**Professor of Medicine, Baylor College of Medicine AN EMBRYOLOGIC INTERPRETATION THE SPECTRUM OF DOUBLE OUTLET RIGHT VENTRICLE:

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1 THE SPECTRUM OF DOUBLE OUTLET RIGHT VENTRICLE: AN EMBRYOLOGIC INTERPRETATION Paolo Angelini, M.D.* and Robert D. Leachman, M.D** SUMMARY After formulating the definition of double outlet right ventricle (DORV) as the persistence of the primitive origin of the trunco-conal structures from the right ventricle, 64 autopsy cases meeting this definition were reviewed. A wide spectrum of anatomic variations of DORV were found. A classification is proposed based upon the type of relationship between the great vessels and upon the presence and type of ventricular septal defect. Twenty-five patients had normally crossed great arteries (13 of which had pulmonic stenosis), nine had transposed vessels and 23 had double muscular conus and a side-by-side arrangement of the great vessels in the frontal view, a relationship called "partial distortion" or atypical transposition. Seven patients had absent trunco-conal septum (common truncus). Only one had intact ventricular septum. Anterior ventricular septal defect was found only in patients with partial distortion or common truncus. Angiographic and surgical correlations are presented. Because of their great heterogeneity, the need for detailed description of the anatomy of the individual case of DORV is emphasized. INTRODUCTION The term double outlet right ventricle (DORV) was introduced by Witham (1957) to include every type of cardiac malformation characterized by the origin of both great arteries from the right ventricle; however, similar cases had been described previously by von Rokitansky (1875), Eisenmenger (1897), Spitzer (1923), and others who used different terminology. The wide spectrum of synonyms employed for this malformation illustrates the confusion existing about its nature: "origin of both great vessels from the right ventricle" (Neufeld, 1961), "transposition type II" (Spitzer, 1951), "tetralogy of Eisenmenger" (Saphir, 1941), "complete dextroposition of the aorta with ventricular septal defect" (Braun, 1952), in addition to "double outlet right ventricle" (Witham, 1957). The Eisenmenger's heart, as opposed to the Eisenmenger's syndrome, has been considered by some authors (Saphir, 1941; De la Cruz, 1956) as a true DORV. The case described had a large membranous type ventricular septal defect (VSD) and mild dextroposition of the aorta without double muscular conus (Eisenmenger, 1897). More recently in the literature and prac- From the Cardiology Division of the Texas Heart Institute, Houston, Texas, and the Department of Medicine, Baylor College of Medicine, Houston, Texas. *Staff Cardiologist, St. Luke's Episcopal and Texas Children's Hospitals **Professor of Medicine, Baylor College of Medicine Presented in part at the Sixth European Congress of Cardiology, Madrid, Spain. Address for reprints: Robert D. Leachman, M.D., St. Luke's Episcopal Hospital, P.O. Box 20269, Houston, Texas Cardiovascular Diseases, Bulletin of the Texas Heart Institute, Vol. 3, Number 2,

