What Is a Ventricle?Robert H. Anderson, MD, and Siew Yen Ho, PhDDepartment of Paediatrics, Imperial College School of Medicine at National Heart and Lung Institute, London, England

Background. The concept of “one and a half” ventric-ular repair is to use “half” a ventricle to support thepulmonary circulation. The component makeup of anyventricle needs clarification for us to understand thenature of the so-called half ventricle.

Methods. The components of normal and abnormalventricles are reviewed.

Results. Normal ventricles possess an inlet, an apicaltrabecular component, and an outlet. This tripartite ap-proach is also logical in the description of congenitallymalformed ventricles. Rudimentary and incomplete ven-

tricles lack one or more of its component parts, and areusually hypoplastic. The location and morphology of therudimentary ventricles correlate with the disposition ofthe atrioventricular conduction system.

Conclusions. Recognition of the ventricular compo-nents permits determination of ventricular morphologyand guidelines for the location of the atrioventricularconduction axis.

(Ann Thorac Surg 1998;66:616–20)© 1998 by The Society of Thoracic Surgeons

The major expansion in the use of the Fontan proce-dure and its derivatives such as the cavopulmonary

connection has focused surgical attention on ventricularmorphology. Nowadays, the surgeon is increasingly ex-ploring the option of incorporating “half” a ventricle inthe pulmonary circulation as part of the so-called oneand a half ventricle repair. This, of course, begs thequestions of what is a ventricle, and can we have half aventricle? This is more so because from the introductionof the procedure, surgeons have tended to describe thehearts submitted to repair by the Fontan option as“univentricular.” The logician would surely question thepossibility of manufacturing one and a half ventriclesfrom a so-called univentricular heart! In reality, most ofthe hearts repaired in this fashion possess one big andone small ventricle [1], so the real question devolves onthe nature of the small ventricle in hearts with double-inlet left ventricle, tricuspid atresia, or pulmonary atresiawith an intact ventricular septum. That will be the focusof this review.

How Do We Analyze Normal Ventricles?

Any definition of chambers within the ventricular massmust start with a consideration of the ventricles of thenormal heart, because congenitally malformed hearts arederived from the building blocks of normality. Tradition-ally, the normal ventricles were considered to possess asinus and a conus [2, 3], yet it is difficult to discernanatomic evidence supporting the presence of two suchcomponents (Fig 1). In contrast, as pointed out by Goorand Lillehei [4], it is an easy matter to perceive thenormal ventricles as possessing an inlet, an apical trabec-

ular component, and an outlet. As we will see, thistripartite approach to ventricular analysis also permitslogical description of congenitally malformed ventricles,

Presented at the Workshop on “One and One-Half Ventricle Repairs,”Gubbio, Italy, Dec 6–7, 1996.

Address reprint requests to Prof Anderson, Department of Paediatrics,Imperial College School of Medicine at National Heart and Lung Insti-tute, Dovehouse St, London SW3 6LY, UK.

Fig 1. These illustrations, taken from the same heart, show how themorphologically right (a) and left (b) ventricles can simply be de-scribed as having inlet, apical trabecular, and outlet components.Note the marked difference in the patterning of the apical trabecula-tions in the two ventricles. (AV 5 atrioventricular; Pulm. 5 pulmo-nary.)

© 1998 by The Society of Thoracic Surgeons 0003-4975/98/$19.00Published by Elsevier Science Inc PII S0003-4975(98)00574-8

something that is much more difficult to achieve whenusing the concept of sinus and conus [5]. Although theventricles themselves can readily be categorized in termsof three components, however, the tripartite approachdoes not lend itself as readily to description of the normalventricular septum. This is because, in the normal heart,as a consequence of the deeply wedged location of thesubaortic outflow tract, much of the inlet of the rightventricle is separated by the muscular ventricular septumfrom the left ventricular outlet. And, because of theextensive free-standing muscular subpulmonary infun-dibulum, very little of the normal muscular septumseparates the outflow tracts in a fashion that justifies thedescription of a normal muscular outlet septum. In thenormal heart, therefore, it is preferable only to recognizethe overall muscular septum, and to distinguish it fromthe relatively small membranous septum. The atrioven-tricular conduction axis is then sandwiched betweenthese two normal ventricular septal components, withthe axis itself branching on the crest of the muscularventricular septum.

