Ann Thorac Surg 1998;65:1381-1387
© 1998 The Society of Thoracic Surgeons
The Surgical Anatomy of the Left Ventricular Outflow Tract in Hearts With Ventricular Septal Defect and Aortic Arch Obstruction
Yuichi Shiokawa, MDa,
Anton E. Becker, MDa
a Department of Cardiovascular Pathology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands
Accepted for publication November 22, 1997.
Address reprint requests to Dr Shiokawa, Department of Cardiovascular Surgery, Fukuoka Children’s Hospital, 2-5-1 Tojin-machi, Chuo-ku, Fukuoka 810, Japan
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Abstract
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Background. Profound understanding of the left ventricular outflow tract (LVOT) anatomy is crucial to improve surgical results in patients with aortic arch obstruction, ventricular septal defect, and subaortic stenosis.
Methods. We studied the morphology of the LVOT in 32 postmortem hearts with aortic arch obstruction and a ventricular septal defect. In case of subaortic obstruction, the length of the subaortic muscular component was measured anteriorly and posteriorly within the left ventricle.
Results. Seven of the 32 hearts had no subaortic stenosis. Nine had aortic override, which caused LVOT narrowing. Sixteen hearts contained a subaortic shelf, downstream to the ventricular septal defect, which deviated into the left ventricle in 15. In 10 of these the shelf was muscular; in 6 it was a fibrous ridge. In cases with a muscular shelf, the posterior part was significantly shorter than the anterior part (p < 0.004). In 9 hearts the LVOT was further narrowed because of the abnormal relationship between the mitral valve and the subaortic shelf.
Conclusions. The present study confirms the complexity of LVOT stenosis in aortic arch obstruction and ventricular septal defect and provides a better understanding of the options to achieve surgical relief.
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Introduction
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The management of patients with a ventricular septal defect (VSD) associated with aortic arch obstruction remains a major surgical challenge [1, 2]. The results are particularly poor in patients in whom critical subaortic stenosis is associated with interrupted aortic arch (IAA) [1–4]. Most patients with aortic arch obstruction and a VSD are critically ill immediately after birth and, consequently, they need surgical treatment during the neonatal period. Furthermore, in these very sick neonates one would like to go for primary repair of aortic coarctation, and even of IAA, together with closure of the VSD in the same session [3–6]. The inevitable result of this approach is that at the time of initial repair the surgeon has to cope with the problem of subaortic stenosis when present [4–6]. The initial enthusiasm for this one-staged procedure, which included resection of the subaortic muscular component, has been tempered somewhat because of high early mortality [6] or some degree of aortic insufficiency [4]. In fact, it appears that the aortic valve is much more easily jeopardized than anticipated and that "safe" resection of the subaortic obstruction depends on the length of the outlet septum [7].
Obviously, therefore, there is a need for a further detailed analysis of the left ventricular outflow tract (LVOT) in hearts with VSD, subaortic narrowing, and aortic arch obstruction.
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Material and methods
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The study is based on 32 heart specimens with VSD and aortic arch obstruction. All had the usual atrial arrangement, concordant atrioventricular connections, concordant ventriculoarterial connections, and balanced ventricles. Seven hearts had a VSD associated with IAA; 5 had the interruption between the left subclavian artery and the left common carotid artery (so-called type B) and 2 had the interruption distal to the left subclavian artery (so-called type A). The remaining 25 hearts had a VSD with coarctation of the aorta.
In hearts with subaortic obstruction the components contributing to the narrowing downstream to the VSD were analyzed (see the next section on definition of terms). In case of a subaortic muscular shelf separating both left and right ventricular outlets, the superoinferior length of the muscle was measured between the lowest attachment of the aortic valve and the upper margin of the VSD, as seen from the left ventricular side. These measurements were taken anteriorly, at the level of the anterior margin of the VSD, and posteriorly (Fig 1). In the same hearts we measured the width of fibrous continuity between the aortic leaflet of the mitral valve and the aortic valve, which was compared with the measurements of the maximum width (see Fig 1).