2 tice of clinical cardiology, the tendency has been toward restricting the usage of the term DORV to those hearts in which both vessels originate from the right ventricle (RV) and are separated from the mitral valve by muscular tissue (Neufeld, 1962; VanMierop, 1963; Dayem, 1967; Edwards, 1968). Our experience with anatomically-proved cases of DORV at the Texas Children's Hospital is reviewed and a critical appraisal of the subject is presented. DEFINITION AND EMBRYOLOGIC CONSIDERATIONS In our opinion, DORV is a descriptive term which should be interpreted to mean that both great arteries originate from the morphological RV, without reference to any spatial arrangement of the great arteries or to their relationship with the atrioventricular (AV) valves, or to the presence and location of a VSD. The logical basis of this broad definition is offered by the following embryologic considerations. In the early human embryo (5.2 mm), after cardiac loop formation, the cono-truncus is anterior and to the right with respect to the primitive ventricle, communicating proximally only with the bulbus cordis. At this stage the primitive ventricle (from which the left ventricle [LV] develops) has connection only with the bulbus cordis and no arterial outlet. To attain normal mature development, in which the LV has a direct outlet into one of the great vessels, migration of the trunco-conus to the left is necessary in order to allow normal alignment with the primitive interventricular septum. This is probably attained by two different events: an absolute shifting of the trunco conus to the left and the widening of the primitive ventricle, which relatively displaces the primitive ventricular septum to the right. The simultaneous widening and the right shifting of the A-V canal does not directly affect the relationship between conal and ventricular septa, as it concerns only the inlet (or sinus) of the ventricles and not the outlet. Although the specific cause has not yet been definitively established (absence of reabsorption of the conoventricular flange or primary failure of the embryogenetic movement or hypoplasia of the primitive ventricle), persistence of a primitive relationship of the trunco-conal structures with the ventricles will result in DORV and a LV with no possible outlet other than a VSD. The failure of conoventricular alignment is the essential error in the embryogenesis of human hearts with DORV: the free conal wall of these hearts is completely positioned to the right of the ventricular septum or aligns with it. It is of interest that Gessner and Van Mierop (1970) have recently reported the experimental production of DORV by mechanically inhibiting the left shifting of the trunco-conus in the early embryo. CLASSIFICATION (Table I) The diagnosis of DORV is established anatomically only on the basis of the origin of both great vessels from the morphologic right ventricle. The case in which one of the great arteries overrides the septum should be classified as transitional between that in which the great vessels originate entirely from separate ventricles (orthoposition) and that of DORV). The multiple variations in the anatomy of DORV require a classification which will distinguish the important differences. Since failure of alignment of the trunco-conus with the ventricular sep- 128

3 tum is the hallmark of DORV, variations in the anatomy of these structures should form the basis for a classification. The trunco-conal derivatives, in anatomic terms, include the ascending aorta, the pulmonary trunk and their respective infundibula and valves. The possible trunco-conal variations include common truncus arteriosus, transposed great arteries, normally crossed great arteries, and the aorta and pulmonary artery in side-by-side relationship with double muscular conus. The anatomic diagnosis of these variations is the same in DORV as when they occur in the usual orthoposition. Common truncus arteriosus exists when a single vessel leaves the heart and supplies the systemic and pulmonary circulations (Fig. 1.1). The spatial relationship between the great vessels has been a well-established criterion to recognize abnormalities of the trunco-conal derivatives. 1/I7 j * '4. Fig. 1. Diagrams illustrating DORV with common truncus arteriosus (1) and tetralogy of Fallot with extreme dextroposition of the aorta (2). In both forms some conal musculature (C) is recognized. Top: frontal view. Bottom: base of the heart. T=tricuspid valve; M = mitral valve. 129

4 Normally related (crossed) and transposed great vessels are described accordingly. The first is characterized by the crossing of the great arteries in both frontal and lateral views and by the presence of a single subpulmonary muscular infundibulum (Fig. 2.1). Transposed great vessels have a parallel course in both frontal and lateral projections and have only a subaortic muscular infundibulum (Fig. 2.3). A third type of relationship is characterized by a side-by-side course of the great arteries in the frontal view, partial overlapping in the lateral view, and the presence of bilateral (sub-aortic and subpulmonic) muscular infundibula (Fig. 2.2). This entity was originally named "partial distortion" (De la Cruz, 1956), and more recently has been called "double conus" or "atypical transposition" (Van Praagh, 1966). Others have confused it with DORV (Neufeld, 1962). We will use the term "partial distortion" because it is the oldest and best defines the difference between this entity and transposition or normally related great vessels. We are aware of the current discussion on the embryology of the various trunco-conal arrangements. Goor (1972), in his recent commentary on the conotruncos, emphasized that the conotruncal ridges were in spiral orientation, "like riflings in a gun barrel" from their first appearance. Whether there are torsional effects to explain the alignment of the great vessels in a side-by-side, anteroposterior (transposed), or crossed relationship, or whether the truncoconal ridges form abnormally from the beginning is unknown. Until these I Fig. 2. Diagrams representing DORV with normally related great vessels (1), partial distortion (2), and transposition of the great vessels (3). The conal musculature is labeled (C). a = aorta; p = pulmonary trunk. In the case of partial distortion the two most frequent locations of the VSD are exemplified (Type I and II). 130