How Do We Recognize Normal and AbnormalVentricles?

Already we have described the relationships of the rightand left ventricles of the normal heart. But how do wedistinguish between them? And what permits us torecognize their rudiments when the ventricular mass isabnormally constituted? In answering these questions,we are guided by an important philosophical principleestablished by Van Praagh and colleagues [6]. The stim-ulus for the concept was provided by problems that usedto exist in defining “univentricular hearts.” It was enun-ciated when we, with our colleagues, had suggested thathearts with double inlet to a right ventricle could beconsidered “univentricular” [7]. We had derived thissuggestion, illogically as it happens, from anotherpremise put forward by Van Praagh and colleagues [8],namely, that the criterion for distinction of a single

ventricle was the presence of double-inlet atrioventricu-lar connection. In those hearts that we described as beinguniventricular examples of a right ventricle [7], there wasunequivocally another chamber present within the ven-tricular mass (Fig 2). Van Praagh and colleagues [6]pointed out, quite rightly, that this second chamber couldreadily be identified as a rudimentary and incompleteleft ventricle because of the structure of its apical com-ponent, which they described as the ventricular sinus.Because of this, they argued that malformed ventriclesshould be recognized morphologically on the basis oftheir most constant component, and they dubbed thisprinciple the “morphologic method.” For the hearts inquestion, namely, those with double-inlet right ventricle,it is clear that the apical trabecular component is the mostconstant part of the left ventricle, because most examplesof double-inlet right ventricle also possess double outletfrom this dominant ventricle (see Fig 2). The principle ofthe morphologic method is equally applicable, nonethe-less, to hearts that possess double inlet to a dominant leftventricle, with its fine apical trabeculations, because therudimentary and incomplete chamber can be recognizedas a right ventricle on the basis of the coarse trabecularpattern of its apical component (Fig 3). Thus, by usingthis principle of defining ventricles according to thepattern of their apical part, we can recognize abnormalventricles as being morphologically left or morphologi-cally right. Concentration on apical trabeculations alsopermits recognition of the rarest ventricular pattern, thatof a solitary and indeterminate ventricle. In this lattermalformation, which truly produces a univentricular ar-rangement, the apical component is uniformly coarse,much coarser than a dominant right ventricle (compareFigs 2a and 4). Thick muscle bundles percolate through-out the ventricle, permitting distinction from those heartswith a huge ventricular septal defect. In the latter mal-formations, a remnant of the apical muscular septumseparates the apical trabecular components of the mor-phologically right and left ventricles.

Fig 2. These illustrations show thecoarsely trabeculated right ventricle(a) connected to both atriums (dou-ble inlet) and supporting both arte-rial trunks (double outlet). The api-cal trabecular component of the leftventricle (b), in posteroinferior posi-tion, forms an incomplete and rudi-mentary ventricle. (AV 5 atrioven-tricular; VSD 5 ventricular septaldefect.)

617Ann Thorac Surg 11⁄2 VENTRICLE ANDERSON AND HO1998;66:616–20 VENTRICULAR MORPHOLOGY

Are Hearts With Double-Inlet AtrioventricularConnection Univentricular?

As we have discussed, the traditional definition of asingle ventricle, or a univentricular heart, was the pres-ence of double-inlet atrioventricular connection. Thisapparently stems from the precedent of Taussig [9], whonamed such hearts “single ventricle with rudimentarychamber,” and implicitly denied the second chamber itsventricular status. This approach was subsequently em-braced by Van Praagh and colleagues [8], but theseinvestigators, without specifying any reason, denied theuniventricular accolade for hearts with atrioventricularvalvar atresia. The concept subsequently received en-dorsement from Edwards [10], but again no reason wasgiven why hearts with atrioventricular valvar atresiawere excluded from the univentricular category. This wasthe situation when our colleagues and ourselves studiedthe structure of the most frequent prototype of tricuspidatresia [11]. Apart from the obvious morphologic similar-ities between the rudimentary ventricles in hearts havingdouble-inlet left ventricle and those with tricuspid atresia(Fig 5), morphometric analysis showed that what differ-ences existed between the morphology of the smallventricles depended on the ventriculoarterial rather thanthe atrioventricular connections [12].

