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Uncommon Conal Pathology in Complete Dextrotransposition of the Great Arteries with Ventricular Septal Defect* M . Quero l i d n e z , M.D., and V . Pdrez Martinez, M.D. The anatomic and clinical findings are reported in five cases of complete dextrotrausposition of the great arteries with an uncommon and varied c o d pathology, different from the usual conal architecture of classic complete dextrotranspositionof the great arteries. The following points summarize the most important findings: (1) The ventricular septal defect was due to a positional anomaly or a deficiency of the conal septum in all of these cases. (2) The conal free wall distribution was different from that designated as characteristic of clansic complete dextrotraqmWon of the great arteries, ie, in three cases, a persistence of the subpulmonary conal free wan preventing the pulmonary-mitral fibrous continuity. (3) A positional deviation of the conal septum inside the conus resulted in obstructive anomalies of the subpuE monary outtlow tract in two cases and of the subaortic in another. (4) There was overriding of the conal septum of the ventricular septum in two case& (5) In keeping with item 4, the observation of the pulmonary outflow tract of three additional cases of classic complete dextrotransposition of the great arteries with intact ventricular septum sqggesb a possible origin of the fibromuscular subpulmonary sten& in these patients: the abnormal fusion line between the ventricular septum and a malorientated conal septum. Some important angiocardiographic characteristica of these cases are also shown. The surgical impHcations of these particular anatomic characteMc8 are disc& he conus cordis has long been a cardiac structure Tin which cardiologists and pathologists have shown a great interest, and alterations in its morphology and/or position have been recognized in various forms of congenital cardiac defe~ts."~ Among the main alterations reported are: (1) a malalignment between the conal and the ventricular septa giving rise, in certain malformative c o m p l e x e ~ , ~ "a~ ventricular septal defect and an ovemding of one of the great arteries of the ventricular septum; (2) an unequal partitioning of the conus by an eccentrically located conal septum resulting in a narrowing of one of the outflow tracts as may occur in some cardiac malformation^;^^ and (3) an abnormal reabsorption or persistence of certain areas of the conal free wall, a fact that, for many authors and for a long time, has been considered of utmost importance in explaining the changes in the conal morphology and position found in certain forms of congenital heart di~ease.'-~-'~'~"' In this report we consider as complete dextrotransposition of the great arteries only those cases in which two-thirds or more of the aortic and pulmonary orifices originated in the morphologically and topographically right and left ventricle, respectively. The term "conus cordisn applies to the outflow tracts of both ventricles situated under the semilunar valves. It derives from the primitive conus cordis once it has been partitioned by the conal septum and the portion pertaining to the left ventricle has been reabsorbed. The "conal septumn is a muscular septum that divides the primitive conus cordis in the definitive aortic and pulmonary outflow tracts. When viewed from the subaortic outflow tract in cases of complete dextrotransposition of the great arteries. the conal septum lies beneath the right and left coronary leaflets of the aortic valve. For descriptive purposes, the conal septum may be said to be delimited by imaginary lines running vertically through the midpoints of the right and left coronary leaflets of the aortic valve. The subaortic conal free wall is composed of the remainder of the subaortic conus that lies outside the two imaginary lines. In most cases of complete dextrotransposition of the great arteries the conal septum divides the primitive conus cordis in a subaortic and a subpulmonary conus located over the right and left ventricle, respectively. The subaortic conus is generally well developed, being muscular all around, as much in the area of the conal septum as in that of the conal free wall. Located between an anterior and rightsided aortic valve and the tricuspid leaflets, it prevents the fibrous continuity between them. The subpulmonary conus has been reabsorbed during the expansion of the conus cordis toward the left ventricle, being muscular only at the region