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6 various malformations can be produced experimentally, the controversy will remain. However, the classification used in this paper corresponds to the observed anatomic variations and thus seems to be useful in describing the different arrangements. In all cases in which a trunco-conal septum has developed, there may be uneven septation at the expense of one of the two vessels, resulting in stenosis or atresia of either the aorta or the pulmonary trunk (Table I). The other important feature in this classification of DORV is the type of ventricular septal defect if present. The defect may be positioned in the antero-superior portion of the ventricular septum ("supracristal" Type I), in the intermediate portion ("membranous" Type II), in the posterosuperior portion ("A-V canal type" Type III), or in the lower two-thirds ("muscular" Type IV), as originally described in otherwise normal hearts by Becu et al. (1956). In addition to position, the functional size of the VSD has been stressed (Lavoid, 1971), and may vary from a non-functioning tiny orifice to complete absence of the septum (common ventricle). MATERIAL A review of 900 heart specimens in the Pathology Department of the Texas Children's Hospital*, yielded 64 examples fulfilling our criteria of DORV. Those specimens of tetralogy of Fallot with marked dextroposition of the aorta, although a form of DORV, did not appear so different from the usual tetralogy to require a detailed anatomic description in this paper. This complex comprises a sizable percentage of the cases of tetralogy of Fallot. Of 109 specimens with tetralogy of Fallot, 13 cases were observed to have extreme dextroposition of the aorta and were considered examples of DORV. The diagnosis of "extreme dextroposition of the aorta" refers to cases in which the aorta lies completely to the right of the left border of the interventricular septum. The different types of DORV encountered in this series are presented in Tables II-V, where the associated defects are also summarized. All the specimens were "D-Loop" and the following description is based upon the assumption of "D-Loop", although the same classification could be applied to "L-Loop" hearts. ANATOMIC DESCRIPTION A. Absence of trunco-conal septum (i.e. truncus arteriosus) 7 cases In the case of complete absence of trunco-conal septation, the aortic and pulmonary outflows and trunks are common;i however, if the persistent truncus arteriosus originates from the RV, DORV occurs embryologically as well as functionally. In all seven cases, a large VSD was present and located in the anterior portion of the ventricular septum. A membranous septum could not be identified in any of these seven specimens. In every case of this group, a muscular band originating from the posterior aspect of the truncal valve and directed to the lateral aspect of the free wall of the RV was observed (Fig. 3). The dimension of this muscular structure *We gratefully acknowledge the collaboration of Harvey S. Rosenberg,, M.D., Director of Laboratories, Texas Children's Hospital, for allowing us to review his collection of cardiac specimens. 132

