Thesurgical anatomy of conduction tissues · disposition ofthe conduction tissues can be predicted...

Thorax 1983;38:408-420

The surgical anatomy of the conduction tissuesROBERT H ANDERSON, SIEW YEN HO, ANTON E BECKER

From the Department of Paediatrics, Cardiothoracic Institute, Brompton Hospital, London; and theUniversity of Amsterdam Department of Cardiovascular Pathology, Interuniversity Institute, AcademicMedical Centre, Amsterdam, Holland

ABSTRACT On the basis of our collective experience we have reviewed the disposition of thecardiac conduction tissues as they might be observed by the surgeon in both normal and abnormalhearts. The sinus node lies subepicardially in the terminal sulcus; because of its variable bloodsupply the entire superior cavoatrial junction is a potential danger area. There are nomorphologically discrete tracts extending through the atrial tissues between sinus andatrioventricular nodes. The atrioventricular node, the atrial extent of the atrioventricularconduction axis, is contained exclusively within the triangle of Koch. The axis penetrates throughthe central fibrous body and branches on the muscular ventricular septum immediately beneaththe interventricular component of the membranous septum. The landmarks to these structures aredescribed as they might be seen through the right atrium, left atrium, and aorta. Consideration isthen given to the surgical anatomy of the abnormal muscular atrioventricular connections thatunderscore the ventricular pre-excitation syndromes. Finally, rules are developed whereby thedisposition of the conduction tissues can be predicted with accuracy in congenitally malformedhearts, in the settings of both normal and abnormal chamber connections. The most importantvariables in this respect are alignment between the atrial and ventricular septal structures and thepattern of ventricular architecture present.

Understanding of the conduction tissues of the hearthas had a chequered history since their discovery atthe end of the last century. A contentious topic attheir birth, they have continued to excite interestand disagreement up to the present time. Notwith-standing this, on one aspect all are agreed. If thecardiac surgeon is to leave the patient with a nor-mally beating heart, he must be aware of the exactdisposition of these tissues and his incisions, retrac-tion, or suction must scrupulously avoid them.Equally, he must know the abnormal dispositions ofthese tissues as found in congenitally malformedhearts and must again treat these areas with consid-erable respect if he is to avoid postoperativearrhythmia, a particularly worrying complication ofthe surgical treatment of congenital heart disease. Inthis review we describe the anatomy of these tissuesas we understand them, illustrating them as they willpresent to the surgeon at operation. We first

Address for reprint requests: Professor RH Anderson, Departmentof Paediatrics, Cardiothoracic Institute, Brompton Hospital, Lon-don SW3 6HP.

Accepted 9 February 1983

describe their disposition in the normal heart andthen discuss rules for predicting their position inmalformed hearts.

Surgical anatomy of normal conduction tissues

THE SINUS NODEThe sinus node, or pacemaker, is a small, cigar-shaped structure which lies in the terminal groove,formed by the lateral junction of the superior cavalvein with the remainder of the right atrium. Almostalways the node lies in the groove to the right of thecrest of the atrial appendage (as viewed with theheart in the surgical position-figure 1). It usuallyhas an extensive tail which runs subepicardially inthe groove towards the junction of the inferior cavalvein with the atrium. The sinus node cannot be re-cognised unequivocally with the naked eye, butsometimes a whitish patch of tissue is visible in thegroove at this site. The entire junction of thesuperior cava with the right atrium should, however,be treated with the utmost respect. This is becauseon occasion the sinus node can extend over the crestof the atrial appendage into the interatrial groove, a

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The surgical anatomy of the conduction tissues

Sinus node in terminal groove

Fig 1 The usual position of the sinus node as it might beviewed by the surgeon operating from the right side oftheoperating table.

Horseshoe node

Terminal groove

Fig 2 The occasional "horseshoe" position (about 10% ofcases) of the sinus node.

20 infant nodes studied byserial sectioningArtery was

in front 7casescircle 4cases

Sinus node artery behind 9cosestrom right coronary 55%. /

left coronary 45% / /ascends throughinteratrial groove

Superior Inferiorvena vena

Cava Cava

Extent of potential "danger area"

Fig 3 The variability in the course ofthe artery to the sinusnode in relation to the superior cavoatrial junction.

pattern described as usual by Hudson.' We foundthis horseshoe arrangement (fig 2) to be the excep-tion rather than the rule.2 Also of significance wasthe major variability we noted in the course of thenodal artery (fig 3). The prominent nodal arteryoriginates from the origin of the right coronaryartery in about 55% of cases and from the left coro-nary artery in the rest.3 We noted further variationin its course relative to the superior cava. It enteredthe terminal groove either in front of the cava orbehind it, or else it formed an arterial circle aroundthe cavoatrial junction. These patterns were not dis-tinguishable with the naked eye. It thereforebehoves the surgeon to treat the entire superiorcavoatrial junction with the utmost respect if he is toavoid damage to the sinus node and postoperativearrhythmic sequelae. The entirety of the potentialdanger area is illustrated in figure 3.