of the conal septum. A subpulmonary conal free wall is, therefore, absent in the majority of the cases of complete dextrotransposition of the great arteries, a fact accounting for the fibrous continuity that is found in most cases between the posterior and left-sided pulmonary valve and the mitral leaflets. Nevertheless, in some cases of transposition of the great arteries, a subpulmonary conal f.ree wall may persist, preventing the mitral-pulmonary fibrous continuity 'From the Servicio de Cardiologia PediAtrica, Clinica Infantil La Paz, Madrid, S ain. Manuscript r e c e i v s September 4, 1973; revision accepted March 26. Reprint requests: Dr. Quero Jimenez, Clinics Znfantil LA Paz, Madrid,Spain CHEST, 66: 4, OCTOBER, 1974 UNCOMMON CONAL PATHOLOGY 411 Downloaded From: http://publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/20957/ on 05/06/2017 which characterizes the cases with classic complete dextrotransposition of the great arteries. In them, the subpulmonary conal free wall is composed of the subpulmonary muscular conus, with the exception of the region of the conal septum. In this paper, the term "septal band" has been used to describe a morphologic feature of the right ventricular aspect of the ventricular septum characterized by: (1) smooth appearance; (2) a superior concave border, the posterior end of which receives the attachment of the papillary muscle of the conus; and (3) an inferior extremity receiving the medial end of the moderator band. In most cases of complete dextrotransposition of the great arteries there is a normal junction between the conal septum and the superior concave extremity of the septal band, the line of fusion between both structures being sometimes recognizable in heart tissue of young patients. When a ventricular septal defect is present, it is most often subpulmonary and involves the membranous ventricular septum. In this report, the term "classic complete dextrotransposition of the great arteries" will be reserved for those cases with the morphologic characteristics described above, including the spatial relationships between the ventricles and both great arteries as stated in the above definitions. The morphologic characteristics used to define classic complete dextroh-anspositionof the great arteries were found in 61 cases (88.4 percent) of a total of 69 cases of this malformation examined at necropsy in our hospital. They were lacking in the remaining eight cases (11.6 percent). The study of five cases of this last group is the main object of this remrt. In all five patients an uncommon and varied conal pathology was found, very Werent from the morphologic charae teristics of classic complete dextrotranspmition of the great arteries described above. For the purpose of understanding the specific type of amal alteration present in every case the following data were studied: ( 1 ) position and ventricular origin of the great arteries; ( 2 ) the conal septum and conal free wall; (3) the septal band; ( 4 ) associated obstructive lesions; and (5) position and boundaries of the ventricular septal defect. Particular attention was paid to the relationship between the septal band and the papillary muscle of the conus, the moderator band and the anterior papillary muscle. The morphology of the superior, concave, bifid border of the septal band was considered useful for its recognition. The papillary muscle of the conus, usually situated on the posterior division of the superior end of the septal band, was considered a useful and reliable structure in locating the septal band. Nevertheless, in some cases, chordae tendinae similar to the papillary muscle of the conus may be found on the conal septum, or certain muscular portions of it. Such a malformation is frequently found in dextmposition of the anterior extremity of the conal septum which, in these circumstances, is often prolonged in an anomalous muscle bundle that extends to the right of the septal band, ending in the anterior free wall of the right ventricle.17 In these cases with anomalous muscle bundle which may receive some short chordae tendinae similar to those of the papillary musde of the wnus and conceal the septal band for which it may be mistaken, impotCant e m may be made in the interpretation of the anatomic nature of the ventricular septal defect. To avoid mistakes it is necessary to identify the septal band, looking for it in the right aspect of the ventdcdar septum of which the septal band is a component, unlike the above-mentioned anomalous musde bundle, which