7 was variable and in some cases it was quite diminutive. It was always located behind the VSD. The truncal valve was always in fibrous continuity with the mitral valve, through the VSD. B. Presence of trunco-conal septum (57 cases) 1. Normally crossed great vessels: Twelve cases had DORV and the great arteries were arranged as in the normal heart. There was an anterior pulmonary artery with a muscular infundibulum that crossed the posterior aorta (without muscular infundibulum) from right to left and from ventral to dorsal. The only difference between these hearts and those with isolated membranous VSD was the complete dextroposition of the- aorta. In all these specimens a VSD was present in the type II position, with the exception of only two cases in which the defect extended to the type III (A-V canal TABLE II. Double Outlet Arteries Right Ventricle With Normally Related Great Case No. Ao:PA ratio VSD VSD:PA ratio Associated Defects (type) (Cardiac) 1 Ao = PA II <1.0 PFO-MA-HLV 2 Ao < PA II 1.0 ASD II 3 Ao = PA III >1.0 CAVC-HLV 4 Ao = PA II <1.0 ASD II-MA-HLV-TAPVR 5 Ao-= PA II <1.0 ASD II-MA-HLV 6 Ao = PA II <1.0 PFO-MA-HLV 7 Ao < PA II <1.0 PFO-MA-HLV-HAo-TAPVR-PDA 8 Ao = PA II 1.0 ASD II-TA-CoAo 9 Ao < PA II >1.0 HAo-PDA 10 Ao = PA II <1.0 ASD II-MA-PDA-TAPVR 11 Ao < PA II & III >1.0 ASD II-CAVC-AoPW-HAo-PDA 12 Ao = PA II & III >1.0 Htx-SA-CAVC-TAPVR-HLV AoPW = aorto-pulmonary window; ASD II = atrial septal defect, secundum type; CAVC = common atrio-ventricular canal; HAo = hypoplastic aorta (ascending and/or aortic arch); HLV = hypoplastic left ventricle; MA = mitral atresia; PFO = patent foramen ovale; SA = single atrium; TA = tricuspid atresia; TAPVR = total anomalous pulmonary venous return (various types). 133

8 TABLE III. Double Outlet Right Ventricle With Partial Distortion of The Great Arteries Case No. Ao:PA ratio VSD VSD:PA ratio Associated Defects (type) (Cardiac) 1 Ao = PA I Ao = PA II > 1.0 HAo-PDA 3 Ao < PA II <1.0 ASD II-MA-HLV-CoAo 4 Ao. PA II <1.0 PFO 5 Ao = PA II < 1.0 PFO-MA-HLV-PDA 6 Ao = PA II <1.0 PFO-MA-HLV-PDA 7 Ao < PA II & III >1.0 PDA 8. As = PA II <1.0 PFO-MA-PDA-BPV 9 Ao = PA II 1.0 PDA 10 Ao = PA II Ao = PA II >1.0 PFO-PDA-CoAo 12 Ao = PA II 1.0 PFO 13 Ao = PA II 1.0 SPS 14 Ao< PA II 1.0 HAo-PDA 15 Ao> PA II & III >1.0 SA-JAA-IPS 16 Ao < PA II <1.0 CT-HLV 17 Ao < PA I >1.0 ASD II 18 Ao< PA II & III >1.0 PFO 19 Ao = PA NONE --- PFO-MA-HLV 20 Ao < PA I > Ao = PA II 1.0 PDA-CoAo 22 Ao = PA II >1.0 Htx-SA-CAVC-TAPVR 23 Ao = PA I 1.0 PFO-PDA CoAo = coarctation of the aorta; BPV = bicuspid pulmonic valve; Htx = heterotaxis; JAA = juxtaposition of atrial appendages; SPS = supravalvular pulmonic stenosis. Others as in Table II. 134