THE INTERNODAL CONDUCTION MECHANISMThe nature of the atrial myocardium between sinusand atrioventricular nodes has engendered moredisagreement than any other part of the cardiacconduction mechanism. When it was discussed atlength by the German Pathological Society in 1910,4the consensus was that the impulse was conductedbetween the nodes through plain atrial myocardium,there being no insulated and isolated tracts of con-duction tissue extending between the nodes. Ourresults5-7 completely endorse this original consen-sus. Those who have since postulated the existenceof specialised tracts89 have done so on the basis offinding scattered cells with histologically specialisedcharacteristics among the broad bands of atrialmyocardium between the nodes. Nowhere havethese protagonists of "specialised internodal con-duction" shown the existence of insulated conduc-tion pathways comparable with the ventricular bun-dle branches, the latter being the paradigm of a con-duction system. Furthermore, electrophysiologicalstudies have shown that, from the standpoint ofconduction, the sinus impulse passes to the atrioven-tricular node along broad fronts,'0-'2 the substratesfor these fronts being determined by the overallgeometry of the right atrium.'3 The intraoperativemapping experiments of Wittig et all4 confirmed theexistence of those broad fronts of atrial activity, par-ticularly after excision of the atrial septum. Theyfailed to show any narrow tracts responsible exclu-sively for the spread of excitation. This does notmean that the surgeon has freedom to make inci-sions at will in the atrial myocardium and then anti-cipate normal postoperative interatrial conduction.Although the mapping experiments militate againstspecialised conduction tracts, they do show thatinteratrial conduction occurs preferentially along

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Anderson, Ho, Becker

Oval fossa

Siteo, sinus nodeTerminal crest

nodeOrifice of inferior caval veinTerminal crest

Fig 4 The anatomy of the right atrium is such that thevenous orifices and oval fossa divide the myocardium intobroad muscle bundles. These bundles, specifically theterminal crest and the margins of the oval fossa, act as theroutes ofpreferential conduction between the sinus andatrioventricular nodes; and it makes sense to avoid theseareas, as well as the sites of the node, during surgery.

the prominent muscle bundles, notably the terminalcrest, the anterior lip of the oval fossa, and the sinusseptum. These are the most direct routes betweensinus and atrioventricular nodes (fig 4). If at all poss-ible they should be preserved at operation. Certainlyduring atrial surgery great efforts should be made tokeep one of those preferential routes intact ifabnormal postoperative atrial conduction is to beavoided. 14

THE ATRIOVENTRICULAR NODE ANDPENETRATING BUNDLEThe surgical landmarks to the atrioventricular nodeare those of the triangle of Koch (fig 5). If tension isplaced on the Eustachian valve (when present), thenthe fibrous extension of this structure (the tendon ofTodaro) can be traced into the sinus septum, whereit can be seen extending leftwards and cephalad toinsert into the central fibrous body. A triangle canthen be constructed between the tendon of Todaro,the attachment of the septal leaflet of the tricuspidvalve to the septum, and the orifice of the coronarysinus. The atrioventricular node and its transitionalcell zones are contained entirely within this triangle.If the tendon of Todaro cannot be identified themiddle of the sinus septum can be taken as the atrialboundary of the triangle.The penetrating bundle passes through the central

