Br Heart J 1982; 47: 419-29

Anatomical-embryological correlates in atrioventricularseptal defectSALLY P ALLWORK*

From the Department of Surgery, Division of Cardiovascular Disease, Royal Postgraduate Medical School,Hammersmith Hospital, London

SUMMARY Recent embryological studies have supported the consideration that the ventricular sep-

tum is multifocal in origin. These data have also provided excellent correlation of the morphology ofmalformed hearts with their embryology. In particular, atrioventricular septal defect correlatesaccurately with these observations on ventricular septation. Many of the names given to atrioven-tricular septal defect (for example ostium primum, persistent atrioventricular canal, endocardialcushion defect) indicate attempts at correlating the anatomy with embryology. None of these hasbeen very convincing.

In the light of this uncertainty, this review considers briefly the anatomy of the malformation andits ontogeny, and presents a hypothesis of the development of atrioventricular septal defect.

Although there is almost always a communication above the atrioventricular valves, the malforma-tion lies in the ventricular, not the atrial septum. Hearts with inlet septal defect without interatrialcommunication represent one end of the spectrum of anomalies, and those with common atrioven-tricular orifice, in which Fallot's tetralogy or single outlet heart may be associated, mark the otherend. The outflow tract malformations are not randomly associated, but are points in a huge range ofcardiac malformations.

Atrioventricular septal defect is a cardiac malforma-tion characterised externally by an abnormally shortposterior (diaphragmatic) ventricular surface.' Inter-nally there is a gap between the concave inferior rimof the atrial septum (which is usually well developed)and the atrioventricular valves. There may be a com-

mon atrioventricular valve, usually with five or sixleaflets of which the two major ones, anterior andposterior, bridge the ventricular septum.2 Often thereare two valves, each with three leaflets3 (Fig. la, b).The left component has an abnormally orientatedanterior leaflet which is separated medially from theseptal or posterior leaflet by an unsupported divisionwhich is usually called a cleft. Between the anterior

i and septal leaflets, at the obtuse margin, is the lateralleaflet (Fig. lb). When there is a bridging leaflet bet-ween either the anterior and lateral, or posterior andlateral leaflets, it produces an accessory orifice, and iscommon in atrioventricular septal defect. 'I The

*Supported by the British Heart Foundation.

Accepted for publication 27 November 1981

419

leaflets themselves are often poorly developed andmay have a verrucous appearance.The short external diaphragmatic surface of the

ventricular mass is reflected internally by a pro-nounced disproportion between the inlet and outletlengths of the left ventricle. In normal hearts thesetwo measurements are essentially the same.' Inatrioventricular septal defect the outflow tract of theleft ventricle is long and narrow compared with anormal heart (Fig. 2a, b) and the left anterior leafletinserts immediately behind and beneath the aorticvalve so that the latter is displaced. (Whether thevalve is really displaced has been debated at length46but the appearance is that of displacement.) It has lostits wedged position between the atrioventricular val-ves and the ventricular septum.78 Sometimes aorticoutflow tract obstruction occurs, and muscular obs-truction may be further aggravated by hypoplasia ofthe aortic valve.9Pulmonary outflow tract obstruction is especially

associated with a common valve with bridgingleaflets2 and may be the result of the infundibularseptal derangement characteristic of Fallot's tetral-

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Fig. I (a) The atrioventncular valves in atrioventncular septal defect (AVSD) seenfromthe right atrium (RA). The atrial septum (AS) is intact and its concave nmforms the "roof 'ofthe AVSD. The valve leaflets are attached at the same level to the summit ofthe muscularseptum, but the anterior (AL) and posterior (PL), or septal (SL) bridging leaflets areseparated by the "cleft" (atrowed). LLL, lateral left leaflet, ARL, anteriorright leaflet; CS,coronary sinus; FO, fossa ovalis; LFO, limbus fossa ovalis. (b) Same heart, left atrial(LA) view. The left valve does not resemble a normal mitral valve. It has three leaflets,anterior, septal or posterior, and lateral. The abnormally orientated anterior leaflet isseparated by the cleft (arrowed) from the septal or posterior bridging leaflet. LAu, leftauricle. Other abbreviations as in Fig. 1(a).