On the basis of our first study, we had proposed thatthe typical variant of tricuspid atresia should also be categorized as a “univentricular heart” [11]. This pro-

posal failed to win unanimous approval, and rightly so,because typical tricuspid atresia clearly possesses a dom-inant left ventricle along with a rudimentary and incom-plete right ventricle. In tricuspid atresia, the right ventri-cle is incomplete because of the total lack of the rightatrioventricular connection and the inlet component ofthe right ventricle (see Fig 5a). It would have been muchmore preferable if, from the outset, we had accepted thebiventricular nature of “classic” tricuspid atresia. Wecould then have argued, by comparable analysis, thatdouble-inlet left ventricle is similarly biventricular, therudimentary right ventricle being incomplete in thisinstance because both inlets are committed to the dom-inant left ventricle (see Figs 3b, 5b). A similar situationwould exist with double-inlet right ventricle, which co-exists with an incomplete and rudimentary left ventricle(see Fig 2). Of the hearts that possess a double-inletatrioventricular connection, therefore, only the exampleswith an indeterminate ventricle are truly univentricular.In all hearts with double inlet, nonetheless, the atrioven-tricular connection certainly is univentricular, becausethe atrial chambers are connected only to the dominantor solitary ventricle [13].

Do All Hearts With Atrioventricular ValvarAtresia Have Univentricular Connections?

The analysis that shows that hearts with double-inletventricle have a univentricular atrioventricular connec-tion shows equally well that some examples of atrioven-tricular valvar atresia must also fall within this category,but not all. Hearts can have tricuspid atresia because themorphologically tricuspid valve is present but imperfo-rate, and can show this arrangement even when the

Fig 3. These illustrations show double inlet to a dominant left ven-tricle (a). In this circumstance, it is the apical trabecular componentof the right ventricle (b) that forms the basis of an incomplete andrudimentary ventricle in the anterosuperior position, in this instancewith the pulmonary (Pulm.) trunk arising from the rudimentaryventricle (concordant ventriculoarterial connections). (AV 5 atrio-ventricular; VSD 5 ventricular septal defect.)

Fig 4. This illustration shows a truly solitary ventricle, which hasvery coarse trabeculations of indeterminate morphology throughoutits apical component. (AV 5 atrioventricular; Pulm. 5 pulmonary.)

618 11⁄2 VENTRICLE ANDERSON AND HO Ann Thorac SurgVENTRICULAR MORPHOLOGY 1998;66:616–20

tricuspid valve is deformed by Ebstein’s malformation. Inthis setting, the distal component of the right ventriclecan be remarkably similar to the small ventricle seen in“classic” tricuspid atresia. This is because the imperfo-rate valvar membrane is formed at the distal extent of theinlet component, sequestering the apical trabecular andoutlet components as the so-called functional right ven-tricle. The right ventricle, therefore, can be small evenwhen it possesses all its component parts. Such anarrangement with ventricular hypoplasia is seen in thoseexamples of tricuspid atresia in which the imperforate

valvar membrane is formed at the atrioventricular junc-tion (Fig 6).

Simply because a ventricle possesses all its componentparts does not mean that it will always be of normal size.This principle is exemplified by hearts showing pulmo-nary atresia with an intact ventricular septum, where thespectrum of ventricular cavity size is well-explained onthe basis that all the right ventricles possess their antic-ipated three parts, but that mural hypertrophy “squeezesout” the ventricular cavity, the squeeze starting at theapex and progressing to involve also the infundibularcomponent. The spectrum of cavity size in pulmonaryatresia with intact septum can be well understood on thebasis of the tripartite structure of the right ventricle, butit is a mistake to describe the ventricles as being “uni-partite” or “bipartite.” All possess their three parts, butlack the normal volume of their cavities. The answer tothe initial question, therefore, is that all hearts withatrioventricular valvar atresia do not have univentricularatrioventricular connections, and neither do most heartswith arterial valvar atresia and an intact ventricularseptum. Hypoplastic ventricles need to be analyzed interms of not only their component make-up, but alsotheir size. It is a rule that all hearts that possess univen-tricular atrioventricular connections will, of necessity,have one dominant and one rudimentary and hypoplas-tic ventricle. Some hearts with biventricular atrioventric-ular connections will also have one of the two ventricleswith a comparably hypoplastic cavity. The size of a ventricleis not dependent solely on its component make-up.

How, Then, Can We Define a Ventricle?

