Ann Thorac Surg 2003;75:1502-1505
© 2003 The Society of Thoracic Surgeons
Original article: cardiovascular
The conduction system in hearts with pulmonary atresia and intact ventricular septum
Aneel Ansari, MBBSa,
Diane Goltz, MDa,
Karen Patricia McCarthy, BSa,
Andrew Cook, BSb,
Siew Yen Ho, PhDa*
a Department of Paediatrics, Imperial College, Faculty of Medicine, and Royal Brompton & Harefield NHS Trust, London, United Kingdom
b The Cardiac Unit, Institute of Child Health, London, United Kingdom
Accepted for publication November 20, 2002.
* Address reprint requests to Dr Ho, Department of Paediatrics, National Heart & Lung Institute, Imperial College, Dovehouse St, London SW3 6LY, United Kingdom.
e-mail: yen.ho{at}imperial.ac.uk
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Abstract
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BACKGROUND: Although a regular course of the conduction system is anticipated in hearts with pulmonary atresia and intact ventricular septum (PAIVS), it has never been demonstrated anatomically. In view of one of the surgical options in repair being debulking of the right ventricular wall, it is important to establish the location of the major conduction pathways.
METHODS: Four hearts belonging to fetuses aged 18, 20, 22, and 29 weeks were examined. Entire hearts were serially sectioned and step sections were stained. The sinus node and atrioventricular conduction bundles were identified and their locations described.
RESULTS: The conduction system in hearts with PAIVS is similar to that in normal hearts, although there was a minor variation in one of them. This was a heart affected by Ebstein malformation of the tricuspid valve. In this case, the right bundle branch terminated abruptly soon after its origin.
CONCLUSIONS: Confirmation of the regular course of the cardiac conduction system is important to the development of strategies for surgical repair of PAIVS.
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Introduction
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Pulmonary atresia with an intact ventricular septum (PAIVS) is a rare and complex range of congenital cardiac malformations constituting only 1% to 4% of congenital heart malformations [1], and hence affecting approximately 1 in 70,000 births [2]. In this malformation, the whole of the right ventricle is involved to a greater or lesser extent [3]. Morphologic classification of PAIVS, although initially divided into two groups [4], has been recently revised by Bull and associates [5] to aid surgical management of this condition. Although right ventricular hypoplasia is a common feature of PAIVS, along with right ventricular hypertrophy and a dysplastic tricuspid valve with smaller than normal diameters, the extent by which these variables are affected is very much dependent on the tripartite structure of the right ventricle. The newer classification [5] groups hearts according to the presence of the cavity in three portions of the right ventricle, namely, the inlet, the trabecular, and the infundibular (outflow) parts. Obliteration of the cavity in one or more portions, usually the infundibulum, may preclude biventricular repair but may be suitable for a "one-and-a-half ventricular repair" [6]. Cases with a well-formed infundibulum may be prepared for biventricular repair by establishing antegrade flow through the right ventricle, allowing it to grow [7]. "Right ventricular overhaul" either as an intermediate step [8] or as part of the definitive repair [9] has been shown to be effective in creating an adequate ventricular cavity.
Although numerous studies have been carried out to determine the disposition of the conduction system in hearts affected by various congenital malformations, we are not aware of any formal studies on hearts with PAIVS. Even though Freedom [10] and Davies and colleagues [11] anticipated that the specialized atrioventricular conduction tissue would be normally positioned in this malformation, the present study aimed to either support or refute this hypothesis by examining four heart specimens.
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Material and methods
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Four formalin-fixed hearts from fetuses aged 18, 20, 22, and 29 weeks were retrieved from the specimen archive of the Guy’s Hospital Medical School, London. These were examined grossly before processing for histology. All four hearts were dehydrated in alcohol, embedded in paraffin wax, and sectioned serially at 10-µm thickness transversely, parallel to the cardiac short axis. Every 50th section was stained using elastic van Gieson stain, which marked elastic and fibrous tissues particularly well. The sinus node, identifiable as a group of small myocardial cells set in a fibrous matrix, was located in each heart. The compact atrioventricular node was recognized by its composition of interconnecting fasciculi of small cells sited adjacent to the central fibrous body. By following the sections in sequence, the conduction system was traced from the atrioventricular node to the penetrating bundle of His, the branching bundle, and the proximal portions of the left and right bundle branches. The spatial relationship of the conduction bundles to the other cardiac structures was noted.
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Results
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Morphologic examination of the four hearts not only confirmed the presence of pulmonary atresia with intact ventricular septum but also showed the extent of involvement of the right ventricle and outflow tract. The oldest of the fetal hearts, at 29 weeks gestational age, was found to have a well-developed but hypoplastic right ventricle, with all the three parts of this chamber being present but terminating into an atretic pulmonary outflow tract (Fig 1a).
In contrast, the youngest of the hearts, at 18 weeks gestational age, had a well-developed pulmonary trunk that led to an identifiable but nonpatent pulmonary valve (Fig 1b). The right ventricular outflow was severely stenotic due to muscular hypertrophy. This heart was also found to have a coexistent Ebstein anomaly, with the hinge of the septal leaflet of the tricuspid valve being displaced towards the apex. The heart of 22 weeks gestation had muscular atresia of the right ventricular outlet and the pulmonary trunk was cord-like and atretic (Fig 1c). The fourth heart of 20 weeks gestation also had a narrow right ventricular outflow tract that tapered to a blind-end immediately underneath the imperforate pulmonary valve, and the trabecular as well as the infundibular parts of the right ventricular cavity were obliterated by muscle (Fig 1).
