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Double Outlet Right Ventricle: In-Depth Anatomic Review Using Three-Dimensional Cardiac CT Data

Double outlet right ventricle (DORV) is a relatively common congenital heart disease in which both great arteries are connected completely or predominantly to the morphologic RV. Unlike other congenital heart diseases, DORV demonstrates various anatomic and hemodynamic subtypes, mimicking ventricular septal defect, tetralogy of Fallot, transposition of the great arteries, and functional single ventricle. Because different surgical strategies are applied to different subtypes of DORV with ventricular septal defects, a detailed assessment of intracardiac anatomy should be performed preoperatively. Due to high spatial and contrast resolutions, cardiac CT can provide an accurate characterization of various intracardiac morphologic features of DORV. In this pictorial essay, major anatomic factors affecting surgical decision-making in DORV with ventricular septal defects were comprehensively reviewed using three-dimensional cardiac CT data. In addition, the surgical procedures available for these patients and major postoperative complications are described.


INTRODUCTION
. However, cardiac MRI is limited in evaluating cardiac anatomic details due to low spatial resolution, and lengthy examination is disadvantageous, especially in young pediatric patients requiring sedation or general anesthesia.
Cardiac MRI has been subsequently performed using cardiac CT, which offers higher image quality and accessibility due to high spatial resolution and short scan time [4][5][6]. Three-dimensional (3D) printing technology has recently been used to characterize the morphologic details of DORV [7][8][9][10]. Because the quality of source images determines the quality of 3D-printed heart models, cardiac CT is the most commonly used imaging modality in 3D printing for congenital heart disease [11]. However, 3D virtual heart models used for advanced visualization are a type of shaded surface rending technique. A substantial amount of information, including material density and fine spatial details, must be lost from the original data [11]. In addition, cardiac walls in hollow models usually do not replicate true anatomy, but they are commonly just outlines of cast models. In contrast, the volume rendering technique used for 3D illustration of cardiac CT provides highfidelity reproduction of cardiac anatomy. In particular, the transparent-lumen volume or cinematic rendering technique is useful for demonstrating intracardiac anatomy [11,12] https://doi.org/10.3348/kjr.2021.0248 kjronline.org and is therefore suitable for assessing anatomical details of DORV. In this article, a wide spectrum of cardiac anatomy in DORV is systematically described based on a literature review and single-center experience over 20 years using 3D cardiac CT data.

Ventriculoarterial Connection
In DORV, the one great artery and more than half of the other, the so-called "50% rule," originates from the morphologic RV. According to this definition, the 50% rule is based on the circumference of the annulus of the overriding semilunar valve (Fig. 1) [2]. However, measuring the annular circumference is impractical in real practice, and thus, the annular diameter is typically measured instead (Fig. 2). Notably, there is a gray zone in the 50% rule, in which the degree of overriding is very close to 50%, and the diagnosis of DORV has to be uncertain (Fig. 3). Some authors may use the "150% rule" in DORV, which corresponds to the sum of the one great artery (100%) plus half of the other (50%), or use the term "200% DORV" when both great arteries arise from the morphologic RV [13]. Although it is not the diagnostic criteria for DORV, normal aortic-mitral fibrous continuity is frequently lost by varying sizes of the intervening muscular tissue in DORV, particularly in cases with the great malposed arteries (Fig. 4).

