Ann Thorac Surg 2004;77:2163-2166
© 2004 The Society of Thoracic Surgeons
Original article: cardiovascular
Congenitally corrected transposition: size of the pulmonary trunk and septal malalignment
Amir-Reza Hosseinpour, MD, FRCS(C-Th)a,
Karen P. McCarthy, BSa,
Massimo Griselli, MD, FRCSa,
Babulal Sethia, FRCSa,
Siew Yen Ho, PhD, FRCPa*
a National Heart and Lung Institute, Imperial College and Royal Brompton Hospital, London, United Kingdom
Accepted for publication November 25, 2003.
* Address reprint requests to Dr Ho, National Heart and Lung Institute, Imperial College, Dovehouse St, London SW3 6LY, UK
e-mail: yen.ho{at}imperial.ac.uk
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Abstract
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BACKGROUND: In hearts with congenitally corrected transposition of the great arteries, the atrial and ventricular septums are usually malaligned. This is associated with an abnormal location of the atrioventricular conduction system. However, well-aligned septums and normally positioned atrioventricular conduction tissues have been found in a few cases, some of which also happened to have a small or atretic pulmonary trunk. Our aim was to determine whether septal alignment could be predicted on the basis of the size of the pulmonary trunk. Potentially this could provide a guide to the disposition of the atrioventricular conduction tissues.
METHODS: We examined 14 hearts. In all cases there was usual atrial arrangement. We carried out histologic examination of the sites of the atrioventricular conduction tissues in 1 heart.
RESULTS: We found a statistically significant correlation between the size of the pulmonary trunk and the degree of septal malalignment. In the presence of a small or atretic pulmonary trunk, the septums were well aligned. Our histologic study of one such specimen revealed dual atrioventricular nodes connecting to a sling of conduction tissue. This suggests that presence of a postero-inferiorly situated atrioventricular conduction bundle in addition to an anteriorly located bundle may be anticipated when the septums are well aligned.
CONCLUSIONS: The presence of a small or atretic pulmonary trunk in congenitally corrected transposition of the great arteries is associated with good septal alignment. This should alert the surgeon to the possibility of a slinglike arrangement of the atrioventricular conduction system.
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Introduction
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Hearts with congenitally corrected transposition of the great arteries (ccTGA), also known as l-TGA, usually have malalignment between the atrial and ventricular septums [1]. This septal malalignment produces a wedge-shaped space between the two septums (Fig 1).
Contiguous with this space is the left ventricular outflow tract (LVOT) leading to the pulmonary trunk.
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Fig 1. Diagrammatic views of the base of the heart illustrating the malalignment of the atrial and the ventricular septums. The wedge-shaped space between the atrial and the ventricular septums lodges the pulmonary outflow tract. A well-developed pulmonary trunk is associated with obvious septal malalignment (left panel). A small or atretic pulmonary trunk is associated with minimal malalignment (right panel). (A = aorta; AS = atrial septum; Inf. = inferior; L = left; M = mitral valve; P = pulmonary trunk; R = right; Sup. = superior; T = tricuspid valve; VS = ventricular septum.)
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We wish to determine whether the size of the pulmonary trunk correlates with the degree of septal malalignment in ccTGA. That is to say, whether a small or atretic pulmonary trunk would be associated with well-aligned septums (Fig 1). This hypothesis aroused our curiosity for three reasons. First, good septal alignment has been seen in some cases of ccTGA [1–4]. But some of those hearts had a small or absent pulmonary outflow tract. Second, good septal alignment is considered normal in ccTGA with mirror-imaged arrangement of the atrial appendages (situs inversus) [5–8]. The majority of cases on which this statement is based also had a stenotic or atretic pulmonary outflow tract. Third, septal malalignment is pertinent because it provides the basis for the abnormal (anterior) position of the conduction axis in ccTGA that have usual arrangement of the atrial appendages (situs solitus) [1]. Indeed, in the few reported cases of ccTGA where there was good septal alignment, including those with mirror-imaged atrial arrangement (situs inversus), the conduction axis was in the regular (posterior) position and formed a conduction sling with the abnormal system [1–8].
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Material and methods
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We examined 14 heart specimens with ccTGA. The arrangements of the atrial appendages were as usual (situs solitus) in all. The external diameter of the pulmonary trunk (PT) was measured at the level of the sinutubular junction in 12 cases with patent pulmonary valves. The arterial diameter was measured at 1 mm above the ventriculo-arterial junction in the 2 cases with pulmonary atresia. In order to standardize the measurements, this was recorded as a proportion of the external diameter of the aorta (Ao), which was also measured at the sinutubular junction.