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Fig 1. Schematic display of the left ventricular outflow tract showing the measurements taken. In case of a subaortic shelf the superoinferior length was measured between the lowest attachment of the aortic valve and the upper margin of the ventricular septal defect (VSD) (white arrows). The width of fibrous continuity between the mitral valve (MV) was measured at the site of aortic (Ao) valve continuity (a) and compared with the maximum width (b).
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Definition of terms
In hearts with a subaortic VSD the muscular components that normally form the right ventricular infundibulum have sprung apart. As in tetralogy of Fallot it is apparent that the term "crista" or supraventricular crest can no longer be used, and hence the naming of the various muscle bundles becomes vital [7].
We will use the following definitions:
- Outlet septum. This is the muscular shelf that separates the subaortic outflow tract from the right ventricle. Basically, this is a small, muscular structure, which in the normal heart is almost inconspicuous and located in the most medial aspect of the ventriculoinfundibular fold (VIF) (see below). In hearts with a VSD in the subaortic position, however, the outlet septum may be recognized as a separate structure, confluent with the right ventricular infundibulum. The latter structure is not septal, but represents a sleeve of free-standing right ventricular myocardium, separated from the aortic root by fibrofatty tissue.
- Ventriculoinfundibular fold. This is defined as any muscle separating the atrioventricular valves from the arterial valves and, as such, represents the inner curvature of the heart. The very beginning of the VIF is often ill-defined, but as it gets closer to the arterial outlets the fold becomes more distinct. In hearts with a VSD and tricuspid-mitral-aortic valvar continuity, the VIF has tapered out at the level of the membranous septum. In other hearts with a VSD, however, the VIF may extend onto the left ventricular side, thus separating the tricuspid from the aortic valve, and occasionally may extend so far as to produce muscular separation between the mitral and aortic valves. Although variable in extent it always fits the definition alluded to above.
- The anterolateral muscle bundle. The anterolateral muscle bundle consists of muscular trabeculations that extend along the anterolateral wall of the LVOT, which may run up to the level of the aortic valve. In the initial description the muscle bundle was described as partially intervening between the aortic valve and the mitral valve [8]. In this study, therefore, the latter characteristic was not considered a prerequisite for the naming of a left ventricular anterolateral trabeculation.
Statistical analysis
All data were expressed as mean ± standard deviation and statistically analyzed using the Wilcoxon single-ranks test. Results were considered statistically significant if the p value was less than 0.05.
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Results
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Types of ventricular septal defect and associated anomalies
Of the 32 heart specimens, 20 had a perimembranous VSD, 11 had a muscular VSD, and 1 had a doubly committed juxtaarterial VSD (further categorization of these VSDs, according to the parts of the ventricular septum involved, is shown in Table 1).
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Table 1. Categorization of Ventricular Septal Defects in 32 Heart Specimens With Aortic Arch Coarctation and Interrupted Aortic Arch
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Mitral stenosis due to a miniaturized valve apparatus was found in 6 hearts, 3 of which had a muscular trabecular VSD without apparent LVOT obstruction. Eight of the 32 cases had a bicuspid aortic valve, but none showed signs of aortic stenosis.
Types of subaortic stenosis
The 32 hearts were classified initially into those with and those without subaortic stenosis. There were 7 hearts without apparent subaortic stenosis, all of which had muscular VSDs extending into the trabecular part of the ventricular septum. Three of these had mitral stenosis due to a miniaturized mitral valve.
The remaining 25 hearts showed some form of LVOT obstruction. In 9 hearts the narrowing was caused by overriding of the aortic valve, which had caused narrowing of the subaortic outflow tract proximal to the VSD (Fig 2). The degree of aortic override ranged from 10% to 45% (20% on average) of the aortic circumference. Nine of these hearts had associated aortic coarctation; none showed IAA.
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Fig 2. (A) Right ventricular aspect of a heart with ventricular septal defect, coarctation of the aorta, and aortic override. Note prominent ventriculoinfundibular fold (VIF) fusing with the rightward deviated outlet septum (star), creating override of the aortic orifice (arrow). (B) Left ventricular aspect of same heart showing marked override of the aortic valve. Because of this rightward position, the left ventricular outflow tract is narrowed proximal to the ventricular septal defect (Ao = aorta.)