fibrous body at the apex of the triangle of Koch,perforating the fibrous tissue to pass directly into theleft ventricular outflow tract. In this position thebundle is more or less directly related to the mitral,tricuspid, and aortic valves and the membranousseptum and is at risk during surgery on any of thesestructures. The overall anatomy and interrelation-

~~~~~~~~t:~~~~~~~~~~~~~~~~~~~~rn~~~~~~~~~~~~~

,7 sf ,f

Fig 5 Landmarks of the triangle of Koch as viewed by thesurgeon.

Fig 6 Dissection viewed in surgical orientation showingthe relationships of the atrioventricular node and bundle tothe aortic, mitral, and tricuspid valve rings.

ships of these structures and the penetrating bundleare shown in the dissection in figure 6. From thesurgical standpoint, the left atrial landmark to this

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Aortic incisionIncision intoleft atrium

Right coronary leaflet

Branching/ bundle

\Sca lops ofmural leaflet

Fig 7 Relationship ofthe atrioventricular node and bundleto the mitral valve as seen by the surgeon approachingthrough the left atrium.

area is above and cephalad to the posteromedialcommissure of the mitral valve (fig 7). As seen fromthe tricuspid valve, the danger-area is between theapex of the triangle of Koch and the anteroseptalcommissure. As viewed through the aorta, thedanger area is directly beneath the commissurebetween the non-coronary and right coronary aorticleaflets (fig 8).

VENTRICULAR CONDUCTION TISSUESIn contrast to the atrial conduction mechanism,there is no doubt that an insulated conduction sys-tem distributes the cardiac impulse through the ven-tricles. Cutting this ventricular system abolishesconduction through it. Having penetrated into thesubaortic outflow tract, the atrioventricular bundledivides into right and left bundle branches.Although usually situated astride the muscular ven-tricular septum, between it and the membranousseptum, the division can occur on the left ventricularaspect of the septum.'5 The branching occurs along alength of the bundle, the left bundle branches tumbl-ing down the smooth left side of the septum, whilethe right bundle branch continues in the general

Non-coronary leaflet

Fig 8 Relationships ofthe ventricular conduction tissues tothe aortic valve as viewed by the surgeon approaching via anaortotomy.

direction of the axis of the node, penetrating thebranching bundles. Three major divisions (anterior,septal, and posterior) can be made out in thedescending fan of left bundle-branch cells, but thethree are intimately interconnected.

In contrast to the fan-like left bundle branch, theright bundle branch is a narrow, cord-like structure.It generally passes intramyocardially to reach theright side of the septum, running through the post-erior limb of the septomarginal trabecula and pas-sing beneath the medial papillary muscle complex. Itthen extends to the right ventricular apex within thebody of the trabecula, ramifying at the ventricularapex.

Surgical anatomy of congenitally malformed con-duction tissues

The conduction tissues can themselves be the sub-

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ject of congenital lesions, such as congenitally com-plete heart block or ventricular pre-excitation.Alternatively, the conduction tissues can be anatom-ically normal but be disposed in abnormal fashionbecause of congenital lesions of the heart itself.Although congenitally complete heart block is ofrelatively little importance surgically, the conduc-tion tissues in ventricular pre-excitation and congen-ital malformations themselves are of major impor-tance.

Surgical anatomy of ventricular pre-excitation

Ventricular pre-excitation can be produced by sev-eral anatomical substrates, the commonest beingparticularly amenable to surgical treatment.'6 Pre-excitation of the Wolff-Parkinson-White type (shortPR-broad QRS-delta wave) is produced byaccessory atrioventricular connections, which canreadily be divided by surgical operation. The rarerform of Wolff-Parkinson-White syndrome, which isdue to accessory nodoventricular or fasciculoven-tricular fibres ("Mahaim" fibres) is not in itselfamenable to surgical treatment, but may on occasionrequire treatment by division of the atrioventricularbundle. This is probably best done by cryothermy,'7the landmarks of the triangle of Koch being used asthe guide to the penetrating bundle. Pre-excitationof the Lown-Ganong-Levine variety (short PR-normal QRS) is due either to atriofascicular fibres orto intranodal bypass tracts. As yet, surgical treat-ment in these cases is confined to division of thepenetrating bundle in cases of intractable tachycar-dia. It is therefore the classical Wolff-Parkinson-White type of pre-excitation which will most fre-quently require surgical treatment. Here knowledgeof the anatomy of accessory atrioventricular connec-tions greatly facilitates their successful surgical divi-sion.Accessory atrioventricular connections can be

