Atrioventricular system univentricular type rudimentary

Thorax, 1980, 35, 123-127

Atrioventricular conduction system in univentricularheart of right ventricular type with right-sidedrudimentary chamberCATHARINA E ESSED, SIEW YEN HO, STEWART HUNTER, ANDROBERT H ANDERSON

From the Departments of Paediatrics and Surgery, Cardiothoracic Institute, Brompton Hospital,London, and Freeman Road Hospital, Newcastle upon Tyne

ABSTRACT The conduction tissue in a univentricular heart of the right ventricular type witha right-sided rudimentary chamber was studied. Both an anterior and conventional node werefound, the anterior node being positioned in the atrial septum very close to the conventionalnode. Between the two nodes, a sling of conduction tissue passed through the annulus fibrosusbut was not related to the trabecular septum. A non-branching bundle descended on toa free-running trabecula in the main ventricular chamber, the trabecular septum itself beingdevoid of conduction tissue. We believe it is likely that this trabecula represents the trabeculaseptomarginalis of the normal right ventricle. It has recently been suggested that during de-velopment the primordium of the trabecula septomarginalis is the structure which carries theconduction tissue from the atrioventricular node (whatever its position) to the trabecularseptum. The present findings seem to support this.

It is now well established that the atrioventricularconduction tissue in univentricular hearts of leftventricular type penetrates from an anterior node,while the course of the non-branching bundle isdependent on the position of the rudimentarychamber.13 It is also known that in univentricularhearts of right ventricular type the penetratingbundle takes origin from the conventional(posterior) node, and the non-branching bundledescends on the trabecular septum, which in thesehearts extends to the crux cordis.4 However, thusfar univentricular hearts of right ventricular typehave been studied only when the rudimentarychamber of left ventricular type was left-sided.The question remains concerning the conductiontissue disposition in these hearts when the rudi-mentary chamber is right-sided. This is becausesuch hearts are closely related to cases of con-genitally corrected transposition, and in the latteranomaly an anterior node and conduction systemare found, despite the fact that the septum extendsto the crux.5

Address for reprint requests: Dr RH Anderson, Department ofPaediatrics, Cardiothoracic Institute, Brompton Hospital, FulhamRoad, London SW3 6HP.

This doubt is endorsed by cases recentlyexamined with congenitally corrected transpositionand straddling of the right atrioventricular valve,hearts which show features intermediate betweenthose of classically corrected transposition anduniventricular hearts of right ventricular type withright-sided rudimentary chamber., In the heartsstudied with straddling valves, two had anteriorconduction tissues alone while another had bothregular and anterior nodes with a ventricular"sling" of conduction tissue.6In view of the uncertainty produced by these

considerations, we have studied an example ofuniventricular heart of right ventricular type withright-sided rudimentary chamber, to try to estab-lish the precise disposition of the atrioventricularconduction tissues.

Case report

The patient was a male child born of healthy un-related parents after a full-term normal pregnancy.The delivery was uncomplicated. There had beenno contact with infectious diseases or drugs duringthe pregnancy. The infant was admitted to hospital

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Catharina E Essed, Siew Yen Ho, Stewart Hunter, and Robert H Anderson

on the sixth day of life because of cyanosis. Chestradiography showed a normal sized heart withpulmonary oligaemia, and the electrocardiogramwas interpreted as showing right ventricular hyper-trophy with some right atrial enlargement. TheT waves were noted to be flattened in all leads.Although the patient was not in heart failure, heexhibited continuing cyanosis despite oxygentherapy and became increasingly dyspnoeic. Hetherefore underwent cardiac catheterisation onthe day of admission. The catheter was advancedfrom the right saphenous vein to the rightatrium, then passed to a ventricular chamber andthence to the aorta. The catheter could also bepassed through an atrial septal defect, across adifferent atrioventricular valve, and into the sameventricular chamber and the aorta. The pul-monary artery was never entered, and it was notpossible to detect a second ventricular chamber.Angiograms confirmed a double inlet to a coarselytrabeculated ventricular chamber of right ventri-cular morphology and indicated the presence ofpulmonary atresia, with small pulmonary arteriesfilling through a ductus arteriosus. An aorto-pulmonary anastomosis was considered as a neces-sary operation, but the infant suffered cardiacarrest and died before this could be performed.At necropsy it was shown that there was situs

solitus of the atria. Both aortic arch and apexwere to the left. The right atrium was distendedbut received normal systemic venous drainage.The foramen ovale was patent. The pulmonaryveins drained normally into the left atrium. Bothright and left atrioventricular orifices com-municated in their entirety with the same ven-tricular chamber. This chamber had a trabecularpattern of right ventricular type (fig la). Within