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Fig. 2 (a) The left ventricular outflow tract in the normal heart. The inlet septum (IS) andits atrioventricular component, indicated by the dotted line, give "depth" to the outflowvtract The atrioventricular septum extends from the membranous part of the septum (MS) tothe crux cordis permitting a posterior recess (arrowed). The anterior mitral leaflet (AML) isnormnally orientated, and extends from the base of the left aortic leaflet (L) to thenon-coronary leaflet (NC). Thus the aortic valve is "wedged" by the inlet septum and themitral valve, so that it lies beside rather than anterior to the mitral valve. Ao, aorta; LVOT,left ventricular outflow tract; R, right aortic leaflet. (b) The left ventricular outflow tract inAVSD. The narrowness is partly the result ofthe deficiency of the inlet septum and absenceof its atrioventricular component (note the septal autachment of the anterior leaflet (AL) ofthe left valve) and partly of attenuation of the outlet septum (OS). The loss of the normnal"'wedge" position of the aortic valve is shown by the position of the anterior leaflet whichextends from the left aortic leaflet to the commissure between non-coronary and right (R)leaflets. The membranous part of the septum is small, but intact. APM, anterior papillarymuscle group. Other abbreviations as in Fig. 1.

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ogy. Double outlet right ventricle, ventriculoarterialdiscordance, and single outlet heart are also some-times represented in atrioventricular septal defect.10'12The atrial septum is usually well developed in

atrioventricular septal defect; the anticipated normalanatomical elements are represented and atrial septa-tion is often complete, especially in those with "par-tial" or "transitional" defects (Fig. la and b). Patencyof the foramen ovale, however, occurs with some fre-quency, as in the normal heart, and secundum atrialseptal defects are common. In view of the frequentfinding of a normal atrial septum it is, perhaps, sur-prising that so many investigators have concluded thatthe development of this structure is at fault inatrioventricular septal defect. The defect itself is thegap between the inferior rim of the atrial septum andthe atrioventricular valves. The vertical dimension ofthis defect is approximately the same as the dispropor-tion between the inlet and outlet length of the ven-tricular septum measured in the left ventricle (Table1). These data strongly suggest that the anomaly con-cerns the inlet ventricular septum rather than theatrial septum.The foregoing paragraphs are not intended to be an

exhaustive description of atrioventricular septaldefect, but to indicate the spectrum which can accu-rately be correlated with the ontogeny of the ventri-cles and the ventricular septum.

Historical background

Atrioventricular septal defect was first described byPeacock'3 who considered imperfect ventricular sep-tation to be involved in the anomaly. In view of recentembryological findings, Peacock's view is of someinterest, as many of those who followed him consi-dered only defective or arrested septation to beresponsible. The history of the terminology and thegross classification are summarised in Table 2.

Table 2 Nomenclature and morphology in atrioventricularseptal defect

Peacock'3 1846

Rokitansky'4 1875

Watkins and Gross'6 1955

Wakai and Edwards'7 1956

Bedford et al. 18 1957Brandt et al. '9 1972

Piccoli et al.' 6 1979

Imperfection of atrial andventricular septa

Atrioventricular canal defectPartial -VSDComplete +VSDEndocardial cushion defectOstium primum ASDPersistent common AV canalPartial separate atrioventricular

valvesTransitional almost separate

atrioventricular valves±small VSD

Complete common atrioventricularvalve

Atrioventricular defectEndocardial cushion defectPartial -VSDComplete +VSDPartial separate atrioventricular

valvesComplete common atrioventricular

valve

ASD, atrial septal defect; VSD, ventricular septal defect.

With respect to "partial" and "complete" defects,most authors have adhered to Rokitansky'sl4 divisionbetween those with interatrial communication only(partial) and those who had an interventricular defectas well. 15-19 More recently, however, the two termshave been used to distinguish between separatedatrioventricular (partial), and common orifice(complete),5-6 but this distinction has not gained gen-eral currency, especially in surgical practice.2 3 Wakaiand Edwards20 described a third type of atrioventricu-lar septal defect which they termed "transitional" or"intermediate". These hearts had two "cleft"atrioventricular valves and a "narrow bridge of valvu-lar tissue in the midline (which) joins the anterior halfof each of the two cleft atrioventricular valvularleaflets with their respective posterior halves justabove the ventricular septum". In four of the sixhearts which they described there was a small inter-ventricular communication as well. Thus the "transi-

