to be restrictive in the absence of obstruction to flow within the aortic arch. This obstruction is then almost always further exacerbated by the fact that the isthmus inserts into the area of the arch lined by the muscular tissues of the arterial duct [9]. A ductal "lasso" will further complicate the surgical arena. It is also likely that additional obstruction will be present at the infundibular level within the rudimentary and incomplete right ventricle [10]. The finding of a restrictive ventricular septal defect, therefore, should be the trigger for searches for additional obstructive lesions at the levels of the subaortic infundibulum and the aortic arch.
The location of the atrioventricular conduction axis
There are several surgical options for bypassing the obstruction produced by a restrictive ventricular septal defect when the atriums connect only to a dominant left ventricle. One of these options is surgical enlargement of the defect itself [10]. It is tempting to attempt such enlargement by resection of the muscular outlet septum. This part of the ventricular septal complex never contains the atrioventricular conduction axis, so resection of this part does not carry the danger of producing iatrogenic heart block. Unfortunately, however, the hearts with a dominant left ventricle and discordant ventriculoarterial connections rarely have an extensive infundibular component in the rudimentary right ventricle [11]. Furthermore, most of the subaortic infundibulum that does exist in the rudimentary right ventricle is a free-standing muscular sleeve rather than representing a muscular septum between the subaortic and subpulmonary outflow tracts. It is difficult, therefore, adequately to resect this area without either damaging the leaflets of the arterial valves or else exiting from the heart. It is far safer, and more efficacious, to attempt to remove part of the apical trabecular septum. This then adds the problem of accomplishing this maneuver safely without damaging the atrioventricular conduction axis. This can usually safely be achieved because, in hearts with the usual atrial arrangement and with the atriums connected exclusively to a dominant left ventricle, the course of the conduction axis is constant irrespective of whether there is a double inlet or absence of one or other atrioventricular connection, and is constant irrespective of the location of the rudimentary and incomplete right ventricle [12]. This is because the apical trabecular septum, which carries the atrioventricular conduction axis, never extends to the crux of the heart. Always, therefore, there will be an anomalous atrioventricular node located anteriorly within the atrioventricular junction. This will be in the anterior quadrant of the right atrioventricular valve [12] or, when the right atrioventricular connection is absent, at the site of the dimple in the muscular floor of the right atrium [13]. The atrioventricular bundle then descends from this right-sided atrioventricular node to reach the crest of the muscular apical trabecular septum. When the rudimentary right ventricle is right-sided, the bundle descends in vertical fashion. When the rudimentary right ventricle is left-sided, the bundle encircles anteriorly the outflow tract from the dominant left ventricle as it courses to reach the apical septum (Fig 5). In either event, it is safe for the surgeon to remove a wedge of apical septum closest to the left-sided delimiting branch of the left coronary artery (Fig 6) without fear of damaging directly the atrioventricular conduction axis [10]. It is possible for hemorrhage and edema to track into the bundle branches should the resection be extensive, but removal of the apical muscular wedge should not, in itself, endanger the conduction axis. The only caveat when performing this procedure is to ensure that the ventricular septal defect is in the usual location. If such a resection is attempted when the defect is in the apical muscular septum (see Fig 3), then there is more danger of damaging the conduction-axis.
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In hearts with univentricular atrioventricular connection, subaortic obstruction is almost always caused by a restrictive ventricular septal defect when the atriums are connected exclusively to a dominant left ventricle and the ventriculoarterial connections are discordant. Then, irrespective of the specific atrioventricular connection and irrespective of the location of the rudimentary and incomplete right ventricle, relief of the obstruction can be achieved by surgical enlargement of the septal defect. The major impediment to success will be the danger of damaging the atrioventricular conduction axis. The position of the axis, nonetheless, is constant in these various lesions. It can be avoided if a wedge of muscular apical septum is removed in the area of the defect closest to the left delimiting coronary artery.
Acknowledgments
This work was supported by the British Heart Foundation.
References
This article has been cited by other articles:
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  A. A. Lotto, R. Hosein, T. J. Jones, D. J. Barron, and W. J. Brawn Outcome of the Norwood procedure in the setting of transposition of the great arteries and functional single left ventricle Eur. J. Cardiothorac. Surg., January 1, 2009; 35(1): 149 - 155. [Abstract] [Full Text] [PDF]
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 Y.-T. Lan, R.-K. Chang, and H. Laks Outcome of patients with double-inlet left ventricle or tricuspid atresia with transposed great arteries J. Am. Coll. Cardiol., January 7, 2004; 43(1): 113 - 119. [Abstract] [Full Text] [PDF]
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