the right and posterior wall of this main chamber,a small rudimentary chamber with left ventricularpattern was found. The septum separating thechambers extended to the crux and was delimitedexternally by the anterior and posterior descendingcoronary arteries (fig lb). The rudimentary cham-ber communicated with the ventricle through asmall septal defect, but had no direct inlet oroutlet. There was a single outlet from the mainventricle, this artery being an aorta. Its orificewas positioned in front of the two atrioventricularorifices and the aortic valve was supported by acomplete muscular infundibulum (fig la). Therewas atresia of the pulmonary outflow tract andtrunk, the remnant of the trunk being positionedto the right and slightly behind the aorta. Thepulmonary arteries arose from a confluence fedthrough a narrow ductus arteriosus.Within the main chamber a prominent muscular

trabecula separated the atrioventricular valves,giving tension apparatus to each valve, and ex-tended upwards to the underside of the atrialseptum (figs la, 2a). On gross examination white"streaks" reminiscent of the conduction tissuewere seen on the left aspect of the trabecula.The diagnosis was: (1) solitus-double inlet

ventricle-single outlet heart, (2) univentricularheart of right ventricular type with right-sidedtrabecular pouch, and (3) pulmonary atresia.A block of tissue was removed which contained

the trabecular pouch, the right atrioventricularorifice, the trabecular septum, the prominent mainchamber trabecula cut at its attachment to thethe trabecular pouch, the right atrioventricularcular orifice, and the lower part of the atrialseptum (fig 2a, b).

It was sectioned perpendicular to the trabecular

YV VatVeUin' chamnber)

Desc.Fig 1 (a) The main chamber with

At C. trabecular pattern of right ventricular_ryl~Mtype and the position of the trabecula in

relation to the right and leftatrioventricular orifices and the aorta.(b) The rudimentary chamber of leftventricular type. Note that the trabecularseptum extends to the crux cordis,indicated by the posterior descendingcoronary artery and the atrial septum.

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A trioventricular conducting tissue in UVH of RV type

septum, using the technique of Smith et al.7 Thesections were 10 microns thick, and every twenty-fifth section was mounted and stained with atrichrome technique.

Results

A conventional node was found in its expectedposition at the apex of the triangle of Koch

(fig 3a). It formed a penetrating bundle whichreached the crest of the free-running mainchamber trabecula (fig 3b). However, immediatelythe bundle ascended again through the fibrousannules to become an anterior nodelike structure(fig 3c). A sling was therefore produced in thebase of the annulus fibrosus on the top of thetrabecular attachment. The anterior nodelikestructure was positioned very close to the posterior

Fig 2 (a) The main chamber view of theblock of tissue removed for study of theconduction tissue. Note the "streaks" onthe trabecula. A section has been takenthrough the area marked "x-x," andcorresponds with fig 4. (b) Therudimentary chamber view of the sameblock of tissue. The position of theconventional and anterior nodes with theconduction tissue sling are marked. Thesections taken through the areas marked"a-a," "b-b," "c-c," correspondrespectively with figs 3a, b, and c.

Fig 3 (a) The posterior node with normal relationship to the tendon of Todaro. (This section correspondswith the mark "a-a" in fig 2b). (b) The penetrating bundle from the conventional node in the annulusfibrosus. The anterior part of the sling is seen passing through the annulus towards the anterior nodecorresponding with the mark "b-b" in fig 2b. (c) The anterior node at its junction with the sling. Beneaththe sling is seen the slender non-branching bundle extending downwards towards the trabecula, correspondingwith the mark "c-c" in fig 2b.

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Catharina E Essed, Siew Yen Ho, Stewart Hunter, and Robert H Anderson

node within the atrial septum (fig 2b). From thesling a slender bundle was found to descend alongthe right and anterior side of the trabecula, with-out branching. Along the left margin of thetrabecula below the attachment of a chorda ofthe left atrioventricular valve, further bundlelikestructures were seen (fig 4). These, however, hadno contact with the remaining conduction tissues.The left-sided structures corresponded to the"streaks" observed on gross examination (fig 2a).No conduction tissue was found in the trabecular

septum. The overall distribution of conductiontissue as it might have been viewed by thesurgeon approaching through the right atrium isshown in fig 5.

Discussion

Fig 4 The conduction tissue on the left side of thetrabecula. This conduction tissue corresponded to thewhite "streaks" seen on gross examination,corresponding with the mark "x-x" in fig 2a.

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: L:e !,

=<.../....

Fig 5 A rtist's impression of the disposition of theconduction tissue in the heart studied as it wouldhave appeared to the surgeon looking through the

right atrium.

Previous investigations have shown that whileanterior conduction systems are the rule inuniventricular hearts of the left ventricular type,'-3the atrioventricular conduction system in uni-ventricular hearts of right ventricular type takesorigin from a conventionally situated node.4 Ourpresent investigation shows that the conductionsystem is still posterior, originating from a con-ventional node, in right ventricular univentricularhearts even when the rudimentary chamber isright-sided. This is of interest because such heartscan be considered "close cousins" to congenitallycorrected transposition, an anomaly in which theconduction tissue is nearly always anterior.5 How-ever two connecting nodes were found in ourpresent case forming a ventricular sling of con-duction tissue. The difference between this slingand previously reported ventricular slings whichhave recently been reviewed by Wenink8 is that inour case the sling was not related to the crest ofthe trabecular septum. Indeed, the trabecularseptum was devoid of conduction tissues. Suchslings unrelated to a septal structure have beenobserved before in two cases of univentricularheart of indeterminate type without rudimentarychamber.9 In these hearts the sling of conductiontissue was found in the posterolateral wall of theventricle. The surgical significance of our case isthat the bundle descended down a prominent free-running trabecula. The heart would have been im-possible to septate without sewing to this structure.The conduction tissues would therefore have beenat considerable risk should surgical repair havebeen attempted.

