Ann Thorac Surg 1998;65:509-514
© 1998 The Society of Thoracic Surgeons
Original Articles: Cardiovascular
Surgical Anatomy of Aorto–Left Ventricular Tunnel
Siew Yen Ho, PhD,
Michaela Muriago, MD,
Andrew C. Cook, BSc,
Gaetano Thiene, MD,
Robert H. Anderson, MD
Department of Paediatrics, Imperial College School of Medicine at the National Heart & Lung Institute, London, England, United Kingdom
Accepted for publication July 30, 1997.
Dr Ho, Department of Paediatrics, Imperial College School of Medicine at the National Heart & Lung Institute, Dovehouse St, London SW3 6LY, UK (e-mail: yen.ho@ic.ac.uk).
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Abstract
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Background. Owing to the rarity of aorto–left ventricular tunnel, surgical experience with this condition is generally limited. The anatomic configuration remains to be clarified in the light of better understanding of the normal aortic root.
Methods. Two autopsied hearts with aorto–left ventricular tunnel were examined and compared with four normal heart specimens. The normal hearts were sectioned in a variety of planes to display the ventriculoarterial junction.
Results. The leaflets of the pulmonary valve in both normal and abnormal hearts have semilunar attachments to a sleeve of freestanding ventricular musculature, the infundibulum. An extensive fibrofatty tissue plane then interposes between the freestanding infundibulum and the aortic sinuses. The aorto–left ventricular tunnels in the abnormal hearts pass within this tissue plane. The aortic orifice of the tunnel is distal to the level of the sinutubular junction, whereas the ventricular orifice is located within the interleaflet triangle between the right and left aortic sinuses.
Conclusions. Aorto–left ventricular tunnels bypass the normal ventriculoarterial junction but do not penetrate the septal musculature. This has implications for the fine-tuning of surgical repair.
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Introduction
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First described as "aorto–left ventricular tunnel" by Levy and colleagues [1] in 1963, this rare congenital malformation is an abnormal channel that begins in the ascending aorta, bypasses the aortic valve, and leads into the left ventricular cavity. In view of its rarity, surgical series are generally small. However, with prenatal detection and early recognition [2] [3], more cases are likely to be seen for surgical correction. Various pathogenetic mechanisms have been proposed to account for the entity. These include an abnormality of the coronary artery [1] [4], a malformation of the bulbus cordis [5], weakness of the aortic wall [6] [7], and intrauterine rupture of an aneurysm of the sinus of Valsalva [8] [9] [10]. Each of these proposals, however, implies a different structural basis for the tunnel. Although the collective review of Hovaguimian and colleagues [11] elegantly demonstrated the potential variants, the anatomic relationships of this intriguing lesion to the surrounding cardiac structures warrant closer examination, particularly in regard to its relationship to the ventricular septum. With this study, we aim to clarify the surgical anatomy of aorto–left ventricular tunnel.
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Material and Methods
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We studied four normal hearts and two hearts with aorto–left ventricular tunnel. One of the malformed hearts has been discussed in a previous study [2]. Gross examination of the heart specimens was combined with sectioning in a variety of planes to delineate precisely the junction between the arterial valvar leaflets and the ventricles. Serial histologic sections of the heart previously examined [2] were restudied.
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Results
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Normal Hearts
The aorta possesses well-developed sinuses that are distinct from the tubular ascending portion. The junction between sinusal and tubular portions is marked by a slight thickening of the aortic wall at the sinutubular ridge (Fig 1a). The junction of the sinuses with the left ventricle is, in part, to the musculature of the ventricular septum and, in part, to the fibrous tissues made up of the membranous septum and the area of valvar continuity with the aortic leaflet of the mitral valve. When the aorta is opened, this junction is seen as a line—the anatomic ventriculoarterial junction (Fig 1b). The hingeline of the semilunar leaflets crosses this circumferential ventriculoarterial junction, producing three nearly triangular-shaped areas of fibrous tissue, the interleaflet triangles, on the ventricular aspect [12]. These areas are thinner than the adjacent aortic wall. On the epicardial aspect, the triangles relate to different cardiac structures, reflecting their relationships to extensions of the pericardial space. The interleaflet triangle between the left and right coronary sinuses is adjacent to the tissue plane between the freestanding subpulmonary infundibulum and the aorta. Although this triangle points toward the corresponding triangle between the facing sinuses of the pulmonary valve, the two are not directly adjacent because of the raised and oblique plane of the pulmonary valvar orifice relative to the aortic valve.
