Malalignment of the interventricular septum with atrioventricular septal defect: its implications concerning conduction tissue disposition

Reassessment of the Location of the Conduction System in Atrioventricular Septal Defect Using Phase-Contrast Computed Tomography

The location of the atrioventricular conduction axis in the setting of atrioventricular septal defect has previously been shown by histology and intraoperative recordings. We have now reassessed the arrangement using phase-contrast computed tomography, aiming to provide precise measurements so as to optimize future surgical repairs. We used the system based on an X-ray Talbot grating interferometer using the beamline BL20B2 in a SPring-8 synchrotron radiation facility available in Japan. We analyzed 18 specimens. The atrioventricular node was found within a nodal triangle 1.7 mm from the coronary sinus, with 95% confidence intervals from 1.45 to 2.0 millimeters. The depth of the node from the right atrial endocardium was 1.0 mm, with 95% confidence intervals from 0.73 to 1.34 mm. The overall length of the scooped-out ventricular septum was 30.8 mm, with 95% confidence intervals from 27.5 to 34.1 millimeters. The length from the inferior atrioventricular junction to the take-off of the right bundle branch was 12.8 mm, with 95% confidence intervals from 11.12 to 14.38 mm, giving a ratio of 0.43 for the extent of the axis along the inferior septum, with 95% confidence intervals of 0.38-0.48. The length of the non-branching bundle was 6.6 mm, with 95% confidence intervals from 5.57 to 7.7 mm. The proportion of septum occupied by the non-branching bundle was 0.22, with 95% confidence intervals from 0.18 to 0.26. Our findings confirm previous histological studies, extending them by providing precise measurements to guide placement of sutures during surgical repair.

Anatomy of common atrioventricular junction with complex associated lesions

The essence of the lesion increasingly described as atrioventricular septal defect is the presence of a common atrioventricular junction. In most instances, the common junction is itself shared in more or less equal fashion between the cardiac chambers, producing the so-called balanced arrangement, which can be considered the default option. Complexity can be produced at various levels within this standard lesion. The most complex malformations are seen in the setting of visceral heterotaxy. Greatest complexity is seen with right isomerism. This always includes totally anomalous pulmonary venous connection, even when the pulmonary veins return to the heart. Still further complexity is often added by the presence of pulmonary stenosis or atresia. Imbalance can involve either the atrial or ventricular chambers. Imbalance at atrial level produces one form of double outlet atrium, but typically with balanced ventricles. Ventricular imbalance represents spectrums extending either to double inlet left or right ventricle through a common atrioventricular valve. Complexity at the level of the ventriculoarterial junctions is seen in the form of abnormal ventriculoarterial connections, notably tetralogy of Fallot or double outlet right ventricle. In these settings, the superior bridging leaflet is free-floating. Hypoplasia of the left atrioventricular valve is part of right ventricular dominance and is often associated with the so-called parachute malformation. Dual orifice is also a problem. In both these lesions, the zone of apposition between the bridging leaflets is the effective inlet to the left ventricle.

Morphology and Morphogenesis of Atrioventricular Septal Defect With Common Atrioventricular Junction

For many years, the lesions now often described as atrioventricular septal defects were considered to represent atrioventricular canal malformations or endocardial cushion defects. It was also long recognized that patients with the so-called ostium primum defect should be included in this category. The phenotypic feature of these hearts is the presence of a common atrioventricular junction, as opposed to separate right and left atrioventricular junctions. The presence of the common atrioventricular junction underscores the associated phenotypic features, such as the presence of a trifoliate left atrioventricular valve, which has no resemblance to a cleft mitral valve; unwedging of the subaortic outflow tract; and disproportion between the inlet and outlet dimensions of the left ventricle. These features are comparable in patients having the so-called partial, intermediate, and complete variants of the malformation. Anatomical differentiation depends on the morphology of the leaflets of the common atrioventricular valve that bridge the ventricular septum. If these bridging leaflets are fused one to the other, then there are dual orifices, rather than a common orifice, within the common atrioventricular junction. The relationships of the bridging leaflets to the septal structures determine the potential for shunting across the atrioventricular septal defect, which can occur at atrial and ventricular levels or exclusively at either atrial or ventricular level. Rarely, the atrioventricular septal defect may close spontaneously. Recent evidence from studies of cardiac development shows that rather than being an endocardial cushion defect, the malformation results from failure of ingrowth into the developing heart from the dorsal mesenchymal tissues.