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The remaining 16 hearts showed narrowing of the LVOT downstream to a VSD; 9 of these had associated aortic coarctation, and all 7 cases with IAA belonged to this category. These 16 hearts were further categorized on the basis of the architectural arrangements of the subaortic components causing obstruction.
In 8 of the 16 hearts, subaortic narrowing was due to the leftward displacement of a subaortic shelf of muscle, considered to represent the outlet septum, which had fused along its anterosuperior margin with an anterolateral muscle bundle (Fig 3). The abnormal position of the outlet septum, together with the anterolateral muscle bundle, produce a "bulge" in the subaortic outflow. Hence, the bulge represents predominantly left ventricular anterior free wall, rather than the interventricular septum. In 2 other hearts there was distinct leftward displacement of the outlet septum, but without a bulge. In another 2 hearts, a distinct bulge was present, which had fused with a leftward deviated shelf of tissue in subaortic position, separating the outlets and thus representing the outlet septum, but with a length (measured from the aortic valve cusp to the free edge) of less than 2 mm and, moreover, in part fibrous rather than muscular (Fig 4). A similar condition of the outlet septum was found in 3 other hearts, but here the leftward deviated shelf was not associated with a bulge in the LVOT (Fig 5). Finally, 1 heart showed a distinct bulge but without leftward deviation of the outlet septum. The latter, in this particular heart, was again less than 2 mm in length and for its greater part fibrous in nature.
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Fig 3. (A) Right ventricular aspect of a heart with ventricular septal defect and coarctation of the aorta. The ventricular septal defect is perimembranous and in the outlet septum of the right ventricle (black star indicates right ventricular infundibulum). The ventriculoinfundibular fold (VIF) extends slightly through the ventricular septal defect into the left ventricular outflow (see also B) and fuses with the infundibulum (black star). (B) Left ventricular aspect of the same heart shown in part A. The subaortic muscular component (white star) shows leftward deviation and fuses with an anterolateral muscle bundle (ALM) in the anterolateral wall of the left ventricular outflow tract; this configuration produces a "bulge" in the subaortic outflow tract. The width of the subaortic shelf is large anteriorly, but it attenuates posteriorly. The arrow indicates the site of fusion between right VIF and the subaortic shelf. (Ao = aorta; LV = left ventricle; MV = mitral valve; PA = pulmonary artery; RV = right ventricle.)
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Fig 4. (A) Right ventricular aspect of a heart with ventricular septal defect and interruption of the aortic arch. The ventricular septal defect is perimembranous and confluent (extends into all parts of the muscular ventricular septum). The ventriculoinfundibular fold (VIF) enters the left ventricle (LV) through the ventricular septal defect and gradually attenuates. A bulge in the anterolateral wall of the left ventricular outflow tract can be seen through the ventricular septal defect (arrow). The tear in the anterior leaflet of the tricuspid valve is an artifact. (B) Left ventricular aspect of the same heart shown in part A. The anterior bulge (black star) is better seen from this view. The subaortic component deviates leftward and fuses with the "bulge." It meets the aortic leaflet of the mitral valve (MV) in its middle part. The attenuated VIF fuses with the posterior insertion of the subaortic shelf (white arrow). (Ao = aorta; PA = pulmonary artery; RV = right ventricle.)
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Fig 5. (A) Right ventricular aspect of a heart with ventricular septal defect and interruption of the aortic arch. The ventricular septal defect is perimembranous and extends into the inlet septum. (B) Left ventricular aspect of the same heart shown in part A. No septal bulge is found. A leftward deviated fibrous ridge is present immediately underneath the aortic valve and meets the aortic leaflet of the mitral valve (MV) almost halfway. The arrow points to the site of infusion with the ventriculoinfundibular fold (VIF). (Ao = aorta; LV = left ventricle; PA = pulmonary artery; RV = right ventricle.)
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Measurements of the superoinferior length of the outlet septum in these 16 hearts revealed that the length of the anterior part was longer than that of the posterior part in most cases (Fig 6). The difference between the mean values at both sides was statistically significant (p < 0.004).
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Fig 6. Graph showing the superoinferior length of the muscular component underneath the aortic valve, downstream to the ventricular septal defect. The anterior and posterior measurements are shown for each heart. The mean value of the posterior length is significantly shorter than that of the anterior length (p < 0.004). (IAA = interrupted aortic valve; CoA = aortic coarctation.)