9 type) position. The 13 cases with tetralogy of Fallot also had normally crossed great vessels but pulmonary infundibular and valvular stenosis were present. These cases are obviously a variant of tetralogy of Fallot in which both great vessels originate entirely from the right ventricle. In only two cases the usual fibrous continuity of the aortic and mitral valves was interrupted by a partial subaortic muscular infundibulum (Fig. 5). This was in the form of an aortic subvalvular muscular band that was interrupted dorsally where the mitral-tricuspid fibrous continuity was preserved. The medial portion of this band reached the ventricular septum and continued into the septal band. The spatial relationship between the great arteries was normal. 2. Partial distortion of the great vessels (double conus or atypical transposition): Twenty-three cases of DORV were found to have this type of relationship between the arterial trunks. The pulmonary trunk and the aorta had a parallel course side-by-side, in the frontal view; in the lateral view the pulmonary infundibulum and valve appeared slightly anterior to the aortic. Each of the great vessels had a completely muscular infundibulum, the anterior portion of which was in continuity with the right ventricular free wall. The two infundibula were separated by a muscular band ("interconal septum") and were posteriorly limited by two other bands: the subaortic joined the lateral free wall of the right ventricle and the subpulmonic the right side of the ventricular septum. TABLE IV. Double Outlet Right Ventricle With Transposition of The Great Arteries Case No. Ao:PA ratio VSD VSD:PA ratio Associated Defects (type) (Cardiac) 1 Ao = PA II & III >1.0 PFO-HLV-PDA 2 Ao = PA II & III >1.0 ASD II-CAVC-IPS 3 Ao = PA II <1.0 PFO-HLV 4 Ao = PA II 1.0 DMO-IPS-BPV 5 Ao = PA II <1.0 PFO-HLV 6 Ao > PA II 1.0 HLV-HPA 7 Ao < PA II <1.0 ASD II-HAo-PDA 8 Ao = PA II <1.0 PFO-MA-HLV 9 Ao > PA II --- SA-Htx-MA-HLV-PDA HPA = hypoplasia of pulmonary artery; IPS = infundibular pulmonic stenosis; PVA = pulmonary valve atresia. Others as in Tables II and III. 135

10 In this type of DORV the VSD was most frequently (18 or 23 cases) in the membranous position (type II, behind the subpulmonary infundibulum. In a minority of cases (4), the pulmonary valve was partially overriding the VSD. This type of VSD was anterior to the insertion of the interconal septum into the ventricular septum (type I position). The pulmonary valve was never found to be in fibrous continuity with the mitral, through this VSD. This type of DORV with partially distorted great arteries and type I VSD is traditionally known as "Taussig-Bing complex."30 3. Transposition of the great arteries: In nine cases of DORV the anatomy of the great vessels and conal derivatives were the ones commonly observed in uncomplicated transposition of the great arteries. The aortic valve and muscular infundibulum was located to the right and anterior to the pulmonary valve (which had no muscular infundibulum). The two vessels did not cross in the frontal nor in the lateral views (Fig. 9-11). A VSD was always present in the type II position beneath the pulmonic valve. The pulmonic valve was in fibrous continuity with the mitral valve through the VSD. C. Aortic or pulmonary hypoplasia or stenosis. Twenty-nine cases of DORV had unequal sizes of the arterial trunks, not considering the normal moderate dilatation of the pulmonary artery caused by increased blood flow. Four cases with normally related great vessels had mildly diminu- TABLE V. Double Outlet Right Arteriosus Ventricle With Common Truncus Case No. Type of VSD VSD:PA Associated Defects Truncus (type) trunk ratio 1 II I --- TAPVR-HLV-MA 2 I I 1.0 RRSA 3 II I --- Right Aortic Arch 4 I I < II I --- Right Aortic Arch 6 I I >1.0 PFO-DMV 7 I I <1.0 HAo Common truncus is classified according to Collett and Edwards. DMV = deformed mitral valve; RRSA = retroesophageal right subclavian artery. Others as in Table II. 136