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Fig 1. (a) A 29-week fetal heart with a tripartite hypoplastic right ventricle, comprising inlet (I), apical trabecular (T), and outlet (O) components. The outlet tapered to muscular atresia. (b) The youngest case, at 18 weeks gestation, had a well-developed pulmonary trunk (PT) leading to a nonpatent pulmonary valve. (c) This heart from a fetus of 22 weeks gestation had muscular atresia of the outflow tract and a cord-like atretic pulmonary trunk (PT). (d) The cavity of the right ventricle in this heart of 20 weeks gestation was severely reduced by muscular hypertrophy. A narrow infundibulum led to the imperforate pulmonary valve (arrow).
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Although differences were noted in the appearance of the right ventricle and pulmonary trunk, the position and course of the conduction system were found to be similar. Examination of the serial histologic sections revealed the sinus node at the junction of the superior caval vein and the right atrium extending from the crest of the appendage laterally towards the orifice of the inferior vena cava, in normal subepicardial location.
The atrioventricular node was situated in its regular location toward the apex of the triangle of Koch, as determined by the endocardial landmarks of the right atrium. At the apex of the triangle of Koch marked by the insertion of the tendon of Todaro into the central fibrous body, the atrioventricular node continued into the penetrating bundle of His that passed toward the ventricular septum (Fig 2a).
Surrounded by fibrous tissue, the course of the atrioventricular bundle could be traced as it continued a path between the membranous and muscular ventricular septum, until finally bifurcating into the proximal portions of the left and right bundle branches (Fig 2b,c).
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Fig 2. Photomicrographs showing the atrioventricular conduction system in the heart of 20 weeks gestation. (a) The atrioventricular node (AVN) was sited in its regular location, and continued into the penetrating bundle of His (AVB) passing towards the ventricular septum (Masson’s trichrome stain, x40). (b) The atrioventricular bundle (BB), enclosed by fibrous tissue, was located on the crest of the muscular ventricular septum (Masson’s trichrome stain, x16). (LV = left ventricle; RV = right ventricle.) (c) The branching bundle separates into the proximal portions of the left (LBB) and right bundle branches (RBB) (Masson’s trichrome stain, x16).
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The left bundle branch was found to run from the atrioventricular bundle onto the subendocardium of the left ventricle. It was composed of a fine sheet of cells, overlying the septal musculature. The right bundle branch descended from the branching bundle, as an intramyocardial cord-like structure within the septum, emerging as a subendocardial structure, at the base of the medial papillary muscle in the septomarginal trabeculation.
Only two variations were noted. First, in the heart that was associated with Ebstein anomaly, the right bundle branch did not take an antero-superior route like it did in the other three hearts, but instead descended intramyocardially and terminated within 2 mm of its origin. Second, we noted a prominent left inferior extension from the compact atrioventricular node in the case of the heart of 22 weeks gestation.
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Comment
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Although pulmonary atresia with an intact ventricular septum is a rare congenital malformation, it is a clinically significant condition, with treatment being based mainly on surgical interventions. Our study confirmed the distribution of the conduction system in hearts affected by PAIVS to be similar to that seen in normal hearts. Hearts in which the inlet, trabecular, and infundibular parts are still present, providing the right ventricular size and function are adequate, usually require pulmonary valvotomy with or without infundibular resection to restore normal outflow from the right ventricle. Because the septum is not involved during such procedures, the proximal parts of the conduction system are unlikely to be affected. Similarly, palliative shunts such as the Waterson shunt and modified Blalock-Taussig shunt are remote from the conduction system.
On the other hand, biventricular repair in hearts with muscle-bound right ventricles involving excavation of the musculature [8, 9] to achieve a satisfactory cavity can encroach upon the conduction tissues. If right ventricular overhaul is required, damage to the proximal portions of the atrioventricular conduction system can be avoided by keeping away from the upper part of the ventricular septum and the septomarginal trabeculation. On a similar theme, should there be associated left ventricular outflow obstruction, note should be taken of the subendocardial course of the branching atrioventricular bundle and left bundle branch, the same as in the normally structured heart.
Studying the course of the bundle branches in the heart affected by PAIVS and Ebstein anomaly of the tricuspid valve revealed a short right bundle branch. This is in accordance with our previous study of the atrioventricular junction in Ebstein malformation [12] that described the absence or underdevelopment of the right bundle branch in four out of five hearts, suggesting that this was related to the abnormal development of the septal leaflet and the medial papillary muscle. If Ebstein anomaly is noted in addition to PAIVS during surgical correction, and an obstructive anterior leaflet is present, surgical treatment may necessitate the excision of the entire tricuspid valve apparatus as an extra step of a definitive operation in addition to those discussed above [13].
In conclusion, the exact surgical procedure used is dependent on the extent of right ventricular involvement, status of the pulmonary and tricuspid valves, and coronary arterial abnormalities [14]. Nevertheless, knowledge of the location of the conduction system is a prerequisite to prevent its damage during any operation, which could lead to further compromise of cardiac contractility and function.
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Acknowledgments
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We acknowledge the contribution of research funding by the Royal Brompton & Harefield Hospital Charitable Fund.
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References
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Abstract
Introduction