is more related to the anterior right ventricular free wall (Fig la). The pulmonary outeow tracts of three additional cases of classic complete tramposition of the great arteries with intact ventricular septa were also studied in order to illustrate a hypothetic origin of the fibromuscu1a.r subpulmonary stenwis. subaortic muscular conus (SACFW) separating fibrous tissue of aortic ( AV ) and tricuspid valves ( TV ) . Obserue anterior extremity of conal septum ( CS ) ending in anomalous muscle bundle ( AMB) located anteriorly and to right of septal band ( SB). Note ventricular septal defect (VSD) between conal septum above and septal band below. APM: anterior papillary muscle; MB: moderator band; SACFW: subaortic conal free wall. ( l b lower). Left ventricular cavity showing subpulmonary conal free wall (SPCFW) separating fibrous tissue of pulmonary ( W ) and mitral valves (MV). Note conal septum ( CS ) ovemding ventricular septum through ventricular septal defect (VSD). Posterior portion of this defect (mrow 1 ) is subaortic, anterior defect ( a m 2 ) being apparently subpulmonary. There is, however, no direct connection between right ventricle and pulmonary artery due to interposition of anomalous muscle band (Fig la: AMB) between. Apparent cleft in anterior mitral leaflet ( ) is only d a c t u a l tear. 412 QUERO JIMENEZ, PERU MARTINEZ Downloaded From: http://publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/20957/ on 05/06/2017 CHEST, 66: 4, OCTOBER, 1974 The patient is a four-month-old boy. The external view discloses the aorta located to the right and slightly posterior to the pulmonary artery. There is a persistence of the conal free wall, both below the aorta and the pulmonary artery preventing fibrous continuity between both semilunar valves and the atriwentricular fibrous tissue. The conal septum wemdes the ventricular septum as m case 1, and divides the conus into two different sized parts: one, a narrower subpulmonary in the anterior and left area of the conus, and the other, a larger subaortic corresponding to the remaining area of the amus. The ventricular s e ~ t adefect l is limited bv the superior concave border of the-septal band below, a& the conal septum and both semilunar valves above. As in case 1, the anterior end of the conal septum ends in an anomalous muscle band situated anteriorly and to the right of the septal band. FIGURE 1 ( c , upper). General arrangement of great arteries, conal free wall (CFW), canal septum (CS) and superior border of septal band (SBSB) of case 1. Aorta (Ao) arises almost entirely from right ventricle located to right and slightly posterior to pulmonary artery (PA), which originates from left ventricle. Posterior conal free wall (CFW) prevents fibrous continuity between great arteries and atrioventricular valves (TV and MV). C o d septum ovemdes superior border of septal band, crossing ventricular septal defect (dark shadowed) from right ventricle, where anterior extremity ends in anterior conal free wall, to left ventricle, fusing with posterior conal free wall. Note how hole between two branches of septal band is divided by conal septum into two ventricular septal defects, posterior one in relation to aorta and an anterior one to pulmonary artery. A: anterior, P: posterior, R: right, L: left. (Fig l a and b reproduced with permission of Coeur ). The patient is a two-month-old boy. In the external examination, the aorta is to the right and slightly posterior to the pulmonary artery. The internal findings reveal both the entire subaortic and subpulmonary conal free wall and the conal septum and there is a lack of fibrous continuity between the aorta and the tricuspid and the pulmonary and mitral valves measuring 2 and 4 mm, respectively ( Fig la, l b ) The conal septum ovemdes the interventricular septum ( Fig lc), with its posterior and anterior junctions with the conal free wall m the left and right ventricle, reqectively. The anterior end of the conal septum terminates in a muscular bundle located to the right of the septal band, for which it might be mistaken. The ventricular septal defect, partially obstructed by the ovemding conal septum (Fig lb), is situated above the superior concave border of the septal band and below the conal septum ( Fig l c ) . . CHEST, 66: 4, OCTOBER, 1974 The patient is a three-and-one-half-month-oldboy. The aorta is anterior and slightly to the left of the pulmonary artery, and there is a persistence of subaortic and subpulmaary conus (Fig 2u. 2b). The canal septum has developed