11 tive sizes of the aorta, while 13 had pulmonary infundibular and valvular stenosis (tetralogy of Fallot with DORV or extreme dextroposition of the aorta). Only one case with partially distorted great arteries had pulmonary stenosis, while seven had mild degrees of aortic hypoplasia. Two cases with transposition had a diminutive pulmonary artery (one had valve atresia) and one case had a slightly diminutive ascending aorta. D. Ventricular septal defect. A ventricular septal defect was always found in our series, with the exception of a single case in which mitral atresia co-existed and the left ventricle was a blind, small cavity without any gross communication with the other cavities, a complex anomaly discussed in the literature (Edwards, 1952; McMahon, 1964; Ainger, 1965; Davachi, 1968). The size of the VSD was usually moderate to large in our cases. No example of "common ventricle" (extremely large ventricular septal defect) was found. The evaluation of the obstruction to flow at the VSD level is approximate in autopsy studies. An anatomic approximate measure of this feature is obtained by comparing the size of the VSD to that of the pulmonary orifice (Tables II-V), as the flow through the VSD is the same as the pulmonary. Seventeen cases of our series were judged to have a stenotic VSD. Six of them had an associated atrial septal defect and ten a widely patent foramen ovale. In all cases the location of the VSD could be classified according to the classification of Becu et al (1956). The anterior, supracristal VSD (type I) was only found in association with partially distorted great arteries (4 Fig. 3. View of the RV outflow in a case of common truncus arteriosus (T) with extreme dextroposition. The truncal leaflets are shown slightly overriding the ventricular septum above a VSD. A parietal band of the crista supraventricularis is clearly shown. TV = tricuspid valve; PA = pulmonary artery. 137

12 ... : : :..... :.::.: :... :. ::d ::::.. :: :.:::;N:... S:::N:::: :: :.:. -.:..... ::: : {Z E %Z;S -:....S.. 'fx, ::: Fig. 4. DORV with normally related great vessels. The aorta crosses the pulmonary artery posteriorly. No subaortic muscular infundibulum was present. The VSD (not visible) was posterior to the papillary muscle of the conus (p) and through it there was mitral-aortic valve continuity. 138

13 . AO%., k N r 'o op Awmfth- P-1% -'% O.W U. Aokk N.WV Fig. 5. DORV with intermediate characteristics between cases with normally related great vessels and partial distortion. A partial subaortic infundibulum is present (B), into which the tricuspid valve (TV) is inserted. In F, fibrous continuity is shown between the tricuspid and aortic valves. The probe is inserted in the pulmonary outflow tract, beneath the parietal band (PB). I39

14 N o. A -.59i) 13 Fig. 6. DORV with partial distortion of the great vessels. The right ventricle was open to show the pulmonary outflow tract. The interconal septum (IC) is shown ahove the VSI). 140

15 No A MM 1 2 Fig. 7. Same case as in Fig. 6. The aortic outflow tract is shown to have a muscular wall. The VSD is not in direct communication with any outflow, and lies behind and below the interconal septum (IC). This is the usual position of the membranous ventricular septum. PI = pulmonary infundibulum; SA = subaortic infundibulum. 141

16 AO... No.A-59'143 rc:.,& 1 2 Fig. 8. DORV with partial distortion of the great vessels and subpulmonary VSD (Taussig-Bing complex). The parallel course of the great arteries in this frontal view is seen. The interconal septum (IC) separates the subaortic muscular infundibulum (SA) from the pulmonic. 142

17 cases) and common truncus (all of the 7 cases). The VSD in common truncus is not completely homologous to the type I, as it necessarily extends to the membranous septum. VSD's involving the membranous septum (type II) were the most common, being present in 44 of 64 specimens. In three cases the VSD was more extensive and included part of the ventricular septum beneath the anterior leaflet of tricuspid and mitral valve (these cases are classified as types II and III in the tables). A complete A-V canal type of VSD was present in five specimens. Type II VSD was immediately related to the aortic valve in the cases with normally related great arteries, but it was related to the pulmonic valve in cases with transposed great arteries, and to neither valve when there were partially distorted great vessels. In this last case the aortic valve was most commonly located anterior and to the right, but in some specimens it was more posterior and although separated from the VSD by muscular tissue, it was adjacent to it. Fig. 9. External view of a case of DORV with transposition of the great vessels. The aorta is anterior and to the right, and the great arteries have parallel courses. 143