Types of Ventricular Septal Defect
In DORV, VSD is almost always present [2]. The spatial relationship between the VSD and the arterial valves is an essential but not unconditional element for determining an optimal surgical strategy. The VSD is categorized into subaortic (Fig. 5), subpulmonary (Fig. 6), non-committed or remote (Fig. 7), and doubly committed types (Fig. 8) (in order of prevalence), according to the presence or absence of commitment of the VSD to the arterial valves [1,9,10]. The upper margin of the VSD is close to the aortic valve in the subaortic type, the pulmonary valve in the subpulmonary type, and both the semilunar valves in the doubly committed type. Although it is a controversial issue, the VSD is generally suggested as non-committed when the distance between the VSD and arterial valves is greater than the age-matched aortic valve diameter [10]. It is important for a surgeon to know which arterial valve is closer to the VSD in the non-committed type, as biventricular repair is feasible in a subset of DORV with the non-committed VSD. The blood stream from the left ventricle to the adjacent great artery via the VSD may be demonstrated when there is a substantial difference in contrast enhancement between the left and RVs (Fig. 3) [14].
If the VSD involves the membranous septum and the adjacent muscular septum, the so-called "perimembranous" type, the atrioventricular conduction axis courses along the posteroinferior margin of the VSD, which is important in surgical planning to avoid the postoperative occurrence of an atrioventricular block [10]. On cardiac CT, the perimembranous extension may be suggested by identifying fibrous continuity between the leaflets of the tricuspid and mitral valves through the defect (Fig. 9), direct contact of the defect margin to the tricuspid valve annulus along the most cranial aspect of the septal leaflet, and the medial kjronline.org (septal) papillary muscle around the defect (Fig. 10) [10,12,15]. Atrioventricular septal defects are frequently found in DORV, in which the defect often extends toward one or both arterial valves (Fig. 11) [10,16]. An additional muscular VSD is often found in the DORV [10], and the diastolic phase should be obtained for cardiac CT for the accurate depiction of muscular VSD. In cases with extensive muscular defects, the so-called "Swiss cheese" type, the biventricular repair is usually not feasible, and single ventricular repair must be performed. VSD is regarded as restrictive when its diameter is smaller than the normal aortic valve [10], and VSD enlargement is often needed to ensure unobstructed flow through an intraventricular baffle from the left ventricle to the aorta [17]. Rarely, the ventricular septum is intact without defects, accounting for 5% in a multicenter study based on 100 heart specimens [15].

Outlet Septum and Ventriculoinfundibular Fold
Yoo et al. [1] demonstrated that the outlet septum is characteristically fused with the anterior margin of kjronline.org the subaortic VSD ( Fig. 4) and the posterior margin of the subpulmonary VSD (Fig. 5), respectively, in 10 patients with DORV. In cases with subpulmonary VSD and dextromalposition of the great arteries, the outlet septum tends to be parallel to the ventricular septum. It is not directly fused with the VSD margin ( Fig. 12) [10]. In contrast, the outlet septum is likely vertical to the ventricular septum in the normal and levoposition of the great arteries (Fig. 10). The orientation of the outlet septum relative to the ventricular septum in the DORV appears to be primarily influenced by the spatial relationship of the great arteries. The length and thickness of the muscular outlet septum are variable. In the doubly committed VSD, the muscular outlet septum was absent, and fibrous continuity was characteristically found between the semilunar valves ( Fig. 7). The fibrous outlet septum kjronline.org The ventriculoinfundibular fold is a muscular structure of the right ventricular outflow tract interposed between the atrioventricular and adjacent arterial valves [10,12,18] (Fig. 13), which is also variable in the DORV. The extent of the infundibulum, including the outlet septum and ventriculoinfundibular fold, contributes to removing the VSD from the semilunar valve [10]. In addition to the 3D configuration (ranging from vertical to parallel to the ventricular septum) of the outlet septum, the extent of the infundibulum is a crucial element for tailoring an optimal surgical strategy.

Outflow Tract Stenosis
In DORV with the committed VSD, the outflow tract related to the non-committed arterial valve tends to be small ( Fig. 9) [10]. In contrast, non-committed VSDs may not follow this tendency (Fig. 13). Outflow tract stenosis may be combined with arterial valvular stenosis (Fig. 9). Biventricular repair using an intraventricular baffle cannot be performed for outflow tract stenosis. Obstructive aortic arch lesions, such as coarctation of the aorta or interrupted aortic arch, are common in subaortic stenosis and are assumed to be related to reduced aortic flow during fetal life [10,16,19].