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The aortic root of displayed by a longitudinal incision through the left coronary aortic sinus. (a) The sinutubular ridge (broken line) marks the junction between the sinusal and tubular portions of the aortic root. (b) Removal of the aortic leaflets along the hingelines reveals three semilunar arcs, which are crossed by the anatomic ventriculoarterial junction (dotted line). The fibrous triangles are delineated by the broken lines. The area of the membranous septum is indicated by cross-hatching and the conduction bundle, by stippling.
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The triangle between the right and noncoronary sinuses abuts the right atrium and the space between the appendage and the infundibulum. This triangle is continuous with the membranous septum, which, in turn, is adjacent to the branching component of the atrioventricular conduction axis (see Fig 1b). The third triangle, between the left and noncoronary sinuses, abuts the left atrium and the transverse pericardial sinus. Inferiorly, it is continuous at its base with the extensive area of fibrous continuity representing the aortic-mitral curtain. The semilunar hingelines of the aortic leaflets also incorporate ventricular musculature in the aortic sinuses at the base of both coronary leaflets (see Fig 1b).
The pulmonary valve, like the aortic valve, has semilunar hingelines of its leaflets that cross the ventriculoarterial junction (Fig 2a). As the pulmonary outflow tract is completely muscular, being formed by the right ventricular infundibulum, the semilunar arrangement encloses three segments of ventricular musculature within the lower points of each hingeline (Fig 2b). The pulmonary sinuses and the sinutubular ridge are less pronounced than those in the aorta. Again, owing to the semilunar insertion of the leaflets, three fibrous interleaflet triangles can be distinguished. As already described, the triangle between the facing leaflets of the pulmonary valve is offset relative to that between the facing leaflets of the aortic valve. The muscular infundibulum lifts the pulmonary valve above the plane of the aortic valve. Infundibulum musculature, continuous with the supraventricular crest of the right ventricle, forms the medial wall of the pulmonary outflow tract (Fig 2b). On the epicardial aspect, the tissue plane between the infundibulum and the aortic sinuses is filled with fibrofatty tissue. A discrete muscular outlet septum, separate from the remainder of the muscular ventricular septum, is not discernible in the normal heart.
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(a) The right ventricular outflow tract is opened to show the complete sleeve of the muscular subpulmonary infundibulum. The broken lines emphasizes the semilunar hingelines of the pulmonary valve. (b) Circumferential arrangement of ventriculoarterial junction (dotted line) and the fibrous interleaflet triangles (broken lines). The supraventricular crest is continuous with the subpulmonary infundibulum on the medial aspect.
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Malformed Hearts
When a malformed heart is examined externally and from the right, a bulge is seen in the aortic wall immediately behind the pulmonary trunk. This extends from an area above the sinusal portion of the aorta and passes into the tissue plane between the aorta and the pulmonary trunk (Fig 3). This bulge is the aortic part of the tunnel, made up of aortic wall, but the part extending into the fibroareolar plane between the arterial trunks is thin and membranous. The oval entrance of the tunnel is situated in the tubular aorta above the sinutubular ridge, the ridge separating it from the right coronary aortic sinus (see Fig 3b). The right coronary artery in one heart is located just below the sinutubular ridge. The ventricular orifice of the tunnel is more triangular. Extending from the sinutubular junction to the level of the ventriculoarterial junction, it is located between the diverging hingelines of the left and right coronary leaflets. In other words, its location is comparable with the area in the normal heart that would remain after removal of the intercoronary interleaflet triangle. The triangular area in the malformed heart, however, is larger than in the normal situation [12]. In the abnormal hearts, the left medial part of the hingeline of the right coronary leaflet is distorted, and the leaflet itself prolapses (see Fig 3b).
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Heart specimen from a 4-day-old-male infant. (a) The aorto–left ventricular tunnel (accentuated by broken line on one side) passes into the fibroareolar tissue plane between the aorta and the pulmonary (Pulm.) trunk. (b) The aortic (closed triangle) and ventricular (open triangle) orifices of the tunnel are shown. The extracardiac wall of the tunnel is marked with a star. The small arrows point to the sinutubular ridge at the peripheral attachments of the valvar leaflets.
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Viewed through its ventricular orifice, the floor of the tunnel is the musculature of the right ventricle, constituting the area between the subpulmonary infundibulum and the ventriculo-infundibular fold. In essence, this is a depression, not an excavation, into the ventricular musculature. It cannot be equated with a tunnel running through the muscular outlet septum [13], such a septum being prominent in hearts with a ventricular septal defect opening to the outlet portion of the right ventricle. Because the tunnel curves from its inferiorly located ventricular orifice to the more superiorly located aortic orifice, the muscular floor sweeps antero-superiorly rightward to become continuous with the extracardiac roof, the latter being the membranous part of the aortic wall, which blends into the regular aortic wall.