18 ASSOCIATED MALFORMATIONS DORV was found to be associated with four types of malformations more frequently than anticipated by chance: syndrome of hypoplastic left heart (18 cases), endocardial cushion defects (8 cases), coarctation or hypoplasia of the aorta (9 cases), and total anomalous pulmonary venous return (6 cases). The high incidence of hypoplastic left heart syndrome and endocardial cushion defects in cases of DORV seems to indicate that these conditions interfere with the left shifting of the trunco-conus. DORV has been described in the literature with ventricular inversion in situs solitus, and with atrioventricular concordance in situs inversus (Neufeld, 1962; Van Mierop, 1963; Keiser, 1968). In these cases, the truncoconal and ventricular structures are the mirror images of those described above and at times they are confused with double outlet LV. We did not find any similar specimens. DISCUSSION Currently in clinical cardiology there is a widespread tendency to con- F... 1.Smc..i Fi. 9. Th RV s o Fit,. 10. Same case as in Fig. 9. The RV is o-pen to show the aortic muscular infundibulum. 144

19 sider DORV as a malformation characterized by the origin of both the aorta and pulmonary artery anteriorly from the RV, with subaortic and subpulmonic muscular "conus" and an identically high level of both semilunar valves (Neufeld, 1962, Van Mierop, 1963; Dayem, 1967; Edwards, 1968). This simplification corresponds to that which considers "double conus" synonymous with DORV. Our material shows that DORV, literally defined, occurs not only with the above mentioned relationship between the great vessels (partial distortion) but also with normally crossed or transposed great arteries. For analogy, a truncus arteriosus originating entirely from the RV must be included in the same group of malformations even if the presence of a single outlet contrasts with the use of the terminology DORV. The important conclusion to be drawn is that every type of trunco-conal arrangement can be associated with DORV, evidence that we are dealing with two different embryologic events: the left shifting of the truncoconus and its septation. The interrelationship between the great arteries called partial distortion (or atypical transposition), is frequently confused with either normally Fig. 11. Same case as in Fig. 9. The RV is open to show the pulnonic outflow. The tricuspid valve is immediately beneath the pulmonic. The VSD (Type II) is severely stenotic and has a probe in. A second probe goes to the aorta. 145

20 crossed (Neufeld, ) or transposed (Hightower, 1969; Hallman, 1971) vessels. If this anatomic entity has not yet received identity, like normally crossed or transposed great vessels, it is probably because partial distortion has until recently been described only in DORV. Even when an attempt was made to give an embryologic interpretation to partial distortion, it was supposed to be instrinsically linked to DORV. The Taussig- Bing complex, a form of DORV with partial distortion, was considered by Paul et al (1968) to be caused by the simple persistence of a bilateral conus. The presence of a bilateral muscular conus does not necessarily result in dextroposition of the trunco-conal structures. Transitional specimens varying from partial distortion in dextroposition (one type of DORV) to partial distortion in orthoposition have been shown to exist (Lev, 1972; Angelini, 1973). In the latter condition the pulmonary artery has shifted completely to the left of the interventricular septum and originates anteriorly from the LV, the aorta remaining on the right ventricular side, similar to a case described by Van Praagh et al (1971). In an editorial commenting on the above mentioned paper (Angelini, 1973), Van Praagh (1973) denied the need of a special name such as "partial distortion", but agreed to the concept that one type of conotruncal malformation is characterized by a bilateral muscular conus. The occurrence of normal relationship, partial distortion, and transposition of the great vessels with DORV is further demonstration that the term "transposition" is misleading when used to describe the relation between the great vessels and the ventricles (Spitzer, 1923; Ruttemberg, 1964; Van Praagh, 1971). We believe that transposition of the great vessels, like partial distortion, is a malformation of the trunco-conus and should be identified regardless of the position or even the existence of ventricular septum (Van Mierop, 1971; Angelini, 1973). We also believe that the "nonspecific" definition of transposition as any abnormal position of the semilunar valves (Goor, 1973) would be confusing and would require new terminology for the various types of abnormalities. Two types of infundibular arrangements may be found in DORV just as in hearts with orthoposition of the great vessels. The first type is characterized by an anterior muscular infundibulum and a posterior infundibulum which is partly fibrous and allows mitro-semilunar valve continuity. This is the type found in DORV with normally crossed great vessels where the muscular infundibulum is beneath the anterior pulmonary valve, and also in DORV with transposed great vessels where the muscular infundibulum is beneath the aortic valve. The second type of conal anatomy is observed in DORV with partial distortion (or atypical transposition), in which case complete muscular rings are found beneath each semilunar valve. In the normal heart, the parietal band of the crista supraventricularis in all probability is formed from the free wall of the pulmonary conus and from the dextrodorsal conal ridge, so that it is only partially derived from the conal septum. In partial distortion, the muscular septum that separates the aortic from the pulmonary infundibulum is a remnant of the interconal septum alone (both conal ridges). Embryologically this structure is not homologous to the crista parietalis, as currently stated (Neufeld, 1962; Van Mierop, 1963; Paul, 1968). The finding of DORV with mitro-aortic fibrous continuity (cases with 146

