Đang tải... (xem toàn văn)
(BQ) Part 2 book Echocardiography in pediatric and congenital heart disease from fetus to adult presents the following contents: Miscellaneous cardiovascular lesions, anomalies of ventricular myocardium, acquired pediatric heart disease, special techniques and topics.
PART V Miscellaneous Cardiovascular Lesions C H A P T E R 27 Hearts with Functionally One Ventricle Stephen P Sanders Departments of Cardiology, Pathology and Cardiac Surgery, Boston Children’s Hospital, Boston, MA, USA Introduction Hearts with functionally one ventricle comprise only a few percent of congenital heart defects [1], but patients with these hearts utilize a disproportionate amount of healthcare resources because of the complexity of management and the need for repeated interventions and lifelong care [2] Optimal management depends on early recognition and careful observation and planning Treatment principles include meticulous protection of the pulmonary vasculature, ongoing surveillance for and rapid treatment of systemic outflow obstruction, and maintenance of systemic ventricular function by avoiding volume and pressure overload as much as possible Despite attentive and intelligent management strategies, the long-term outlook for these patients is guarded [3] Palliation of patients with functionally one ventricle is likely to be effective for only two to four decades [4] Most, perhaps all, such patients will eventually require an alternative management strategy such as transplantation or mechanical support Many hearts in this category have only one ventricular sinus or body, anatomically as well as functionally [5,6] The other chamber usually present in the ventricular mass is an infundibulum or outlet chamber These include double-inlet left ventricle (DILV), most double-inlet right ventricle (DIRV), most tricuspid atresia (TA), and many mitral atresia hearts Others of this category, perhaps most, have two ventricular sinuses but one or both are unsuitable to function independently [7] Most of these are discussed in other chapters and will only be listed here Although not all cases of these defects have functionally one ventricle, many and must be identified as soon as possible to maximize outcomes Included are variants of hypoplastic left heart syndrome (Chapter 20), hypoplastic right heart syndrome (mostly pulmonary atresia with intact ventricular septum) (Chapter 17), straddling mitral valve (Chapter 14), straddling tricuspid valve (Chapter 13), unbalanced common atrioventricular (AV) canal (Chapter 15), congenitally physiologically corrected transposition of the great arteries (TGA) (Chapter 26), Ebstein anomaly (Chapter 14), heterotaxy syndromes (Chapter 28), and superiorinferior ventricles (SIV) and criss-cross heart Etiology The genetic and/or environmental causes of these defects are poorly understood Most cases appear to be sporadic although familial occurrences have been reported [8–10] There are a few case reports of syndromic association of TA or other functionally single ventricle hearts [11–18] The recurrence risk among first-degree relatives appears to be in the range associated with polygenic inheritance (2–5%) [19–22] Embryologic development Concepts about development of most congenital heart defects are speculative because no one has ever observed active, in vivo development of a defective human heart However, it is possible to infer likely mechanisms from what is known about normal human development and abnormal development in animal models The following brief description of normal development is provided for comparison with the proposed abnormal development in subsequent sections In early looping the heart tube is rather uniform with no clear demarcation of chambers Cells from the second heart field are added to both ends of the heart tube as it elongates and loops [23] The dorsal mesocardium, which initially joins the heart tube throughout its length to prepharyngeal mesoderm, degenerates in its mid portion [24] allowing the elongating heart tube to bend anteriorly and then rightward, called dextral or D-looping [25] As the heart chambers begin expansile growth from the outer curvature of the looping heart tube [26], the AV canal becomes apparent as a constriction between the common atrial chamber and the developing left ventricle and the interventricular foramen as a constriction between the developing Echocardiography in Pediatric and Congenital Heart Disease: From Fetus to Adult, Second Edition Edited by Wyman W Lai, Luc L Mertens, Meryl S Cohen and Tal Geva © 2016 John Wiley & Sons, Ltd Published 2016 by John Wiley & Sons, Ltd Companion website: www.lai-echo.com 511 512 Part V Miscellaneous Cardiovascular Lesions ventricles The AV canal is exclusively aligned with the developing left ventricle and the outflow continues