located around the mitral orifice, around the tricus-pid orifice, or within the septum itself. Left-sidedmitral connections generally produce so-called typeA pre-excitation and have a particularly constantmorphology. They take their origin from the atrialmyocardium just above the mitral annulus and thenrun through the epicardial fat pad on the outside ofthe annulus, hugging it very closely, before insertinginto the ventricular myocardium. An incision placedin the atrial wall above the annulus will not divide aleft-sided connection (fig 9). Dissection in the fatpad on the epicardial aspect of the annulus will benecessary to ablate the connection. It should also beborne in mind that these connections may be multi-ple.

Right-sided connections usually produce so-called


Suggested inasion in left atrium

Fig 9 Incision in the left atrial wall recommended by Sealyfor division ofaccessory atrioventricular connections; theinset shows the anatomy ofthese connections vis-a-vis themitral annulus and indicates how dissection is necessary inaddition to the cut to ensure their division.

type B pre-excitation. These too may be multipleand can extend through the epicardial fat pad toconnect atrial and ventricular myocardium. Thetricuspid annulus, however, is much less well formedthan the mitral and these connections can traversesubendocardially through deficiencies in theannulus. None the less, an incision on the annuluswith dissection in the epicardial fat pad is usuallynecessary to be sure of achieving division of a right-sided connection.

Septal accessory connections are the most difficultto treat, in terms both of localisation and of surgicaldivision.'8 Here use can be made of the tissue planethat extends beneath the coronary sinus towards theatrioventricular node.6 Access to this plane can beobtained from the right atrium and then the atrialseptal myocardium can be liberated from the ven-tricular myocardium with disruption of any acces-sory connections between. Even in the most experthands, however, these connections are the mostdifficult to disrupt. In refractory cases division of theatrioventricular bundle may be required.

Conduction tissues in malformed hearts

ATRIAL CONDUCTION TISSUESVery rarely is the sinus node shifted from its normalposition in the terminal groove. The only occasionson which we have encountered an abnormallylocated sinus node are in association with juxtaposi-tion of the atrial appendages'9 and in left atrialisomerism ("situs ambiguus with polysplenia"20).



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These occasions are likely to be rare in surgical prac-tice. Atrial arrhythmias are, however, well known tobe frequent after atrial surgery, particularly in thetreatment of complete transposition. Although thiscomplication has been attributed to damage to"specialised pathways," as we have indicated above,conduction between the nodes occurs through plainatrial myocardium. The careful studies of Gilletteand his colleagues2' have shown that almost alwaysthese arrhythmias result from sinus node damage,while Ullal et a122 have demonstrated that scrupulousavoidance of the sinuatrial junction can effectivelyabolish postoperative arrhythmias after Mustard'soperation for complete transposition. The secret ofavoidance of atrial arrhythmias is therefore to beaware of the danger area of the sinuatrial junctionand to avoid it meticulously during all surgical pro-cedures.

VENTRICULAR CONDUCTION TISSUESThe ventricular conduction tissues are most at riskduring repair of ventricular septal defects. We willdescribe the relationship of defects and conductiontissues both when the atrioventricular node is nor-mally sited and when it is found in abnormal sites.

Malformations with normally situated atrioventricu-lar nodes"Isolated" ventricular septal defect In all heartswith atrioventricular concordance and in thosehearts with ambiguous atrioventricular connectionand right-hand pattern (d-loop) ventricular architec-ture,20 the atrioventricular node is normally sited atthe apex of the triangle of Koch irrespective of thepresence of defects elsewhere in the heart. Thisencompasses the great majority of lesions requiringsurgical treatment. In these patients therefore theguides to the atrioventricular node and the penetrat-ing atrioventricular bundle are the landmarks of thetriangle of Koch as described above. The guides to

Doaja.:y - ommitted su,-barter defect -De-'eolrnot- ciort', cind purmoncry voaves-, eyXtern{'o Decorme ;erirnerbran s

j-X_scl;Jcur defe -'sCornp.ete y surraind e-i, m :sc'e

:e rnerrZc3-3u.$,e.e,-s:|bc.ortr abuts \on .-entr tesa.bocd y

Fig 10 The basic subdivision ofventricular septal defectsas viewed by the surgeon.

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Node inatrnal septum

Tricuspd leafletnever containsconduction tissue

Septal remnantcovers bundle

Septal crestusual y devoidof conduction tissue

Fig 11 The usual relationship of the non-branchingbundle to the crest ofthe muscular septum in the presence ofa ventricular septal defect; it is always safe to place stitchesin the leaflet tissue of the tricuspid valve.