21 normally related great arteries, transposition or common truncus) is good evidence against the theory that claims DORV results from the persistence of the cono-ventricular flange (Baron, 1971). In our series, the length of the muscular infundibula (subaortic and subpulmonic), was variable in each type of great vessel relationship. This was especially evident in cases with partial distortion, some of which had rudimentary conal musculature beneath either semilunar valve, while others had well-developed bilateral infundibula. A significant observation is the absence of anterior VSD in the great majority of cases of DORV. This type of defect would be expected to occur regularly when there is no alignment of the conal and ventricular septa, as in this anomaly. The anterior and superior ventricular septum is normally formed by the interconal septum, which obviously cannot contribute to its development in DORV. It can be hypothesized that this portion of the ventricular septum is completed in DORV by the left side of the free conal wall. The cases of DORV with anterior VSD (essentially only Taussig-Bing complexes) should then be considered examples of defective left-sided free conal wall, perhaps secondary to a lesser degree of dextroposition of the trunco-conal derivatives. From a clinical point of view the most important anatomic variations in DORV depend on the interrelationship between the great vessels and the position of the VSD. The diagnosis of normally crossed or transposed great arteries is easily made by angiographic means. The identification of those cases with partial distortion is at times difficult. Our anatomical review suggests the following angiographic criteria should be used for the diagnosis of partial distortion: (1) parallel course, side-by-side aorta and pulmonary artery in the frontal view; (2) identical coronal level of the sigmoid valves; (3) overlapping of the origin of the great arteries in the lateral view;; (4) existence of a subaortic muscular conus (usually appearing as a subvalvular filling defect) which also can be described as lack of mitroaortic continuity. This finding is not proof of DORV nor is its absence proof that DORV does not exist. The anatomic variability of the position of the aortic valve with respect to the pulmonary should suggest certain elasticity in the application of the third criterion. We agree with Lev (1972) and Goor (1973) that there is a continuous spectrum of anatomic arrangements between normally crossed and transposed great vessels. Partial distortion should then mean an intermediate ("partial") relationship between those two extremes. The importance of a detailed description of the anatomy in DORV is especially relevant for surgical purposes. For example, transposition of the great vessels in DORV requires a different surgical approach than does normally crossed great arteries in DORV (Kirklin, 1971). The same is true for partially distorted great vessels, where the presence of a subaortic conus and extreme dextroposition of the aorta requires a special tunnel-shaped patching of the VSD if it is of the membranous type (Hallman, 1971; Kirklin, 1971). If the VSD in DORV with partial distortion is subpulmonic, the most logical repair appears to be incorporation of the pulmonary artery into the LV associated with inversion of the atrial circulation by Mustard's procedure (Kirklin, 1971). 147

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