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An abnormal relationship between the mitral valve and the subaortic outlet septum was found in 9 of the 16 hearts. The aortic leaflet of the mitral valve, instead of being in fibrous continuity with the aortic valve over its total length as in normal hearts, met with the outlet septum at some sites (see Fig 4). This caused considerable reduction in width of the fibrous attachment between the two valves and, hence, caused further narrowing of the LVOT. Once expressed as a percentage of the total width of the anterior leaflet of the mitral valve it appeared that in 9 hearts the aortic leaflet of the mitral valve was reduced considerably (Fig 7). Of the 7 hearts with IAA, 5 had such an arrangement, and of the 9 hearts with associated aortic coarctation, 4 had this particular anatomy. There was no significant correlation between this architectural arrangement and the location of the VSD.
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Fig 7. Graph showing the percent width of the aortic–mitral fibrous continuity in hearts with the mitral valve leaflet inserting into the outlet septum. The normal maximal width has been taken as 100%, and the abnormal width has been expressed as a percentage of normal.
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Comment
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Surgical management of subaortic stenosis associated with aortic arch obstruction and a VSD remains a major challenge. This relates to the fact that the anatomy of the LVOT narrowing is complex, whereas the surgeon’s view of the LVOT through the VSD is limited in the small neonatal heart. An aortotomy under these circumstances is not recommended simply because of the critically small diameter of the ascending aorta. Any attempt, therefore, to manage subaortic obstruction in these conditions requires profound knowledge of the anatomy of the LVOT.
The present study, based on postmortem heart specimens, confirms that a variety of cardiac conditions can be associated with aortic arch obstruction, although occasionally (in 7 of 32 hearts in the present series) no intracardiac anatomic cause of the arch anomaly can be found. Of those with an intracardiac abnormality, 9 presented with overriding of the aorta; all of these had associated aortic coarctation and none had interrupted aortic arch. The latter condition, in association with overriding of the aortic valve, is rare [9]. Of all hearts with coarctation of the aorta and VSD in our series, 36% had overriding of the aorta as the main intracardiac feature. This incidence is lower than that reported from other anatomic studies by Anderson and associates [10] and Kitchiner and associates [11], who documented an incidence of 80% and 54%, respectively. A clinical study by Smallhorn and coworkers [12] using cross-sectional echocardiography reported an incidence of 56%. The discrepancies noted almost certainly relate to the bias introduced by referral patterns and the specifics of cardiovascular registries. In our small series, moreover, we did not encounter tissue tags derived from the tricuspid valve as an obstructive mechanism in hearts with aortic arch obstruction, as suggested by Smallhorn and associates [12]. In case such tissue tags do occur, however, one may anticipate proper identification at operation, and it seems rather unlikely that they will cause a problem after closure of the VSD. In fact, reoperation because of residual LVOT obstruction after closure of the VSD is exceedingly rare [11].
The most common intracardiac anomaly associated with aortic arch obstruction appeared to be LVOT narrowing downstream to the VSD. This situation occurred in 16 of the 25 hearts with LVOT obstruction. In 10 of these a leftward deviated outlet septum, which presented as a subaortic muscular shelf, obstructed the LVOT downstream to the VSD. Proper identification of this shelf is crucial in the management of LVOT obstruction. From the right-sided approach this may not always be that readily recognizable. It is important to realize that the VIF, which corresponds to the inner curvature of the heart and certainly is not a septal structure, may taper out at the site of the membranous septum in cases with a perimembranous VSD, or may extend into the left ventricle for some distance. In the latter situation the VIF will separate the tricuspid valve from the aortic valve. At the same time, again viewed from the right side, the right ventricular infundibulum may be mistaken for the outlet septum. As outlined before this is not the case. The major part of the infundibulum is not intervening between the right ventricle and the LVOT, but represents a sleeve of free-standing right ventricular outflow tract musculature. The "true" outlet septum, alluded to above, is positioned behind the infundibulum, inside the left ventricle, and usually with its posterior insertion deeper than its anterior part. Because of the presence of a free-standing subpulmonary infundibulum, there is no overriding of the pulmonary valve despite leftward deviation of the outlet septum, as pointed out by Al Marsafawy and colleagues [7]. Moreover, our measurements have shown that the distance between the aortic valve and the free edge of this subaortic muscular shelf differs markedly from posterior to anterior. In fact, posteriorly it is often extremely small, whereas anteriorly it widens up. This is important if the surgeon considers resection of part of the outlet septum in an attempt to relieve LVOT obstruction. It is of equal importance to realize that the deviated outlet septum is not always fully muscularized. Not infrequently one will see that the posterior part is fibrous whereas the more anterior part is muscular. In some instances (6 of the 16 hearts in the present series) the outlet septum is almost completely fibrous in nature and less than 2 mm in length. Proper identification of this particular configuration is crucial, because resection of this shelf under these circumstances is practically impossible without causing damage to the aortic valve. However, even in cases where there is a distinct muscularized outlet septum, one has to be well aware of the fact that the more posterior one resects the closer one gets to the aortic valve, which in that particular position is readily damaged.