The histologic sections confirmed the gross findings regarding the anatomic relationships of the tunnel. The medial musculature in the right ventricular outflow tract, the floor of the tunnel, is thin. The immediate extracardiac portion of the tunnel, the membranous roof, is composed of layers of fibrous tissue separated by small groups of smooth muscle cells (Fig 4). A thin elastic layer lines the luminal aspect. The elastic layer separates into two layers, between which are increasing fragments of elastic tissue, themselves interspersed with small strands of fibrous tissue as the tunnel blends into the aortic wall. The inner curvature of the tunnel is the aortic wall, which is the sinus of the aorta supporting the hinge of the right coronary aortic valvar leaflet (see Fig 4). The right coronary artery in the heart that has been sectioned arises through a stenotic orifice positioned to the right and superior to the aortic orifice of the tunnel. The branching portion of the ventricular conduction system is located postero-inferior to the ventricular orifice of the tunnel and is distant from the potential surgical field (see Fig 3b).
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Heart specimen from a 22-week fetus. (a) Histologic section longitudinally through the heart. (b) Boxed area in 4a enlarged. The course of the tunnel is traced by the broken line. The aortic and ventricular orifices are indicated by closed and open triangles, respectively. The aortic orifice is in the aortic wall, and ventricular orifice is sandwiched by the right coronary leaflet and the ventricular septum. The obliquity of this section through the septum exaggerates its thickness. The star marks the aortic wall at the commissure between the right and left coronary leaflets. (Elastic van Gieson’s stain.) (vent. = ventricle.)
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Comment
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Despite studying extensively the previous descriptions and also examining hearts ourselves [2], we admit that we found it difficult in the past to appreciate the precise surgical anatomy of aorto–left ventricular tunnel. Our own difficulties are reflected by previous descriptions. Thus, several investigators [1] [7] [8] have commented on the problem in distinguishing an aorto–left ventricular tunnel from a ruptured aneurysm of the sinus of Valsalva. Indeed, Cooley and colleagues [8] argued that earlier reports thought to be instances of the tunnel lesion [1] in adults were more likely, according to their clinical histories, to be instances of sinusal rupture. Rupture of an aortic sinus, however, is not entirely confined to older patients. Neonatal cases are not unknown [3].
Levy and colleagues [1] suggested that the location of the tunnel in the aorta relative to the origin of the coronary artery would provide a distinguishing criterion. A true sinusal aneurysm was said to arise from below the coronary arterial orifice, whereas a tunnel would arise above the orifice [1]. This criterion is not distinctive because as noted by others [6] [7], the origin of the right coronary artery varies from heart to heart in the presence of a tunnel. What does distinguish the entities is the presence of the aortic orifice distal to the sinutubular junction, this feature clearly separating a tunnel from a ruptured sinus of Valsalva (Fig 5a, b). The segment of the sinutubular ridge closest to the aortic orifice may be distorted or displaced by the tunnel, but nevertheless, it continues to mark the distal limit of the sinus. In all hearts we examined and in all except one reported in the literature, the aortic orifice of the tunnel is located above the right coronary sinus. Origin in the outstanding case was above the left coronary sinus in a 19-year-old patient [14].
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Normal heart sectioned longitudinally to show the subpulmonary infundibulum (Subpulm. infund.) separated from the aortic root by the fibroareolar tissue plane (arrowhead). Because the pulmonary (Pulm.) valve is at a higher level than the aortic valve, the facing interleaflet triangles (hatching) are also offset. The interleaflet triangle of the aortic valve shown in this section is that between the right and left coronary leaflets. (b) This heart, in a similar orientation, shows the tunnel lesion with the aortic orifice (closed triangle) above the sinutubular ridge (broken line). The ventricular orifice (open triangle) is in the area normally occupied by the interleaflet triangle. The right coronary (cor.) leaflet prolapses into the orifice. (c) Idealized diagram of specimen, in mirror image for surgical orientation, depicting location of tunnel in extracardiac tissue plane between pulmonary trunk and aorta. The fine hatching (indicated by small arrows) represents fibrous areas, and stippled areas are the arterial walls. (ST junction = sinutubular junction.)