The angle between the atrial and the ventricular septums was determined by inserting a long needle into each septum parallel to its plane but close to the level of the atrioventricular junction, and measuring the angle between the two using a pair of compasses and a protractor (Fig 1). To avoid interobserver variations, all measurements were made by one investigator who performed each measurement twice, on separate occasions. For quality control, a second observer who was blind to the status of the pulmonary trunk checked the locations of the needles.
The measurements were tabulated and tested statistically to determine if a correlation exists between the size of the pulmonary trunk and the degree of septal malalignment. The statistical test used was the Pearson Product Moment Correlation. To establish whether hearts with a PT/Ao ratio of 1 or more had a larger angle of malalignment than hearts with ratios less than 1, we separated the hearts into two groups and performed an unpaired Student's t test.
Histologic studies were made on one specimen in order to locate the atrioventricular conduction system. The specimen had usual atrial arrangement (situs solitus) and good septal alignment in the setting of pulmonary atresia. There were no clinical details available. We excised that part of the septal tissues estimated to contain the proximal portions of the atrioventricular conduction axis. Serial histologic sections were made parallel to the diaphragmatic surface of the heart. The sections were cut at 10 µm thickness and every twenty-fifth section was mounted and stained with Masson's trichrome. The conduction tissues were identified and "traced" towards the superior and the inferior aspects of the heart to establish their locations.
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Results
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Relation between size of the pulmonary trunk and degree of septal malalignment
There is a statistically significant correlation between the diameter of the PT and the degree of septal malalignment in ccTGA (correlation coefficient = 0.85, p < 0.0001) (Fig 2)
. When the PT was small compared to the aorta (PT/Ao ratio less than 0.6), there was minimal septal malalignment (0 to 5 degrees). When pulmonary trunk was larger, septal malalignment was 26 to 45 degrees.
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Fig 2. A graph showing the relationship between the size of the pulmonary trunk (horizontal axis) and the degree of septal malalignment (vertical axis). The diameter of the pulmonary trunk is measured as a proportion of the aorta with both measurements taken at the sinutubular junction.  = pulmonary atresia; • = cases without pulmonary atresia. (PT/Ao = ratio of diameter of pulmonary trunk to the diameter of the aorta.)
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Comparing hearts with a PT-Ao ratio of one and greater to hearts having a smaller PT-Ao ratio showed a significant difference between the two groups (t = –4.2 [95% confidence interval –1.16 to –0.37] p = 0.001).
Histologic examination
This heart had usual atrial arrangement, with ccTGA, and a perimembranous ventricular septal defect (VSD). The root of the pulmonary trunk was a thin fibrous cord, with valvar atresia. The aortic arch was left-sided, with the ascending aorta sited anterior and to the left of the small pulmonary trunk. The atrial and ventricular septums were well aligned with respect to each other. Histologic examination of this heart revealed a normally positioned posterior atrioventricular node and bundle, which descended along the postero-inferior border of the VSD. This coexisted with a hypoplastic atrioventricular node that was located in the anterior margin of the tricuspid valve (Fig 3).
This anomalously located node connected with a slender atrioventricular bundle that extended from close to the base of the atretic pulmonary trunk to descend onto the anterior crest of the ventricular septum. The two atrioventricular conduction bundles formed a sling of specialized tissue surrounding the VSD.
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Fig 3. Specimen with usual arrangement of the atrial appendages and pulmonary atresia. The diagram shows the levels of histologic sections a, b, c, and d. There is a sling of conduction tissues connecting a large posterior AV node to a small anterior AV node. The cut through the posterior AV node in (d) is an artifact. All histologic sections are shown at the same magnification. (Trichrome stain.) (A = aorta; Ant. = anterior; AV = atrioventricular; LV = left ventricle; Morph. = morphologic; MV = mitral valve; P = pulmonary trunk; Post. = posterior; Pulm. = pulmonary; RA = right atrium; RV = right ventricle; TV = tricuspid valve.)
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Comment
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The size of the pulmonary trunk can be a guide to whether there is important septal malalignment in ccTGA in hearts with usual arrangement of the atrial appendages (situs solitus). A small or absent pulmonary trunk is associated with good septal alignment. A larger pulmonary trunk is more likely to be associated with septal malalignment. In determining whether a larger PT/Ao ratio was associated with greater degree of septal malalignment, we took 1 as the ratio separating the two groups. Arguably, we could have taken a ratio of 0.6 and less since this would be more representative of a small pulmonary trunk and indicative of stenosis or atresia. Nevertheless, the ratio of less than 1 still produced statistical significance in the angles of malalignment in this small series. In the clinical setting, the PT/Ao ratio applied to echocardiographic studies could be a predictor of septal alignment.