the ventricular conduction tissue depend on the typeof ventricular septal defect present. Irrespective ofthe ventriculoarterial connection found in heartswith ventricular septal defects, the defects can bedivided into three types: perimembranous, muscu-lar, and doubly committed subarterial23 (fig 10). Thedisposition of conduction tissue varies in these, butbasically depends on whether a defect is perimem-branous or muscular. A perimembranous defect ischaracterised by the fact that the defect abuts on thecentral fibrous body and part of its rim is formed bythe atrioventricular component of the membranousseptum and the aortic valve. This atrioventricularcomponent of the membranous septum is the majordanger area of these defects, since it is the site of thepenetrating bundle. When approached from eitherright atrium or right ventricle this part of aperimembranous defect will be overlaid by theleaflets of the tricuspid valve. The free leaflet tissueof the tricuspid valve never harbours conduction tis-sue. The rule of safety therefore is to place sutures inthis area through the leaflet tissue and not throughits attachment. In this way it will be impossible todamage the penetrating atrioventricular bundle (fig11). In hearts with ventricular septal defects almostalways there is a segment of non-branchingatrioventricular bundle between penetrating andbranching bundles and always in perimembranousventricular septal defects this non-branching bundleis to the right hand of the surgeon when he isapproaching through the atrium (fig 12). The prox-imity of non-branching and branching bundles to thecrest of the septum depends on the precise type of



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Perimembranous-outlet (infundibular) Perimembranous-trobecular

Overallorientatior,

Triangle of Koch

Inferior

9ceriarCO"0 ven ca va

Perimembranous-iNret Muscular-inlet

Fig 12 Disposition of the ventricular conduction tissues inthe different types ofventricular septal defect as viewed bythe surgeon through a right atriotomy.

perimembranous defect encountered.24 When thedefect extends into the inlet septum the bundles are

much closer to the septal crest than when the defectextends into the ventricular outlet regions.

In muscular defects, the entire rim of the defect ismuscular and so the penetrating bundle is no longervulnerable. The danger area in a muscular defecttherefore depends on its position within the septum.The muscular defect in which the bundle is most atrisk is one in the inlet part of the septum whichextends up towards the valve annuli but does notreach the central fibrous body, being separated fromit by a muscular rim. In these muscular inlet defectsthe conduction tissue axis is to the left hand of thesurgeon approaching through the atrium. In otherwords, it is precisely to the opposite side as seen in a

perimembranous defect (fig 12). For this reason it isessential to distinguish perimembranous and muscu-lar inlet defects.Muscular trabecular defects are well distant from

the central fibrous body and the proximal ventricu-lar conduction tissues. They are therefore relativelysafe from the risk of heart block, the only dangerbeing peripheral damage with proximal "tracking"of haemorrhage and exudate.25 Defects in the outletregion with muscular inferior rims are the safestfrom the standpoint of conduction tissues. In thesedefects the posterior limb of the septomarginal


trabecula fuses into the ventriculoinfundibular foldand forms a muscle bundle that separates the centralfibrous body from the right ventricular aspect of thedefect. It also protects the conduction tissues. In allhearts with a muscular rim separating the centralfibrous body from the defect therefore the conduc-tion tissues are safe from all but the most deeplyplaced sutures. This rule holds good for doublycommitted subarterial defects with muscular inferiorrims (we define a subarterial defect as one beneathboth aorta and pulmonary valves-hence its doublycommitted nature). It does not apply when a doublycommitted subarterial defect extends to becomeperimembranous. These rules for conduction tissuedisposition in perimembranous, muscular, and doublycommitted subarterial defects hold good also formalformations with abnormal ventriculoarterialconnections such as double-outlet right ventricle andcomplete transposition.Atrioventricular septal defects The septal defi-ciency in an atrioventricular septal defect("atrioventricular canal malformation," "endocar-dial cushion defect") affects primarily the atrioven-tricular junction. There is an extensive deficiency ofthe inlet part of the muscular ventricular septum,and although the atrial septum is frequently wellformed the atrial septum makes contact with theventricular septum only at the anterior and posteriormargins of the atrioventricular junction-unlike inthe normal heart, or hearts with isolated ventricularseptal defects, where there is usually a more exten-sive area of septal atrioventricular contiguity. Thislack of septal contiguity determines the conductiontissue disposition. The atrioventricular node is situ-ated well posterior in the atrial septum just above itsposterior connection with the atrioventricular junc-tion. A nodal triangle is formed between the lowerrim of the atrial septum (roof of the "ostium primumatrial septal defect") and the posterior attachmentof the atrioventricular valve and the coronary sinus.This nodal triangle is not the triangle of Koch. Thetriangle of Koch does exist in atrioventricular septaldefects but does not contain the connectingatrioventricular node. The nodal triangle justdescribed is, however, the most obvious triangle (fig13) and its apex is an excellent guide to the penetrat-ing atrioventricular bundle. The non-branchingbundle is extensive and is carried on the crest of the"scooped out" inlet part of the muscular septum.The branching bundle is also carried on the crest ofthe septum and the right bundle branch then runsforward on the septum towards the medial papillarymuscle or the septomarginal trabecula (fig 13). Thusfar no mention has been made of so-called "com-plete" versus "partial" atrioventricular septaldefects. This is because the conduction tissue dispos-