from the inner curvature of the heart tube at the rostral end of the developing right ventricle As the heart chambers continue to enlarge, the right side of the AV canal and the right atrium grow faster than the left, allowing the AV canal to expand above the right ventricle and below the right atrium, creating a right ventricular inflow and establishing alignment of the right atrium with the right ventricle through the right side of the AV canal [27] Swellings or cushions develop in the AV canal as well as the outflow [28] At the same time, septation of the common atrium begins with downward growth of septum primum or the primary atrial septum from the superior wall and invasion of the dorsal mesenchymal protrusion or vestibular spine from the posterior-inferior wall [29] These septal structures continue to grow into and septate the common atrium, finally fusing with the two main AV cushions At that point the AV cushions fuse to each other as well, separating the AV canal into right and left halves, aligning the right atrium and ventricle and the left atrium and ventricle, respectively Before completion of atrial septation, the septum primum, near its origin from the superior wall, breaks down forming ostium secundum and allowing continued communication between the right and left atria The fused AV cushions then become draped over the muscular inflow ventricular septum that has developed on their ventricular side [30] Meanwhile the outflow cushions fuse from distal to proximal under the influence of cardiac neural crest cells, dividing the outflow into aorta and pulmonary artery [31] As this occurs the proximal outflow undergoes counterclockwise rotation, as viewed from the ventricles, so that the aorta which is anterior distally comes to lie rightward and posterior proximally, and the pulmonary artery which is posterior distally moves anteriorly and leftward proximally [32] The outflow septum, which developed by fusion and muscularization of the proximal parts of the outflow cushions, then inserts into the limbs of the interventricular foramen, aligning the aorta with the left ventricle and the pulmonary artery with the right ventricle, and closing the interventricular foramen [33] The AV valves develop from the AV cushions by a process that involves thinning, elongation and separation from underlying myocardium by apoptosis of cardiomyocytes [30] The semilunar valves develop from the outflow cushions by excavation and thinning that also involves apoptosis probably initiated by neural crest cells [34] Double-inlet left ventricle DILV includes hearts in which both AV valves are aligned with and connected to one ventricular chamber of left ventricular morphology [6] (Figures 27.1–27.6; Videos 27.1–27.5) Embryologic development DILV is easily envisioned as an arrest of development at the stage where the AV canal is completely aligned with the developing left ventricle Most often DILV is associated with leftward bending or looping (levo or L-looping) of the heart tube, although 30– 35% of the time dextro or D-looping occurs DILV is associated with failure of development and growth of the right ventricular sinus As the AV canal expands toward the inner curvature of the heart tube it becomes aligned with the right atrium However, the expanding edge does not cross the foramen between the left ventricle and outflow as it does in the normal heart It is unclear if confinement of the AV canal to the left ventricle is primary and failure of RV development secondary, or the converse In either case, atrial septation appears to proceed correctly with the septum primum and dorsal mesenchymal protrusion fusing with and inducing central fusion of the main AV cushions to form two AV valves It is interesting that development of the AV valves proceeds rather normally despite lacking an underlying muscular ventricular septum over which to drape themselves The infundibulum or outlet chamber derives from the outflow portion or ascending limb of the heart tube and maintains its primitive connection with the left ventricle, the outflow foramen, which would have been the interventricular foramen had the right ventricle developed In fact, the size of the outlet chamber is somewhat variable and in some cases it appears that a small portion of right ventricular sinus might be present, especially in hearts where the outlet chamber extends inferiorly toward the diaphragmatic heart border Alternatively, the larger size of the outlet chamber could be from expansion of the apical trabecular portion of the infundibulum or outlet chamber Abnormal ventriculo-arterial (VA) alignment, present in the great majority of these hearts, is most likely due to abnormal rotation of the outflow so that the proximal aorta remains anterior and the proximal pulmonary trunk posterior [35,36] This