Septal alignment or malalignment is thought to affect the position of the conduction system [1–8]. In ccTGA with usual atrial arrangement, the atrioventricular conduction bundle is recognized to be in an abnormal (anterior) position. It is thought that septal malalignment results in a gap that prevents a normally located atrioventricular node (at the apex of the triangle of Koch) from reaching the ventricular conduction bundles. The gap is filled with a large membranous septum in cases without a VSD in this location. The ventricular conduction bundles connect to another atrioventricular node through an anteriorly located atrioventricular bundle. Thus, the functionally connected node is formed in an antero-superior position between the "annulus" of the right-sided mitral valve and the os of the right atrial appendage. The penetrating atrioventricular bundle (of His) penetrates the atrioventricular junction at the lateral area of fibrous continuity that exists between the pulmonary and mitral valves. It continues into an elongated atrioventricular bundle that passes around the anterior quadrants of the subpulmonary outflow tract, in the antero-superior wall of the morphologic left ventricle, and reaches the upper part of the ventricular septum. From there, it descends and branches into the left and right bundle branches. Where there is a perimembranous VSD, a common associated malformation in hearts with ccTGA, the conduction bundle passes in its antero-superior margin.
However, we have identified six cases of ccTGA with usual atrial arrangement where the conduction axis was reported to be postero-inferiorly disposed; a posterior positioned atrioventricular node connected to an atrioventricular bundle, which passed postero-inferior to the membranous septum or VSD [1–4, 9, 10]. In three cases, the septums were well aligned [1, 3, 4], whereas in one case the authors described slight septal malalignment [2]. Septal alignment was offered as an explanation for the posterior position of the conduction axis. We noted that in two of these four cases, there was atresia or stenosis of the pulmonary outlet [1, 3] while in one case the wall of the morphologic left ventricle was described as 9-mm thick without comment on the pulmonary outlet [2]. We regard obstruction to pulmonary outflow as significant because of our own finding of a strong correlation between the size of the pulmonary trunk and septal malalignment. Also, we carried out histologic studies in one heart with pulmonary atresia and, likewise, found posteriorly positioned conduction tissues. In contrast, the fourth case had mild subpulmonary stenosis with a large pulmonary trunk [4]. Its septal alignment may be related to its association with straddling of the mitral valve through a large perimembranous defect with tendency toward formation of a conduction sling [10]. In the remaining two hearts reported as having a posterior conduction system [9, 11], the photographs did not show small pulmonary outlets and there was no reference to septal alignment. The penetrating atrioventricular bundle was interrupted in one case [10].
Thus, an antero-superiorly positioned conduction tissue is found in the presence of septal malalignment at the cardiac crux. Conversely, postero-inferiorly positioned conduction tissues are found when the septums are well aligned. When a postero-inferiorly positioned conduction system is present, it is likely to be in addition to, rather than instead of, the antero-superiorly sited system anticipated in ccTGA with usual atrial arrangement [1, 3]. The two systems (anterior and posterior) would then form a sling of conduction tissues described as a "Mo
ckeberg sling" [3, 12]. In the index case of Mockeberg, cited by Bharati and Lev [3], ccTGA was associated with double outlet right ventricle. In this setting it is reasonable to speculate that there is no requirement for a malalignment gap to accommodate the pulmonary trunk in the morphologic left ventricle. A solitary posterior system, however, cannot be ruled out [4].
The limitations of this study must be recognized. First, the specimens had been fixed in formalin solution and extensively manipulated over the years for teaching purposes. Consequently, the degree of septal malalignment observed in these isolated hearts may not be an exact representation of that in vivo. Second, histologic studies were made on only one specimen. This is because previous investigators had already described the position of the conduction axis in such hearts. We, therefore, could not justify the destruction of such few and rare specimens, which are precious teaching material. We sacrificed a heart, which contradicted general concepts in that it had usual atrial arrangement but well-aligned septums. We can now explain that on the basis of pulmonary atresia in that heart, but we needed to know if this was associated with a postero-inferior conduction system, which it was albeit in a sling arrangement with the anteriorly located system already anticipated in hearts with usual atrial arrangement. Third, we had no clinical details of this case to allow for correlations.
In conclusion, we would like to postulate that the presence of a stenotic or atretic pulmonary trunk or outflow should alert the surgeon to the possibility of good septal alignment and a postero-inferiorly (regularly) positioned conduction system in ccTGA with usual atrial arrangement. This "normal" conduction system would be in addition to, rather than instead of, an anterior conduction axis, thus forming a sling of conduction tissues.
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Acknowledgments
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SYH and KPM acknowledge the Royal Brompton and Harefield Hospital Charitable Fund as the source of funding for this work.
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References
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Abstract
Introduction