Apart from the deviated outlet septum as a major cause of LVOT narrowing in hearts with aortic arch obstruction and VSD, this study revealed two additional important conditions. Both occur in association with the former.
The first additional obstructive anomaly encountered in this series was LVOT narrowing caused by a distinct muscular bulge in the anterior wall of the LVOT. This bulge, which basically represents left ventricular anterior free wall, appears as a continuation of an anterolateral muscle bundle within the left ventricle. Anderson and co-workers [13] documented a variety of muscle bundles obstructing the LVOT, including a hypertrophic anterolateral muscle bundle, aberrant subaortic bundle, and a septal bulge. From our observations we are left with the impression that much of these are closely related and represent different degrees of extension of an anterolateral muscle bundle rather than separate entities. The surgical relevance of this observation is that attempts to relieve outflow tract obstruction may include partial resection of this bulge. The surgeon, however, has to be aware of the fact that only the part of fusion between the ridge and the interventricular septum allows vigorous resection, whereas ongoing resection into the left ventricle could cause laceration of the free wall.
The second important observation with respect to LVOT obstruction in these hearts related to the anatomy of the mitral valve. In the normal heart there is fibrous continuity between the mitral valve and the aortic root over a relatively wide area. However, in 9 of our 16 hearts with LVOT obstruction downstream to a VSD we encountered an abnormal configuration. In these hearts the aortic leaflet of the mitral valve inserted in part into the leftward deviated outlet septum. Indeed, measurements of the width of the aortic leaflet of the mitral valve, which contributed to the LVOT, showed a marked reduction (about 40% on the average) in participating in the total circumference of the LVOT. It is of considerable interest, in this respect, that of the 7 hearts with interrupted aortic arch 5 showed such an arrangement and that of the 9 hearts with aortic coarctation 4 presented this particular anatomy. It is obvious, therefore, that the abnormal insertion of part of the aortic leaflet of the mitral valve into the outlet septum contributes to further LVOT narrowing. In our opinion, this particular configuration is of major significance for surgical management of LVOT narrowing in this category of patients.
In conclusion, the anatomic variations causing LVOT obstruction in hearts with aortic arch obstruction and VSD are quite variable. Occasionally there is no obvious anatomic obstruction, and occasionally overriding of the aortic valve should be considered the major intracardiac abnormality. The vast majority of cases, however, present with a complex anatomy of the LVOT. The concept that the obstruction is "simply" caused by leftward deviation of the outlet septum is certainly an oversimplification. Leftward deviation of the outlet septum is an important feature, in itself quite variable, but the condition is often further complicated by muscular narrowing in the anterior wall of the LVOT and an abnormal relationship between the aortic mitral valve leaflet and the abnormally positioned outlet septum itself. Understanding these features is important in obtaining optimal results in managing LVOT narrowing and reemphasizes that in these patients proper identification of the anatomic features is mandatory and subsequently will dictate an individual approach.
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Acknowledgments
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During this study Dr Shiokawa was a Research Fellow from the Fukuoka Children’s Hospital and Medical Center for Infectious Diseases, Fukuoka, Japan.
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References
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Abstract
Introduction