Spatial Relationship between the Great Arteries
The great arteries are abnormally related in almost all cases of DORV [7]. In fact, because of aortic overriding or malposition of the great arteries, the aortic root is almost always less tightly wedged or completely unwedged within the base of the heart and variably displaced from the normal wedging position between the tricuspid and kjronline.org mitral valves. The great arterial relationship is clinically important because almost identical intracardiac anatomy may present differently if the relationship is reversed [10]. Furthermore, according to the spatial relationship between the great arteries, the spatial relationship may be spiral or parallel between both outflow tracts and vertical or parallel between the outlet septum and the ventricular septum.

Ventricular Volumes
For successful biventricular repair of DORV, both ventricular volumes should be adequately large and balanced after surgery. In DORV with a severely hypoplastic left or RV, a single ventricular repair must be performed. In borderline cases, CT-based morphometric parameters, including ventricular volumes and atrioventricular valve size, may help determine bi-or univentricular surgical repair [20,21]. The remaining right ventricular volume after intraventricular baffling may be estimated by inserting a simulated intraventricular baffle into a virtual heart model using computer-aided design software (Fig. 14) [11].

Atrioventricular Valve Abnormalities
Straddling or overriding the atrioventricular valve may preclude biventricular repair of the DORV [10,16,19,22]. In contrast to overriding, which can be evaluated with cardiac CT, straddling may not be evident on cardiac CT (Fig. 15). Therefore, echocardiography is necessary to accurately evaluate the straddling of the atrioventricular valve [16,19,22]. In addition, the close proximity of the septal leaflets of the tricuspid valve to the VSD margin may

Fig. 14. Double outlet RV with non-committed VSD in a 3-year-old girl who underwent P banding and pulsatile bidirectional cavopulmonary shunt.
A. Virtual heart model from three-dimensional cardiac CT data shows both great arteries connected to the morphologic RV. Notably, the aortic valve (pink) and the pulmonary valve (light blue) are demarcated using computer-aided design software. P banding site (arrow) is also noted. B. Virtual heart model after removing the RV anterior free wall demonstrates the spatial relationships between the non-committed VSD (asterisks in B-E) and the arterial valves. The TV (blue) and the MV (red) demarcated using computer-aided design software are also seen. kjronline.org compromise tricuspid valvular function during intraventricular baffling of the VSD, particularly when the VSD involves the inlet portion of the ventricular septum (Fig. 14) [10].

Coronary Artery Anomaly
Surgically important coronary artery anomalies need to be identified preoperatively, and accurate identification can usually be achieved using cardiac CT [23]. As with TOF, the surgical technique for right ventricular outflow obstruction needs to be modified when a major coronary artery crossing the right ventricular outflow tract is present in the Fallot type of DORV [7,24]. In addition, as in TGA, the origins and branching patterns of the coronary arteries should be delineated when arterial switch operation with coronary artery transfer is planned for the biventricular repair of DORV (Fig. 16) [10,17,25]. Commissural malalignment of the semilunar valves also frequently requires a modified coronary transfer technique during arterial switch operation [26].

Twisted Heart with Superior DORV
The atrial and ventricular septa are rarely twisted and malaligned in DORV, which almost always results in kjronline.org unusual interventricular geometry, superior RV-to-inferior left ventricle [27]. In this rare form of DORV, abnormal atrioventricular connection, large complex VSD, and unbalanced ventricular volume are not infrequent; therefore, the biventricular repair is usually not feasible.

Juxtaposed Atrial Appendages
The left juxtaposition of the atrial appendages is not infrequently encountered in DORV. In addition, anomalies are commonly associated with TGA and tricuspid atresia [25]. Embryologically, the anomaly is caused by the underdevelopment of torsion of the primitive cardiac tube [28]. The presence of this anomaly may pose some difficulties during interventional and surgical procedures, including intra-atrial manipulations [28].