rather posteriorly in the conus, its left end joining the posterior division of the septal band (Fig 20, 2b). The posteriorly developed conal septum divides the conus into two infundibula very different in size, the ro(subpulmonary) portion being mu& smaller than the levoanterior ( subaortic) ( Fig 2b). The limits of the ventricular septal defect are: the superior border of the septal band below, the anterior aspect of the conal septum behind, the aortic semilunar valves (the anterior part of the left coronary and the posterior part of the noncoronary l e h ) together with the free wall of the left ventricle above (Fig 2u, 2b). CASE4 The patient is a one-month-old boy. The aorta originates anteriorly and to the right of the pulmonary artery. There is a persistence of the subaortic conal free wall which preventa the fibrous continuity between the aortic and tricuspid leaflets. The posterior subpulmonary conal free wall has disappeared, which allows the fibrous continuity between the pulmonary and mitral valvular tissue. The conal septum is incompletely developed, with the superior part of the posterior conal ridge absent. The resulting infundibular ventricular septal defect is small, being limited by the posterior aspect of the anterior conal ridge in front, the anterior aspect of the subaortic conal free wall behind, the anterior part of the coronary aortic leaflet above, and the inferior normally developed part of the posterior conal ridge below. The patient is a four-month-old boy. The aorta is anterior to the pulmonary artery,the subaortic conal free wall is well developed (Fig 3), and the subpulmonary c o d free wall is nonexistent, allowing fibrous contact between the pulmonary and mitral valves. The conal septum is well developed and, instead of joining the superior extremity of the septal band, ends anteriorly, in the anterior right ventricular free wall (Fig 3). The ventricular septal defect is limited by the superior border of the septal band below and the pulmonary semilunar valves above (Fig 3). Viewed from the left ventricle, it occupies the region of the anterior smooth component of the ventricular septum. The aortic o u t h v tract is n a ~ ~ ~and ~ ea dpreductal , aortic coarctation exists. UNCOMMON COWL PATHOLOGY 413 Downloaded From: http://publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/20957/ on 05/06/2017 BSB FIGURE 3. Right ventricular view of case 5. In this case aorta (Ao) originated entirely from right ventricleand approximately two thirds of pulmonary artery orifice connectedJeft ventricle. Note ventricular septal defect (VSD) locatedbtween the two branches of superior border of septal band (SB). Note how conal septum (CS), instead of 6lling hole between two divisions of septal band, ends in anterior wall of' right ventricle, resulting in a narrowed aortic outflow tract. In case this dextroposition of conal septum coexisted with subaortic narrowing and aortic isthmus hypoplasia. ( R e p r o d u d with permission of Coeur). cases 1, 2, 4, and 5 revealed cardiomegaly and increased pulmonary circulation, with a narrow vascular pedicle in cases 1,4, and 5; the heart size and the pulmonary circulation were normal in case 3, and the vascular pedicle was not strikingly narrow in cases 2 and 3. The electrocardiogram showed an AQRS ranging between 100' and - 135". There was biventricular enlargement in cases 1,2,4 and 5. Isolated enlarge ment of the right atrium and ventricle characterized case 3. In case 1, "q" waves existed in leads 1 and aVL. The cardiac catheterization and angiocardiog- + A FIGURE 2. ( a upper). Note conal septum (CS) joining posterior division (arrow) of septal band (SB). Posterior conal free wall (PCFW) prevents fibrous continuity between aortic (AV)and bicuspid (TV)valves. Boundaries of ventricular septal defect (VSD) are superior border of septal band below, anterior aspect of conal septum behind, and aortic valves above. (2b lower). Note position of aorta (Ao), anterior and somewhat to left of pulmonary artery (PA). Obseroe, also, how conal septum (CS), very posteriorly situated, joins posterior division of superior border of septal band (SBSB). Result of posterior location of a n a l septum is narrowing of pulmonary outflow tract in comparison with aortic. Posterior canal free wall (CFW) persists both beneath aortic and pulmonary valves, preventing fibrous continuity between semilunar and atrioventricular valves (TV and MV). Observe aorta overriding ventricular septum (SB). A: anterior, P: posterior, R: right, L: left. CLINICAL FINDINGS The clinical &dings in the five cases are shown in Table 1. A brief report of the most outstanding features follows: All patients were infant boys, with the age of death ranging between one and four months. Heart failure and hypoxemia were present in cases 1, 2,4, and 5. In case 3, with pulmonary stenosis, hypoxemia was the dominant feature in the clinical picture. The plain radiologic examination of patients in Table I-Main Clinical Findinga i n Five Carer of Complete Dextrotranapdtion of the Great Arterier, d t h Ventricular S e p t d Defect Cam. Age, Sex Clinical No. Mo. 1 2 Picture m X-Ray ECG Position of Great Arteries in Film Fin- ~ HF+++ HS+++ AQRS H+ PC+ + + CVH -A 7 ~ ~ p b AofromRV:PAfrom LV; P A and Ao more pasterior than nornul: subpulmo- 2 3 4 3 m m HF+++ HS++ H++ PC++ VP-HF HS H+ ++ PC I VP-- 4 1 m 6 4 m nary mnun AQRS~OOOAO~IO~RVW~~II d v u l u plane lowthan normd &QRB13n0 Ao from both RAH RVH ventricles. overti* V8. PA from LV; Ao anterior and to left of PA HF+++ HS+++ A ~ ~ s i i o ~ ~ ~ f ~ ~ m ~ v w i t h antenor position H+ PC+ ++ CVH vp& PA is not viaruli.sd HF+++ HS+++ AQRS 1000 H+ PC+++ CVH VPl 'HF: h e u t failure: H: hypoxemia; HS: h e u t sin:PC: pulmonary cimub tion: VP: vrrscular pedicle; CVH: combined ventricular hypertrophy; RAE: right atrial hypertrophy; RVH: right ventricular hypertrophy; Ao: .art.; PA: pulmonary artery; RV: &ht ventride; LV: ldt ventricle; VS: ventricular nptum; 1 : decnased or numwed; :n o d . 414 QUERO JIMENEZ, PEREZ MARTINEZ Downloaded From: http://publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/20957/ on 05/06/2017 -- CHEST, 66: 4, OCTOBER, 1974 FIGURE6. Left ventricular view of a case with transposition of the great arteries and intact ventricular septum. Observe how the overriding conal septum (CS) ends rather posteriorly in the left aspect of the ventricular septum (VS) instead of forming the smooth anterior portion of this latter structure. Note the pronounced fusion line (arrows) between this malorientated conal septum and the ventricular septum. The continuous deposition of fibrin in this irregularity of the left septal surface could be the origin of the fibro-muscular subpulmonary stenosis present in some cases of transposition of the great arteries with intact ventricular septum. septal defect in cases 3 and 5 was the same as in cases 1 and 2, with the superior limit the aortic semilunar leaflets in case 3 and the pulmonary valve in case 5. The posterior location of the aortic in relation to the pulmonary valve (Fig 1 ) in cases 1 and 2 is in perfect accordance with the smaller dimension of the subaortic muscle (posterior conal free wall) as compared with the subpulmonary musculature in both cases. The posteriorly situated aortic valve and the lack of fibrous continuity between the pulmonary and mitral leaflets in transposition of the great arteries, are anatomic peculiarities already reported.'"18 Our contribution to previous studies1518 was not included in the present report. The angiocardiographic demonstration of a posteriorly located aortic valve, as reported previously,18 and a subpulmonary muscle bundle (Fig 4), was possible in case 1. However, they do not invalidate the diagnosis of transposition of the great arteries provided that two-thirds or more of the aortic and pulmonary orifices originate from the right and left ventricle, respectively. The peculiar location of the conal septum in case 3 (Fig 2) accounts, in our opinion, for the most striking origin and position of the great arteries. In fact, it is interesting to note that in the complete dextrotransposition of the great arteries (the pulmonary artery originated entirely from the left ventricle and the aorta, although ovemding the ventricular s e p tum, emerged predominantly from the right ventricle) the aortic valve was anterior and somewhat to the left of the pulmonary valve. Considering the angiocardiographic features of case 3, one could suspect that both great arteries originated predominantly from the left ventricle (Fig 5b). This was a false impression obtained by examining only this angiocardiographic projection, but a careful observation of the frontal view and the heart specimen showed that the aorta, although overriding the ventricular septum, originated p r e dominantly from the right ventricle. It is'worthwhile to note how the conal septum has developed eccentrically inside the conus, its growth more anterior and posterior than normal in cases 2 and 3, respectively. Considering the different