Conduction tissue has previously been found on

free-running trabeculae in univentricular hearts ofindeterminate type'011 and in one of the cases of

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Atrioventricular conducting tissue in UVH of RV type

univentricular hearts of right ventricular type withleft-sided rudimentary chamber.4 What is thenature of the trabecula? It is tempting to speculatethat it is the trabecula septomarginalis. Wenink3has commented on the significance of the trabeculaseptomarginalis in forming the anterior septum inuniventricular hearts of left ventricular type andhas since developed this concept to suggest that thetrabecula septomarginalis is the structure whichcarries the ventricular conduction tissues from theatrioventricular node, whatever its position, to thetrabecular septum (Wenink, personal communica-tion, 1979). We have previously argued that theventricular conduction tissue is carried on thetrabecular septum, and that it is the orientation ofthis septum which determines the position of theconnecting atrioventricular node.12 The presentcase, which has divorce of the trabecula septo-marginalis from the trabecular septum, the latterstructure being devoid of conduction tissue despiteextending to the crux, suggests that our initialconcept was simplistic. Instead it lends strongsupport to the alternative concept advanced byWenink.The findings in the present and other cases4 1011

suggest that the surgeon should suspect that a free-running prominent trabecula may carry conduc-tion tissue in a univentricular heart whenever itextends to make contact with the atrioventricularjunction. In this circumstance the trabecularseptum may be a poor guide to the penetratingbundle even though the septum extends to thecrux cordis.

We are grateful to Dr D Scott of NewcastleGeneral Hospital who performed the initial post-mortem examination of this case and gave uspermission to carry out the further investigations.We are also'indebted to Miss Noelle de Freitas forhelp in the preparation of the manuscript. CEEwas a visiting fellow from the Department ofThoracic Surgery, Thoraxcenter, AcademicHospital "Dijkzigt", Rotterdam, the Netherlands.RHA is supported by the Joseph Levy Foundationand the British Heart Foundation.

References

1 Anderson RH, Arnold R, Thapar MK, JonesRS, Hamilton DI. Cardiac specialized tissue inhearts with an apparently single ventricularchamber. (Double inlet left ventricle.) Am JCardiol 1974; 33:95-106.

2 Bharati S, Lev M. The course of the conductionsystem in single ventricle with inverted (L) loopand inverted (L) transposition. Circulation 1975;51:723-30.

3 Wenink ACG. The conducting tissues in primitiveventricle with outlet chamber: two different pos-sibilities. J Thorac Cardiovasc Surg 1978; 75:747-53.

4 Wilkinson JL, Dickinson DF, Smith A, AndersonRH. Univentricular heart of right ventricular typewith double or common inlet. J Thorac CardiovascSurg 1980; in press.

5 Anderson RH, Becker AE, Arnold R, WilkinsonJL. The conducting tissues in congenitally cor-rected transposition. Circulation 1974; 50:911-24.

6 Becker AE, Ho SY, Caruso G, Anderson RH.Straddling mitral valve in congenitally correctedtransposition of the great arteries. Circulation1980; in press.

7 Smith A, Ho SY, Anderson RH. Histologicalstudy of the cardiac conducting system as aroutine procedure. Med Lab Sci 1977; 34:223-9.

8 Wenink ACG. Congenitally complete heart blockwith an interrupted Monckeberg sling. Eur JCardiol 1979; 9:89-99.

9 Dickinson DF, Wilkinson JL, Anderson KR,Smith A, Ho SY, Anderson RH. The cardiacconducting tissues in situs ambiguus. Circulation1980; in press.

10 Wilkinson JL, Anderson RH, Arnold R, HamiltonDI, Smith A. The conducting tissues in primitiveventricular hearts without an outlet chamber.Circulation 1976; 53:93048.

11 Wilkinson JL, Becker AE, Anderson RH.Morphology and conduction tissue in primitiveventricle with two atrioventricular valves. In:Anderson RH, Shineboume, EA, eds. PaediatricCardiology 1977. Edinburgh: Churchill Living-stone, 1978; 323-38.

12 Anderson RH, Wilkinson JL, Becker AE. Con-ducting tissues in the univentricular heart. InVan Mierop LHS, Oppenheimer-Dekker A, eds.Embryology and Teratology of the Heart andGreat Arteries. The Hague: Martinus Nijhoff,1978; 62-78.

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