On the basis that normal ventricles possess three parts,and that the apical trabecular component is the mostconstant of these parts, a good working definition of aventricle is any chamber within the ventricular mass thathas an apical component. Such ventricles will be normal

Fig 5. These pictures of the rudimentaryand incomplete right ventricle from heartswith (a) double-inlet left ventricle and (b)tricuspid atresia show the remarkable simi-larity in morphology when the ventriclessupport the same arterial trunk (in thesecases the aorta). (AV 5 atrioventricular;Pulm. 5 pulmonary; VSD 5 ventricularseptal defect.)

Fig 6. This illustration shows tricuspid atresia produced by an inper-forate valve interposing between the right atrium and the right ven-tricle. In this heart, the atrioventricular connections are biventricularand concordant and the right ventricle, although small, possesses allof its component parts. (TV 5 tricuspid valve.)

619Ann Thorac Surg 11⁄2 VENTRICLE ANDERSON AND HO1998;66:616–20 VENTRICULAR MORPHOLOGY

when they possess an apical component together with aninlet and an outlet. Normal ventricles are of right or leftmorphology, depending on the trabecular pattern of theapical component, and such right and left ventriclesalways coexist within the same ventricular mass. Normalventricles can be hypoplastic, however, particularly inthe setting of severe stenosis or atresia of their outletcomponent. Abnormal ventricles can possess more orless of their anticipated three components, and are foundin hearts with abnormal segmental connections. Thesolitary ventricle of indeterminate morphology is theonly true univentricular heart, and it usually coexistswith double inlet and double outlet from the solitarychamber (see Fig 4). It can be found, nonetheless, withabsence of one or other of the inlets or outlets. The otherabnormal ventricles will be either of right and left mor-phology, but will coexist within the same ventricularmass. One of the ventricles will be dominant, and almostalways will be the one attached to the atriums. The smallventricle will be incomplete and rudimentary because itlacks one or more of its component parts, and it usuallywill also be hypoplastic.

Rudimentary ventricles will be of right or left morphol-ogy depending on the nature of their apical trabecula-tions, and can themselves be right-sided or left-sided inrelation to the dominant ventricle. Rudimentary andincomplete right ventricles are always located anterosu-periorly relative to their dominant partner, whereas ru-dimentary and incomplete left ventricles are locatedposteroinferiorly within the ventricular mass. Because ofthis, the apical trabecular septum is always malalignedrelative to the atrial septum in hearts with univentricularatrioventricular connection to a dominant left ventricle,so the atrioventricular node is an anterior structure. Inthis setting, the ventricular septal defect can always beenlarged, irrespective of the atrioventricular or ventricu-loarterial connections, and irrespective of the right-sidedor left-sided location of the rudimentary right ventricle,by resecting a wedge of apical septum closest to theobtuse margin of the ventricular mass. The outlet septumcan also be resected if of suitable size, because it nevercarries the conduction axis.

In hearts with dominant right ventricles, the atrioven-tricular conduction axis arises from the regular atrioven-tricular node when the rudimentary and incomplete leftventricle is left-sided, but there will be an anterioratrioventricular node, or a sling of conduction tissue [14],when the rudimentary left ventricle is right-sided, be-cause the ventricular mass will then show left-handtopology. The atrioventricular conduction axis will bebizarrely located in solitary ventricles of indeterminatemorphology [15], because such hearts lack completely theapical trabecular septum, which usually carries the ven-tricular bundle branches.

On occasion, hearts will be found in which a subarte-rial infundibulum is sequestered as a separate compo-nent within the ventricular mass. Within our proposedworking definition, such an infundibular chamber willnot be considered a ventricle because it lacks an apicalcomponent. Sometimes an infundibulum can be seques-

tered together with part of the apical trabecular compo-nent, as in so-called double-chambered right ventricle,but this anomaly represents division of a normally struc-tured right ventricle rather than formation of an abnor-mal ventricle in its own right [16].

Conclusions

Definition of a ventricle, be it normal or abnormal, on thebasis of its apical trabecular component permits logicalrules to be established for determination of its morphol-ogy. This sets the scene for subsequent determination ofthe atrioventricular and ventriculoarterial connections, aswell as establishing guidelines with which to predict thelocation of the atrioventricular conduction axis.

This work was supported by the British Heart Foundation.

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