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The surgical anatomy of the conduction tissuesBracnching and non-brcanching bundleat risk beneath postbridging leaflet

R gint bundle brat risk orl expoa

n

con ti01:r renjr'r- -ar

(a)

415

Right bundle branch penetratesto lie beneath medial papillary

ranch musclese,d,crest

sedcreJ\ tnPenetratingbunfeIe\.ai-. rrxor r,sk at apex

at nodal tr'ang:e

- > z of noda tr ring e iNon-branching bundle and

-I>t 1 left bundle branch on leftventricular aspect of septum

C,// Penetrating bundle through area of

tricuspid -aortic fibrous continuityRight bundle branch at risk

'ss:e Prox rn;ty of sinus to annulus-gfe rch deterrn!res feosb i:ty of leaving

c-cr oGr j sE.:rus dr.nrirng to right atriumrr

Lecving Pcoronary s nusSutu re '!ne ent rely to left atrium is mucn safer

rn leftdoes not cross aixs

(h)Fig 13 Basic disposition of the conductiontissues inatrioventricular septal defects as viewed by the surgeonthrough a right atriotomy: (a) shows the arrangement with a

common orifice and (b) the arrangement with separate rightand left valve orifices. The possible suture lines are

indicated.

ition is essentially the same in the two subgroups.The difference is that in defects with a common

atrioventricular valve the right bundle branch isexposed on the "bare" area of septum found bet-ween the facing edges of the anterior and posteriorbridging leaflets. The non-branching and branchingsegments of the axis are covered by the posteriorbridging leaflet. In those defects in which a connect-ing leaflet tongue produces separate right and leftatrioventricular orifices ("ostium primum atrial sep-tal defects") the tongue also covers the septal crestso that the entire conduction axis is hidden from thesurgeon by valve leaflet tissue (fig 13).Tetralogy of Fallot As in hearts with an isolatedventricular septal defect, the defect in tetralogy ofFallot can be subdivided into perimembranous,muscular, and doubly committed subarterial forms;and the conduction tissue disposition is as described

Ventriculo-infundibular fold fuses withseptomarginal trabecula- safe conduction axisFig 14 The distribution ofconduction tissues in tetralogyofFallot varies according to the type ofdefect present: theupperpanel shows the arrangement with a perimembranousdefect and the lower panel the much safer disposition whenthere is a muscular posteroinferior rim.

for the basic defects. Most defects are perimem-branous in position-that is, they abut directly onthe central fibrous body. They frequently possessmembranous septal remnants in their posterior-inferior quadrants. The danger area is the segmentof aorto-mitral-tricuspid continuity where the axispenetrates. The rule that tricuspid valve leaflet tis-sue never harbours conduction tissue should againguide suture placement. Usually the non-branchingand branching bundles descend on the left ventricu-lar aspect of the septum (fig 14), so the crest of theseptum itself is devoid of conduction tissue. But onoccasion the branching bundle may be found astridethe septum,2627 so it is probably safer always to placeright ventricular sutures away from the actual crest.Figure 15 illustrates these danger points and otherareas around the defect in which the surgeon may beconfronted by problems because of the anatomicalarrangement.

In about a fifth of cases of tetralogy the posteriorlimb of the septomarginal trabecula fuses with theventriculoinfundibular fold to produce a muscularoutlet defect. As with isolated defects, the musclebar thus formed protects the conduction axis and theentire muscular rim of the defect is a safe area (fig14, lower panel). A much rarer variant of tetralogy



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rr +

N

I-

. i ...INC\

f/ :/

Fig 15 Anatomical danger areas of the typical perimembranous ventricular septal defectfound in tetralogy ofFallot.

is when the infundibular (outlet) septum is itselfabsent or grossly deficient. The important point thenwith regard to the conduction tissue is whether thedoubly committed subarterial defect thus producedextends to become perimembranous or has a muscu-lar inferior rim. The conduction tissues will be muchmore at risk should the defect extend to beperimembranous.

Malformations with abnormally located atrioven-tricular nodesThe hearts that possess abnormally located atrioven-tricular nodes are those with atrioventricular discor-dance, those hearts with ambiguous atrioventricularconnection that also have a left-hand (1-loop) pat-tern of ventricular architecture, and hearts with uni-ventricular atrioventricular connection to a mor-phologically left or indeterminate ventricle. Somehearts with univentricular atrioventricular connec-tion to a morphologically right ventricle with aright-sided rudimentary left ventricle also haveabnormal nodes, but these are exceedingly rare andare discussed elsewhere.28 Finally, hearts with astraddling tricuspid valve have abnormally locatednodes.29The rules which govern the location of the