Surgical Procedures
Various surgical options are available for DORV with VSD (Fig. 17). The major determinants for selecting the best surgical option for DORV with VSD are summarized in Table 1. Before definitive surgery, palliative surgical procedures are often performed in patients with DORV (Fig. 17). Pulmonary artery banding was performed to avoid the development of pulmonary vascular disease in patients with pulmonary arterial overflow (Fig. 14). Blalock-Taussig or central shunt is placed to augment pulmonary arterial flow and growth in cases with the reduced pulmonary arterial flow, usually at the age of two weeks or younger. In cases with severe cyanosis, a bidirectional cavopulmonary shunt may be created to improve systemic arterial oxygen saturation without increased right ventricular volume load, generally at 4-6 months.
Biventricular repair largely consists of intraventricular baffling to the aorta, intraventricular baffling to the pulmonary artery plus arterial switch operation, and double-root translocation (modified Nikaidoh operation) (Fig. 17) [17,19]. Intraventricular baffling of the aorta can be performed in subaortic or non-committed VSD closer to the aorta (Fig. 18). If a case is associated with pulmonary stenosis, relief of right ventricular outflow obstruction can be performed simultaneously. To relieve right ventricular outflow obstruction, right ventricular outflow tract reconstruction (Fig. 18) or placement of the RV-to-pulmonary artery conduit (Rastelli operation) may be selected. In subpulmonary or non-committed VSD closer to the pulmonary artery, intraventricular baffling to the pulmonary artery and arterial switch operation can be performed (Fig. 16)  kjronline.org root translocation, in which the normal geometry of the left and right ventricular outflow tracts is restored and at the same time function and growth potential of the aortic and pulmonary valves are preserved, can be performed with right ventricular outflow tract reconstruction (Fig. 19) [29,30].
In patients with DORV in whom biventricular repair is not feasible (for example, a functional single ventricle, swisscheese VSD type, and straddling of the atrioventricular valve) [19], one and a half or univentricular repair is performed, in which the pulmonary circulation is separated from the systemic circulation in both surgery categories. Pulsatile blood flow in the pulmonary arteries is maintained in

Major Postoperative Complications
After biventricular repair of DORV, left or right ventricular outflow tract obstruction occurs in approximately 3.8% to 30.0% of patients (Fig. 20) [17,19]. A muscular conus between the mitral and aortic valves, the length of an intraventricular baffle, and an acutely angulated intraventricular baffle are described as important predictors of postoperative left ventricular outflow tract obstruction (Fig. 20) [17,19]. The subaortic muscular conus demonstrates a dynamic change in its dimension throughout the cardiac cycle. Therefore, the end-systolic dimension should be measured to evaluate left ventricular outflow tract obstruction. After the Rastelli procedure, a RV-to-pulmonary artery conduit is subject to the conduit's gradual narrowing and intramural calcification. The conduit with significant obstruction should be replaced with a new conduit. Rarely, intraventricular baffle leak may occur, and a leak leading to a substantial left-to-right shunt (e.g., Qp/ Qs ≥ 1.8) should be repaired. A. Preoperative oblique sagittal CT image shows a large non-committed VSD (asterisk) closer to the relatively smaller P and subpulmonary stenosis. B. Postoperative oblique sagittal CT image demonstrates the posteriorly translocated aortic root and the anteriorly translocated pulmonary root (double arrow) with transannular patch widening. Both the right and left ventricular outflow tracts appear unobstructed. C. Standard volume-rendered CT image with superior view reveals the dilated main P due to patch widening and the P bifurcation anterior to the A that is typical after the Lecompte maneuver. A = ascending aorta, LA = left atrium, LV = left ventricle, P = pulmonary artery, RA = right atrium, RV = right ventricle, VSD = ventricular septal defect

CONCLUSION
DORV is a unique multifarious cardiac defect that demonstrates extremely varied cardiac anatomy. Therefore, it should not be considered a single entity, and such morphological diversity causes considerable diagnostic and therapeutic challenges in clinical practice. Detailed and comprehensive anatomic characterization of intracardiac anatomy can result in adequate diagnosis and successful surgical decision-making in DORV. In this article, the major anatomic determinants affecting surgical decision-making, common surgical procedures based on these determinants, and major postoperative complications are illustrated in detail using 3D cardiac CT data.

Conflicts of Interest
The author has no potential conflicts of interest to disclose.