Which hearts are unsuitable for biventricular correction?

BACKGROUND

The surgical option of biventricular repair requires two ventricles, each fully capable of supporting the systemic or pulmonary circulation. The morphologic substrates that may preclude some hearts from biventricular repair need to be assessed.

METHODS

Heart specimens were reviewed to assess the morphologic mechanisms that produce an unbalanced ventricular mass and to identify features that would, potentially, be a contraindication for biventricular repair.

RESULTS

Hearts with solitary and indeterminate ventricles, and hearts with essentially solitary ventricles, often have associated abnormalities of venoatrial connections and arrangement of the atrioventricular valves. In the majority of hearts with univentricular atrioventricular connections, the rudimentary and incomplete ventricle of either right or left morphology may be too small to support either the systemic or the pulmonary circulation. Straddling with overriding of the atrioventricular valve, unbalanced atrioventricular septal defect, and gross hypoplasia of one of the ventricles in hearts with biventricular connections are other mechanisms producing ventricular imbalance, which could preclude biventricular repair.

CONCLUSIONS

The morphologic mechanisms that result in ventricular imbalance are mainly related to the sizes and morphology of the ventricles, septal malalignment, valvar morphology, and component make-up of the ventricles. These features will influence decision-making in considering the option of biventricular repair.

The surgical anatomy of ventricular septal defects associated with overriding valvar orifices

This is the second review in a three-part series concerned with the description and categorization of ventricular septal defects. By viewing the defects from the right ventricular aspect, they can be placed into one of three classes: perimembranous, muscular, or doubly committed and juxta-arterial. According to the posteroinferior margin of the third group, these could extend to become perimembranous or muscular. In this review, the complications produced by malalignment of the septal structures associated with overriding of an arterial or atrioventricular valve are described in detail. It shows that although there are problems in defining the extent of any interventricular communication, these ventricular septal defects can be classified with the same categorization as developed for those not associated with overriding. The nosology developed is able to serve as a guide to the surgeon to the site of the specialized axis for atrioventricular conduction.

Surgical significance of morphological variations in the atrial septum in atrioventricular septal defect for determination of the site of penetration of the atrioventricular conduction axis

The morphological variation in the recognized landmarks for the atrioventricular conduction system was studied grossly in 94 hearts with atrioventricular septal defect, assessing 20 hearts with normal atrioventricular septation as a control. In all the hearts with intact atrioventricular septal structures, the tendon of Todaro demarcated the superior boundary of the triangle of Koch. In hearts with atrioventricular septal defect, however, the landmarks for the conduction axis made up a separate nodal triangle. The tendon of Todaro, along with a bridging tendon not found in the normal heart, were variably developed in hearts with atrioventricular septal defect and formed a further triangle unrelated to the axis for atrioventricular conduction. The opening of the coronary sinus was also variable in its location and size. It was the location of the inferior bridging leaflet as it crossed the ventricular septum that was the best surgical landmark to the site of penetration of the atrioventricular conduction axis.

Morphology of the posterior junctional area in atrioventricular septal defects

The location and size of the coronary sinus in hearts with atrioventricular septal defect were investigated in relation to the known disposition of the atrioventricular conduction axis. We examined the morphology in 40 hearts and supplemented this series with two other hearts that had been serially sectioned previously. The coronary sinus received drainage from a persistent left superior caval vein in 5 hearts. Six cases of 40 had malalignment of the septal structures relative to the crux of the heart. In these, the conduction axis was anticipated to course in the position where the inlet ventricular septum met the atrioventricular junction. The coronary sinus terminated in the left atrium in 4 hearts: 2 in the morphological series and 2 that were sectioned for histological studies. The sectioned hearts showed the atrioventricular conduction axis in the usual position for the defect, unrelated to the coronary sinus. The principle that the node and penetrating bundle are located at the intersection of the ventricular septum with the atrioventricular junction holds good despite the variability of the coronary sinus.

Unusual opening of coronary sinus in atrioventricular septal defects

The coronary sinus is an important landmark for the position of the atrial component of the atrioventricular conduction axis and, thus, assumes special relevance to the surgeon in the operating room. We describe here 2 patients with atrioventricular septal defect characterized by an unusual termination of the coronary sinus within the left atrium. We discuss the potential importance of this finding to the disposition and surgical avoidance of the conduction tissues.