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The "ventricular" orifice, when described, has always been located between the hingelines of the right and left coronary aortic leaflets or else has simply been stated to be immediately below the attachment of the aortic valve. Comparison with the normal heart shows that this site is the area occupied by the interleaflet triangle between the left and right coronary aortic valvar leaflets (see Fig 5b). This area, in functional terms, is ventricular, as it lies proximal to the hingelines of the valvar leaflets. Structurally, it is above the anatomic ventriculoarterial junction and is composed of fibrous tissue rather than muscle in the normal heart. In the presence of such a tunnel, the fibrous bar [2] to which the right coronary aortic valvar leaflet is attached is the remnant of the right coronary aortic sinus. In the gross specimen examined, the affected part of the hingeline was irregular compared with the smooth curvature of the remaining sinus. The ventricular orifice, therefore, occupies a larger area than can be replicated simply by removing the fibrous triangle in a normal heart. This is analogous to a ventricular septal defect of perimembranous type, where the deficiency extends into the cardiac septum more extensively than would be replicated simply by removing the membranous septum.
Prenatal echocardiography [2] [3] has established beyond doubt that the tunnel is a congenital malformation. The mechanism of its formation remains unclear. The location and structure of its walls strongly suggest that it is mainly an extracardiac conduit that has been produced by a cleavage in the wall of the affected aortic sinus with detachment of the fibrous interleaflet triangle forming part of its outer wall (Fig 5c). Consequently, part of the right and left coronary sinuses, along with the valvar hinges, are lifted away from their ventricular support. The integrity of the aortic root is maintained by the remaining outer wall of the cloven aortic sinuses. The extracardiac course of the tunnel occupies the space between the subpulmonary infundibulum (in front) and the aortic sinuses (behind) (see Fig 5). Contrary to conventional depictions in textbooks [15] [16], this course does not penetrate the septal musculature. It is in this area that, in some cases, the tunnel bulges into the right ventricular outflow tract, causing the so-called septal aneurysm [11].
Understanding that the tunnel lesion does not represent a defect in the ventricular septum is of particular relevance to surgical repair. The ventricular orifice is not surrounded by firm muscular margins suitable for anchorage of sutures. Closure of the ventricular orifice with a patch rather than by direct suture has been recommended by most surgeons. This avoids distortion of the aortic root and minimizes the development of aortic regurgitation, which is a well-recognized long-term problem [17] [18]. Particular care must be taken when suturing the patch to the irregular hingeline of the right coronary aortic leaflet to prevent damage to the leaflet, which may be dysplastic or prolapsed. Tuna and Edwards [19] drew attention to this area by showing how it predisposes to valvar incompetence and requires further strengthening. Some surgeons [11] [17] have also alluded to "an inherent weakness at the junction between the membranous part of the ventricular septum and the anterior part of the aortic annulus" [11] as a substrate for aortic incompetence. The membranous septum, which is related to the interleaflet triangle between the right and noncoronary aortic sinuses, is, in reality, distant from the margin of the defect. Because the branching bundle of the cardiac conduction system passes between the membranous septum and the muscular ventricular septum [20], it also is a safe distance from the margin of the defect unless the orifice extends well posteriorly. The anterior extensions of the left bundle branch, however, may be at risk [21].
It may prove impossible to close the aortic end of the tunnel, above the sinutubular junction, without relocating the right coronary artery. As we have already discussed, the artery can originate from the wall of the tunnel with some frequency [2] [3] [18]. Closure of the aortic end with a patch has been recommended [17] to avoid placing undue tension on the aortic wall. Hovaguimian and colleagues [11] also recommended closing the body of the tunnel itself, especially in cases with aneurysmal bulging into the right ventricular outflow tract. This should be readily achievable without damage to the surrounding cardiac structures.
Our continuing examination of aorto–left ventricular tunnel shows that its anatomy is more readily understood when one comprehends fully the normal structure of the aortic valve. Although seldom considered in any detail, the interleaflet triangles are part and parcel of the valvar apparatus [12]. Their locations above the ventricular myocardium place them in extracardiac position. Any deficiencies in these locations lead outside the heart. The aorto–left ventricular tunnel can be considered a defect of the aortic root affecting the interleaflet triangle. Surgical repair undoubtedly needs to be tailored for each individual case, but repair that restores the anatomy as near as possible to normality should lessen the risk of progressive aortic regurgitation.
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Acknowledgments
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Doctors Ho and Anderson are supported by the British Heart Foundation together with the Joseph Levy Foundation.
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References
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Abstract
Introduction