atrioventricular node depend, firstly, on the align-

ment and position of the ventricular septum and,secondly, on the pattern of ventricular architecture.The orientation of the ventricular septum is impor-tant because the trabecular septum carries the ven-tricular conduction tissues. The atrioventnrcularnode is developed from the embryonic atrioventricu-lar canal musculature. Hence abnormal hearts havethe potential to develop an atrioventricular nodewherever the trabecular septum makes contact withthe atrioventricular junction (fig 16). A normallysited node is developed only when the muscular ven-tricular septum extends to the crux cordis and is

Fig 16 An atrioventnicular node can be formned at anypoint round the atrioventricular junction where thetrabecular septum makes contact with the junction.

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Danger areafor anterolcteral node Mitral vaive|

I

Dli

Tricngle of Koch does not

usually contain connecting node

Fig 17 Position of the connecting atrioventricular node incongenitally corrected transposition as viewed through aright atriotomy.normally aligned relative to the atrial septum.

Congenitally corrected transposition Congenitallycorrected transposition is the combination ofatrioventricular discordance and ventriculoarterialdiscordance. When this segmental arrangement isfound with usually arranged atrial chambers (sol-itus), the ventricular septum extends to the cruxcordis. But it is not in alignment with the atrial sep-tum because of the wedged position of the pulmo-nary outflow tract. The normal node is unable tomake contact with the ventricular conduction tis-sues. Instead, a node situated anterolaterally in theright atrioventricular orifice beneath the aditus tothe atrial appendage takes over as the connectingatrioventricular node (fig 17). When there is a co-existing ventricular septal defect that is viewedthrough a ventriculotomy, the non-branching bun-dle is seen to encircle anteriorly around the pulmo-nary valve and then to descend cephalad to thedefect (fig 18). In some examples of congenitallycorrected transposition with usual atrial arrange-ment, however, there is better alignment betweenatrial and inlet ventricular septa. Then both nor-mally sited and anomalous nodes can make contactwith the ventricular conduction tissues, producing a"sling" of conduction tissue.30 In congenitally cor-rected transposition with mirror-image atrial cham-bers (inversus), there is almost always good align-ment between atrial septum and inlet ventricularseptum. Despite the atrioventricular discordance, a

normally sited connecting node is then the rule.3233Ambiguous atrioventricular connection Anambiguous atrioventricular connection is foundwhen there are isomeric atrial chambers ("situsambiguus") and each atrium connects to its own

417

Bundle passes anteriorto pulmonary valve andcephalad to ventricularseptal defect

Incision in morphologicallyleft ventricle

Fig 18 Disposition ofthe ventricular conduction tissues incongenitally corrected transposition as viewed via aventriculotomy in the morphologically left ventricle.

ventricle. Then the septum extends to the crux butthe atrial septum is often deficient because fre-quently there is a coexisting atrioventricular septaldefect, usually with a common atrioventricularorifice. In these circumstances it is the pattern ofventricular architecture which determines the dis-position of the ventricular conduction tissues. Whenthere is a right-hand pattern ("d-loop") a normallysituated atrioventricular node is formed. Whenthere is a left-hand pattern ("1-loop") there is eitheran anomalous anterolateral connecting node or else

Lett-sided rudimentary/K&3 right ventricle

Incision in left ventricle

Bundle fram anteriornode

Fig 19 Disposition ofconduction tissues in double-inletleft ventricle with a left-sided rudimentary right ventricle asviewed through an incision in the main ventricular chamber.



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Usual interventricularcommunKation

Non-branching andbranching bundle onlett ventricular aspectof septum

Fig 21 Disposition of the ventricular conduction tissues inrelation to the interventricular communication in hearts withthe atria connected only to the left ventricle as seen from therudimentary right ventricle. The non-branching andbranching bundles are carried on the left ventricular aspectofthe septum and so are away from the operative field. Theypass beneath the usual interventricular communication butwould be superior to the rare apical defect. Only theterminal part ofthe right bundle branch is immediatelysubendocardial in the rudimentary ventricle.

Fig 20 Disposition ofconduction tissues in double-inletleft ventricle with right-sided (upper drawing) and left-sided(lower drawing) rudimentary right ventricle as viewedthrough a right atriotomy.

a sling of conduction tissue.20 These patterns arefound irrespective of the presence of right isomerism(" asplenia") or left isomerism ("polysplenia").