Table 1 Measurements ofinlet and outlet septa in left ventricle, and ofvertical dimension ofdefect in 10 hearts with atrioventricularseptal defect, situs solitus, and concordant ventriculoarterial connections

No. AV valves Septal length (mm) Inlet outlet Vertical dimensiondifference (mm) of defect (mm)

Inlet Outlet

1 Common 28 48 20 202 Separate 30 38 8 103 Common 29 41 12 164 Almost separate 36 55 19 215 Common 36 47 11 126 Separate 35 62 27 287 Separate 40 54 14 168 Common 25 40 15 169 Separate 40 65 25 2510 Common 31 51 20 22

Note: The discrepancy is much the same as the vertical dimension of the defect, and the figures accord with those given by Goor et al., ' that isthat there is about 300/o difference between the two. Thus the inlet septum represents about one third of the septal mass.

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tional" group are like those with "partial" atrioven-tricular septal defect because they have two separatedvalves, but they also resemble "complete" atrioven-tricular septal defect because they have an interven-tricular communication.With respect to the interventricular communica-

tion, the authors considered it to represent a defi-ciency of the membranous part of the septum.'720This structure, however, is usually present and intactin atrioventricular septal defect though it is abnor-mally located5 6 (Fig. 2b).

(1) VENTRICULAR SEPTUMIn order to correlate the features described, it isnecessary to describe the inlet ventricular septum andits atrioventricular component and to review recentembryological studies of ventricular septation.The inlet ventricular septum is that part of the septumwhich separates the inlet portions of the left and rightventricles. It is triangular in shape (Fig. 3) and isidentified in both ventricles as the smooth septumposterior and superior to the papillary muscles. Onthe right ventricular surface its boundaries are thetricuspid annulus and the chordal insertion of the sep-tal leaflet of the tricuspid valve. In the left ventriclethe boundaries are less well defined as the inlet sep-tum coalesces with the trabecular septum, whichextends basally to the membranous part of the ven-tricular septum.The atnioventricular septum: the leaflets of bothatrioventricular valves are attached to the inlet ven-tricular septum; because the tricuspid leaflets areattached slightly more towards the apex than those ofthe mitral valve, a part of the inlet musculature inter-poses between the right atrium and the left ventricle.

Fig. 3 (a) Diagrammatic outline (from a left ventricularangiocardiogram) to show the position of the inlet septum. It iswedge-shaped and occupies the area between the mitral valve andthe outlet septum (broken lines). (b) The same view in AVSD.The left valve (solid line), notched to indicate gap between theleaflets (the cleft), has "descended" into the left ventricle. Thebroken lines represent the deficient inlet septum.

This is the muscular atrioventricular septum. It liesbelow and posterior to the membranous part of theseptum, and is distinct from it (Fig. 2a). It extendsfrom its points of coalescence with the trabecular andmembranous septa to the crux cordis, producing awell defined posterior recess behind the anteriormitral leaflet. This recess not only gives an anteropos-terior dimension to the left ventricular outflow tractbut also contributes to the normal wedged position ofthe aortic valve between the two ventricular inlets andthe atrioventricular valves.7 8Normal cardiac septation: the ventricular septum, as itsadult morphology suggests, is multifocal in origin.21-27Before septation has begun the inlet and outletparts of the ventricular loop are distinguishable fromone another, and both contribute to the two adultventricles.24 As the loop elaborates an inlet-outletforamen becomes recognisable between the two limbsof the loop. This foramen is orientated in such a man-ner that subsequent septation will place its basal por-tion in the left ventricle and the apical part in the rightventricle. According to Wenink27 some of the anteriorleaflet of the mitral valve elaborates from endocar-dium from the apical part. The inlet part of the sep-tum develops from loose trabeculations in the post-erior wall of the embryonic ventricular mass, whilethe outlet septum develops from cushions in the outletpart and subsequent infolding of the walls of the out-let.28By 33 to 34 days (7 5 mm crown-rump) trabecula-

tions appear in the posterior ventricular wall. By 43days (15 mm crown-rump) these have coalesced todivide the inlet portions of the ventricle. This newinlet septum is obvious at 25 mm (53 days), and bythis time the process of undermining of the myocar-dium which will form the atrioventricular valves andtheir papillary muscles is under way.26 By 28 mm thisundermining is well advanced and the trabecular sep-tomarginalis is emancipated from the septal myocar-dium. In the adult heart no tensor apparatus is foundanterior to the trabecular septomarginalis.The endocardial cushions, for so long considered to