location of the pulmonary artery in case 2 (anterior) and case 3 (posterior), it is easily understood how the eccentric growth of the conal septum resulted in pulmonary stenosis in both cases. Unlike the common forms of classic complete dextrotranspositionof the great arteries in which the ventricular septal defect is subpulmonary, the latter lesion was simultaneously subpulmonary and subaortic in cases 1and 2 ( Fig 1), and only subaortic in case 3 (Fig 2). We wonder whether in these cases the surgical techniques aiming to re-establish the continuity between the left ventricle and the aorta1@mwould be more appropriate than the Mustard procedure. An enlargement of the ventricular septal defect enabling the re-establishment of the left ventricularaortic continuity by removing the conal septum musculature, probably should be made in patients with the anatomic characteristics of cases 1and 2, in which the conal septum overrode the ventricular septum obstructing the ventricular septal defect (Fig 1). The ventricular septal defect in case 4 was not due to a malalignrnent between the conal and ventricular septa, but to defective development of the superior part of the posterior conal ridge. It is noteworthy that in case 5 a dextroposed conal septum (Fig 3), resulting in subaortic stenosis, was associated with a preductal aortic hypoplasia as happens more frequently in other forms of congenital heart lesions with dextroposition of the conal septum (double outlet right ventricle, etc). The ventricular septal defect of this patient viewed from the left ventricle involved, as expected, the anterior smooth regi0n'O.l' of the ventricular septum. For years we have been searching for the anatomic basis supporting the statement that the conus in transposition of the great arteries with an anterior and right-sided aorta is the mirror-image of the normal ~onus.~l By the same reasoning, it would seem perfectly logical that the orientation of the conal septum in such cases would also be inverted, with respect to the normal ones. Although infrequently, in some cases we have observed how the conal septum overrides the ventricular septum extending from the right ventricle (in which it is 416 QUERO JIMENU, PERU MARTINU Downloaded From: http://publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/20957/ on 05/06/2017 CHEST, 66: 4, OCTOBER, 1974 FIGURE6. Left ventricular view of a case with transposition of the great arteries and intact ventricular septum. Observe how the overriding conal septum (CS) ends rather posteriorly in the left aspect of the ventricular septum (VS) instead of forming the smooth anterior portion of this latter structure. Note the pronounced fusion line (arrows) between this malorientated conal septum and the ventricular septum. The continuous deposition of fibrin in this irregularity of the left septal surface could be the origin of the fibro-muscular subpulmonary stenosis present in some cases of transposition of the great arteries with intact ventricular septum. septal defect in cases 3 and 5 was the same as in cases 1 and 2, with the superior limit the aortic semilunar leaflets in case 3 and the pulmonary valve in case 5. The posterior location of the aortic in relation to the pulmonary valve (Fig 1 ) in cases 1 and 2 is in perfect accordance with the smaller dimension of the subaortic muscle (posterior conal free wall) as compared with the subpulmonary musculature in both cases. The posteriorly situated aortic valve and the lack of fibrous continuity between the pulmonary and mitral leaflets in transposition of the great arteries, are anatomic peculiarities already reported.'"18 Our contribution to previous studies1518 was not included in the present report. The angiocardiographic demonstration of a posteriorly located aortic valve, as reported previously,18 and a subpulmonary muscle bundle (Fig 4), was possible in case 1. However, they do not invalidate the diagnosis of transposition of the great arteries provided that two-thirds or more of the aortic and pulmonary orifices originate from the right and left ventricle, respectively. The peculiar location of the conal septum in case 3 (Fig 2) accounts, in our opinion, for the most striking origin and position of the great arteries. In fact, it is interesting to note that in the complete dextrotransposition of the great arteries (the pulmonary artery originated entirely from the left ventricle and the aorta, although ovemding the ventricular s e p tum, emerged predominantly from the right ventricle) the aortic