Hearts with univentricular atrioventricular connec-

dion In those hearts in which the atrial chambersconnect with only one ventricle, the orientation ofthe trabecular septum when present is the majordetermining feature. In hearts with a double-inletleft ventricle ("single ventricle with outletchamber") the septum never reaches to the crux.Thus whether the rudimentary right ventricle is rightsided or left sided there is always an anomalousanterolateral atrioventricular node.34 The positionof the rudimentary ventricle affects only the rela-tionship of the non-branching bundle to the pulmo-nary valve (if we assume the presence of ven-triculoarterial discordance).35 When the rudimentaryventricle is left sided the bundle encircles the pul-monary valve to reach the trabecular septum (fig19). When the rudimentary right ventricle is rightsided the bundle descends directly on to the septum,shown in figure 20 as it would be viewed through an

atriotomy and compared with a heart with a left-sided rudimentary ventricle. The important point isthat if the approach is from the rudimentary rightventricle (fig 21) the bundle will always be beneaththe interventricular communication. The same ruleshold good for classical tricuspid atresia. In this ano-maly the atria connect only to the left ventriclebecause the right atrioventricular connection isabsent.36 Because the tricuspid orifice is absent therecannot be an anterior and a normally sited node andthere is only a conglomerate mass of nodal tissue inthe floor of the blind-ending right atrium (fig 22).But when seen from the rudimentary right ventriclethe ventricular conduction tissues are the same asare found with a double-inlet left ventricle. Here it isimportant to distinguish the usual communicationfrom an apical muscular trabecular defect. The bun-dle will pass beneath the usual defect but above amuscular trabecular defect (fig 21). This must beremembered if a defect is to be enlarged surgically intricuspid atresia with ventriculoarterial discordance.

In hearts with a solitary indeterminate ventriclethere is no ventricular septum. An anomalousatrioventricular node is therefore the rule. Usuallythis will be anterolateral and the bundle will descendlaterally into the ventricular wall. Very frequently,however, large free-running trabeculae are found inthe ventricles of these hearts. When these trabeculaeextend to the atrioventricular junction they may

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Position ofatrioventricular node

Atrial septa1 defect Coron

Fig 22 Surgeon's view ofthe right atrial floortricuspid atresia showing the position of the at)node.

Straddling tricuspid valve

Regular node (no contact )

Fig 23 Disposition of the conduction tissue axis in a heartwith atrioventricular concordance and straddling tricuspidvalve as viewed through a right atriotomy.

carry the conduction tissues, either from an anteriornode or on occasion from a normally situatednode.3738 It is also important to distinguish heartswith a solitary indeterminate ventricle from heartswith huge ventricular septal defects. In the latteranomaly a rudimentary ventricular septum will befound extending to the crux. Because of this therewill be a normally sited node and conduction tissues.

Straddling tricuspid valve In hearts with straddlingof a normally positioned tricuspid valve the degreeof override of the straddling valve can vary consid-erably. When the valve is primarily connected to themorphologically left ventricle the heart in essencehas a double-inlet left ventricle and we categorise itas such. Then, as described above, an anomalousnode is to be expected and is indeed present. Butwhen the overriding tricuspid valve is connectedmostly to the morphologically right ventricle theatrioventricular connection is concordant. A nor-

mally sited node would then be anticipated. Theparamount feature, however, is the orientation ofthe septum away from the crux. In these circum-

Muscular floor stances an anomalous node is formed, albeit some-'of right atrium times posterior to the normal node (fig 23).

Condusions

With relatively few rules, the surgical disposition ofthe conduction tissues can be anticipated not only innormal hearts but also in congenitally malformedhearts. The rules demand attention to the sequential

aysinus segmental arrangement, particularly the morpho--iory sinus logy of the atrioventricular junction and the orienta-r in classical tion of the ventricular septum. These guides, how-rioventricular ever, are at best rules and not immutable laws. As

expected with rules, they will lead to correct deci-sions in most cases. But if there is any suspicion of adeparture from the rules it is always wise to check

ilalignment of conduction tissue disposition with intraoperativeseptum identification.

Two of us (RHA and SYH) are supported by grants*noalous node from the British Heart Foundation. We are indebted

to all our friends, colleagues and fellows who havebeen concerned with the findings described herein.We thank them for their many contributions to theconcepts derived and expressed. We also thank MsJoan Otto and Mrs Christine Anderson for theirhelp in the preparation of the manuscript.

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