be precursors of the atrioventricular valves and to playa major part in both atrial and ventricular septation,48are prominent embryonic cell masses which are firstdistinguishable at 29 days (3-6 mm).26 They functionas valves in the embryonic heart29 30 and have regres-sed by the time (53 days) the atrioventricular valveshave elaborated from the ventricular myocardium.This, according to Van Gils,30 is their primary func-tion and they have no role in the septation of the inletsor in forming the adult atrioventricular valves.

(2) ATRIAL SEPTUMThe sequence of events which septate the atria beginsfractionally later than that of the ventricles. The sep-

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Table 3 Timing of events in normal cardiac septation

Gestational Crown-rump Eventage (d) length (mm)

29-30 3-6 Atrioventricular canal, endocardialcushions, inlet outlet foramen

32 5 Septum primum, ostium primum,endocardial cushions

33-34 7-5 Ventriculobulbar septum, posteriortrabeculations visible, right part ofatrioventricular canal in direct contactwith outlet of future right ventricle

36 10 Ostium primum almost obliterated:septum primum degenerates, ostiumsecundum formed

43 15 Posterior trabeculations form inletseptum, some begin coalescence toform atrioventricular valves

46 18 Septum secundum meets septumprimum anteriorly; septum primumforms foramen ovale

53 25 Endocardial cushions rapidly regressing,myocardial undermining emancipatestrabecular septomarginalis andatrioventricular valves

240 Birth Foramen ovale (septum primum)usually closes

tum primum and endocardial cushions are evident at32 days (5 mm) and the ostium primum is almostobliterated by 36 days (10 mm).31 The septumprimum then degenerates to permit a second path-way, the ostium secundum. At about 46 days (18 mm)the septum secundum meets the septum primumanteriorly to form a new opening-the foramen ovale.As is universally recognised, atrial septation is notcompleted until a short time after birth. Cardiac sep-

tation is summarised in Table 3.

Hypothetical development of atrioventricular septaldefect

From the foregoing review of normal septation it ispossible to hypothesise a developmental backgroundfor atrioventricular septal defect.

Although the atrioventricular valve deformities are

of supreme surgical importance, in anatomical termsthey are secondary to and consequent upon the mal-formation of the ventricular septum.

DEFECT OF INLET SEPTUMAbnormal development of the posterior ventricularwall produces a deficiency of the posterior, inlet sep-tum. Because the atrioventricular valves also developfrom the posterior myocardium, they are automati-cally deformed when the posterior septum is deficient.A small degree of posterior maldevelopment mightresult in a modest defect below and behind the valves,usually called a "canal type defect".32 Most "canaltype defects" are in fact excavated malalignment/perimembranous defects,S but where there is a

genuine inlet/outlet discrepancy of the left ventricle,

they are true atrioventricular defects.7 A bigger post-erior septal deficit would carry the valves down intothe ventricles, so the gap (defect) is between them andthe floor of the atrial septum (so-called ostiumprimum atrial septal defect). A huge deficit, readilymeasured anatomically (Table 1), permits the biginterventricular communication extending as far for-wards as the anterior septum. The septal deficiency islarge enough to leave the valves behind, so that themajor leaflets of the common valve float, bridging theseptal deficit. Because the anterior mitral leaflet elabo-rates from the outlet part of the loop, it remains highin the left ventricle, sometimes attached to the post-erior rim of the outlet septum and the aortic valve(Fig. 4a, b).

ADDITIONAL ANOMALIESBecause the inlet/outlet foramen develops before theinlet septum, coincidental maldevelopment of thisregion would just antedate inlet septum deficiency. Asthe foramen is orientated before its septation,myocardial cells are carried to the basal part of the leftventricle. In normal development these contribute tothe anterior mitral leaflet. The same probably occursin atrioventricular septal defect, permitting attach-ment of the left valve beneath the aortic valve whenthe defect is small. If the deficiency of the inlet sep-tum is large, however, then the aortic valve may loseits anchorage in the left ventricle and become a rightventricular structure. Maldevelopment of the outletseptum such as anterior deviation of the outlet (bul-bar) cushions would produce infundibular obstruc-tion such as that in Fallot's tetralogy33 and this maybe exacerbated by muscle bands and abnormal place-ment and development of the trabecula septomar-ginalis.34The final morphology of atrioventricular septal

defect is determined by two factors-the extent ofposterior septal deficiency governs the size of thedefect (and probably the valve morphology too) whilethe degree of maldevelopment of the ventriculo-infundibular cushions dictates the morphology of theoutflow tracts.