valve was anterior and somewhat to the left of the pulmonary valve. Considering the angiocardiographic features of case 3, one could suspect that both great arteries originated predominantly from the left ventricle (Fig 5b). This was a false impression obtained by examining only this angiocardiographic projection, but a careful observation of the frontal view and the heart specimen showed that the aorta, although overriding the ventricular septum, originated p r e dominantly from the right ventricle. It is'worthwhile to note how the conal septum has developed eccentrically inside the conus, its growth more anterior and posterior than normal in cases 2 and 3, respectively. Considering the different location of the pulmonary artery in case 2 (anterior) and case 3 (posterior), it is easily understood how the eccentric growth of the conal septum resulted in pulmonary stenosis in both cases. Unlike the common forms of classic complete dextrotranspositionof the great arteries in which the ventricular septal defect is subpulmonary, the latter lesion was simultaneously subpulmonary and subaortic in cases 1and 2 ( Fig 1), and only subaortic in case 3 (Fig 2). We wonder whether in these cases the surgical techniques aiming to re-establish the continuity between the left ventricle and the aorta1@mwould be more appropriate than the Mustard procedure. An enlargement of the ventricular septal defect enabling the re-establishment of the left ventricularaortic continuity by removing the conal septum musculature, probably should be made in patients with the anatomic characteristics of cases 1and 2, in which the conal septum overrode the ventricular septum obstructing the ventricular septal defect (Fig 1). The ventricular septal defect in case 4 was not due to a malalignrnent between the conal and ventricular septa, but to defective development of the superior part of the posterior conal ridge. It is noteworthy that in case 5 a dextroposed conal septum (Fig 3), resulting in subaortic stenosis, was associated with a preductal aortic hypoplasia as happens more frequently in other forms of congenital heart lesions with dextroposition of the conal septum (double outlet right ventricle, etc). The ventricular septal defect of this patient viewed from the left ventricle involved, as expected, the anterior smooth regi0n'O.l' of the ventricular septum. For years we have been searching for the anatomic basis supporting the statement that the conus in transposition of the great arteries with an anterior and right-sided aorta is the mirror-image of the normal ~onus.~l By the same reasoning, it would seem perfectly logical that the orientation of the conal septum in such cases would also be inverted, with respect to the normal ones. Although infrequently, in some cases we have observed how the conal septum overrides the ventricular septum extending from the right ventricle (in which it is 416 QUERO JIMENU, PERU MARTINU Downloaded From: http://publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/20957/ on 05/06/2017 CHEST, 66: 4, OCTOBER, 1974 possible to recognize its anterior end) to the left ventricle in which its posterior end abnormally fills the hole between the two divisions of the septal band. This filling is accomplished with the conal septum extending in a different direction as it normally does, resulting in certain irregularities in the left ventricular aspect of the ventricular septum (Fig 1, 6 ) that might well correspond to abnormal fusion lines between the ventricular septum and the abnormally orientated conal septum. It is our tentative supposition that the continuous deposition of fibrin in these irregularities" could be the origin of some fibromuscular subpulmonary stenosis observed in cases of transposition of the great arteries with intact ventricular septum (Fig 6 ).* 1 Keith A: The Hunterian Lectures. Lancet 2:433, 1909 2 De la Cruz MV, Da Rocha JP: An ontogenic theory for the explanation of congenital malformations involving the truncus and the conus. Am Heart J 51 :782, 1956 3 Van Mierop LHA, Alley RD, Kansel HW, et al: Pathogenesis of transposition complexes. 1. Embryology of the ventricles and great arteries. Am J Cardiol 12:216, 1963 4 Van Mierop LHS, Wiglesworth FW: Pathogenesis of transposition complexes. 2. Anomalies due to faulty transfer of the posterior great artery. Am J Cardiol 12:226, 1963 5 Van Mierop LHS, Wiglesworth FW: Pathogenesis of transposition complexes. 3. True transposition of the great vessels. 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