Correlates

(1) VENTRICULAR SEPTUM INATRIOVENTRICULAR SEPTAL DEFECTThe anatomical hallmarks of atrioventricular septaldefect are deficiency of the triangular inlet septumand absence of its atrioventricular component. Defic-iency of the inlet septum is shown by the short dia-phragmatic surface, which is represented internally bythe concave appearance of the septal surface(Fig. 4a, b). Absence of the atrioventricular part ofthe inlet ventricular septum is shown by three abnor-

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Fig. 4 (a) Left ventricular mew ofa smallAVSD (the same heart as in Fig. 1). The left valve is attached to the

rim of the oulet septum. The defect of the inlet septum (transilluminated), although quite small, is large enough to

cause the abnormal orientation and attachment of the valve leaflets. PA, pulmonary artery (b) Left ventricular

view of large AVSD There is no inlt Portion and the outlet septum is very attenuated. The inlet deficit extends

above, below and behind the left valve. The anteior leaflet of the valve is atached by secondary chordaetendineae to the rim ofthe outlet septum, while those of the septal leaflet insert into the summit of the remainingseptum. Other abbreviations as in Fig. 1.

malities. These are the abnormal plane of theatrioventricular valve(s), which are attached at thesame point to the septum (Fig. la, b), the loss ofanteroposterior dimension to the left ventricularoutflow tract (Fig. 2a, b), and the abnormal positionof the aortic valve. This structure is no longer wedgedbetween the ventricular inlets, but lies anterior andsuperior to the atrioventricular valve leaflets(Fig. 2b, 4b). All these features indicate defectivedevelopment of the inlet septum and absence of itsatrioventricular component. Specimens without aninteratrial communication ("canal-type" ventricularseptal defects) have in the past been excluded fromthe compass of the malformation under review32 but, ifthere is a disproportion between the inlet and the out-let lengths of the left ventricle, they can be classifiedas examples of atrioventricular septal defect.7

(2) ATRIOVENTRICULAR VALVESBecause the valves develop from the posterior wall ofthe embryonic heart and some elements of the inlet!

outlet cushions they are obligatorily malformed whenthe embryonic structures develop abnormally. Theirmyocardial origin is often suggested by the muscularanomalies of both chordae tendineae and papillarymuscles in atrioventricular septal defect (Fig. 5).Valve morphology accords with the degree of post-erior septum deficit-two orifices usually occur with afairly small defect and the severest form of the mal-formation has the free floating leaflets of the commonvalve sometimes predominantly opening into one orother ventricle and the huge defect both behind,below, and in front of the common valve (Fig. 4b).The plane of the valves or valve, always abnormal inatrioventricular septal defect, is governed in part bythe size of the remnant of the inlet septum. Usuallythe valves are attached to this structure so that they"descend" to an undue extent into the ventricles.Thus there is a gap where the septum should be, andinteratrial or ventriculoatrial shunting occurs throughthis gap.

Ventriculoatrial shunting may also occur through

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p

Fig. 5 The right valve in AVSD, right ventnrcular view. The valve leaflets are supportedmainly by muscle. There are truly tendinous cords only at the nrght lateral-postenrorcommissure (arrowed). Most of the remaining tensor apparatus is muscular. P, pulmonaryvalve; RV, right ventricle; PL, posterior leaflet; ARL, anterior right leaflet.

the "cleft" in the left valve. The disposition of leaflettissue is abnormal in atrioventricular septal defect sothat the left valve leaflets are abnormally orientated.The anterior leaflet itself is not cloven but is alwaysintact-the cleft is the tissue deficit between theanterior and posterior (septal) leaflets of the valve(Fig. 6a, b). The degree of deficit, however, does notnecessarily influence the function of the valve.35

Double orifice left atrioventricular valve resultsfrom an extra deep scallop (cleft) between either theanterior and lateral or the lateral and posterior leaflets,respectively, and a bridge of tissue between the twoleaflets. Deep scallops (clefts) are sometimes found innormal hearts, especially in the tricuspid valve,36 andreflect the undermining process which modelledthem.

Parachute mitral valve, present in about 7% ofcases,37 results from inadequate liberation of thefuture papillary muscles from the posteromedialembryonic wall.

(3) AORTIC OUTFLOW TRACTThe pathognomonic appearance of the left ventricularoutflow tract results from posterior septal deficiency(the "scooped-out" appearance)5 6 and the abnormalattachment, often with short, thick chordae,38 of theanterior left leaflet to the remaining outlet septum.Because this septum does not have its normal smoothcontinuity with the infundibular septum, the myocar-dium in the outflow tract attenuates, giving thecharacteristic tunnel-like outflow tract (Fig. 4b).

The displacement of the aortic valve is probablyreal; if the atrioventricular septum is absent, then theaortic valve is further superior and to the right than innormal hearts, as it is no longer wedged between thetwo atrioventricular valves.

(4) PULMONARY OUTFLOW TRACTFallot's tetralogy and atrioventricular septal defect arean uncommon combination; the hypothesis offeredconcerning the inlet septal maldevelopment does notcontradict the generally accepted views about theontogeny of the infundibular malformation.33 34

(5) ATRIAL SEPTUMThe atrial septum is often normally developed inatrioventricular septal defect, and this also lends sup-port to the view that the malformation concerns theventricular septum. None of the widely acceptedviews about atrial septation is challenged-rather,these views are supported. Defective atrial septationhas never satisfactorily explained atrioventricular sep-tal defect. Despite the usual cognomen "atrioven-tricular canal" the malformation does not remotelyresemble the arrangement in the newly-loopedembryonic heart. Arrested or abnormal developmentof this region cannot explain the malformation of theoutflow tract of the left ventricle, which may be mildor severe, but is always present.Common atrium does, of course, represent

rudimentary atrial septation, and is usually associated

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Fig. 6 (a) The left ventricular inflow in the normal heart. The anterior mitral leaflet issupported by both papillary muscles; the posteror leaflet is characteristicaly scalloped (1, 2,3). The anterior leaflet completely covers the upper third ofthe ventricular septum and leaflettissue is continuous around the annulus. (b) Left ventricular inflow in AVSD. The anteriorleaflet ofthe left valve is almost at right angles to the normal structure in (a) so that the upperthird of the venticular septum is visible. The leaflet is supported only by the anteriorpapiUary muscle group-the remainder of the leaflet inserts into the posterior wall of theoutlet septum. Distinct septal and lateral leaflets are recognised. Although there is valvetissue on the smmit ofthe ventricular septum there is a gap (the cleft, arrowed) between theanterior and septal leaflets. The anterior leaflet is never split-clefts are the spaces betweenthe leaflets. PPM, posterior papillary muscle group; APM, anterior papillary musclegroup. Other abbreviations as in Fig. 1.

Embryology of atrioventricular septal defect 427

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with the more severe forms of atrioventricular septaldefect (Fig. 6). There seems no reason to suppose anymore elaborate mechanism than defective atrial septa-tion to explain this coexistence.

Conclusion

The embryological studies reviewed here shed lighton the developmental background of the ventricularseptum and the atrioventricular valves. The embryol-ogy of the anatomical touchstones of atrioventricularseptal defect has been hypothesised, and thishypothesis challenges neither the accepted viewsabout atrial septation nor about outflow tract mal-development which may coexist.

References

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3 Carpentier A. Surgical anatomy and management of themitral component of atrioventricular defects. In: Ander-son RH, Shinebourne EA, eds. Paediatric cardiology1977. Edinburgh, London, & New York: ChurchillLivingstone, 1978: 477-90.

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7 Anderson RH, Wilkinson JL, Macartney FJ, et al. Theanatomy and conduction tissues of atrioventriculardefects. In: Gallucci V, Bini RM, Thiene G, eds. Selectedtopics in cardiac surgery. Bologna: Patron Editione, 1980:11-35.

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Requests for reprints to Dr S P Allwork, Departmentof Surgery, Royal Postgraduate Medical School,Ducane Road, London W12 OHS.

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