abnormal rotation is also associated with persistence of the outflow foramen between the left ventricle and outlet chamber due to failure of the outlet septum to join the limbs of the outflow foramen [36] Rarely, and essentially exclusively in D-loop hearts, the rotation of the outflow occurs correctly resulting in normally aligned great arteries Even in these cases, however, the outflow septum rarely completely closes the outflow foramen Anatomy The systemic and pulmonary veins and atria are usually normal (for situs solitus) Although reported in situs inversus [37,38], such hearts are extraordinarily rare because situs inversus and DILV are both rare conditions The AV valves are often recognizable as a mitral or tricuspid valve [39,40] The valve near the septal wall of the left ventricle (see later) is often tricuspid in character and the one nearest the free wall more mitral (Figure 27.6) In other cases the valves are symmetrical, resembling each other more than either a mitral or tricuspid valve Hypoplasia and stenosis of an AV valve is seen in up to 15–20% of hearts [40] (Figure 27.7) Rarely a common AV valve is aligned and connected to an isolated left ventricle The left ventricular chamber has a free wall on one side with multiple papillary muscles and a smooth wall on the other that Chapter 27 Hearts with Functionally One Ventricle (a) 513 (b) (c) Figure 27.1 A heart specimen of DILV with transposition of the great arteries {S,L,L} (a) Opened left ventricle The smooth septal wall is to the right of the image and the free wall is to the left The right AV valve (RAVV) is mitral-like with a deep medial leaflet and a shallow lateral leaflet while the left AV valve (LAVV) has three leaflets and attachments onto the inferior septal wall near the outflow foramen (OF), more like a tricuspid valve The pulmonary valve (PV) is between the AV valves Only a right hand (upper left) can describe this L-loop or inverted left ventricle, with the thumb in the AV valves, the fingers in the pulmonary valve and the palm against the septal wall (b) Opened outlet chamber The aortic valve (AoV) is supported by the ring of infundibular muscle that comprises the outlet chamber Part of the LAVV is seen through the outflow foramen (OF) below the AoV (c) Zoomed view of the PV and OF Three mechanisms for subpulmonary obstruction are illustrated here: accessory AV valve tissue (arrow), fibromuscular ridge (∗), and deviation of the outlet septum (dashed line) Note that the outlet septum is nearly perpendicular to the plane of the muscular septum and does not occupy the OF is the left ventricular septal wall (Figures 27.1–27.3) Note that the septal wall is a characteristic feature of the left ventricle and is present even when there is no ventricular sinus on the other side A large muscle bundle, the posterior median ridge, is often present on the inferior or diaphragmatic wall of the left ventricle running from base to apex between the AV valves (Figure 27.3a) An infundibulum or outlet chamber is uniformly associated with the left ventricle (Figures 27.1b and 27.2b) and the AV valve closer to the septum frequently straddles into it [6,40] (Figure 27.3) The connection between the left ventricle and outlet chamber is variously called a bulboventricular foramen, interventricular foramen, or ventricular septal defect In fact, the type and location of the communication is variable [40,41] Most often it is the persisting outflow foramen of the embryonic heart, between the conal or infundibular septum (outlet septum) and the left ventricular septal wall (Figures 27.1 and 27.3) The infundibular or outlet septum is readily identified between the semilunar valves because it is rarely correctly inserted into the muscular ventricular septum (Figures 27.1c and 27.2b) In other cases, the communication is a muscular defect between the mid or apical part of the outlet chamber and the left ventricle (Figure 27.7) The size of the communication is also variable [40–42] A small communication is a frequent cause of obstruction to whichever semilunar root arises from the 514 Part V Miscellaneous Cardiovascular Lesions (a) (b) (c) Figure 27.2 Three echocardiographic views of a heart with DILV and transposition of the great arteries {S,L,L} (a) Apical 4-chamber view showing both AV valves entering the left ventricle (LV) (b) Subxiphoid long-axis view showing the right atrium (RA) aligned with the LV through the right AV valve, the posterior and rightward pulmonary artery (PA) aligned with the LV, and the anterior and leftward aorta (Ao) aligned with the outlet chamber (OC) The outlet septum (▴) is deviated posteriorly and leftward toward the PA The space between the deviated outlet septum and the LV septal wall is the outflow foramen (c) Parasternal short-axis view showing the left (LAVV) and right (RAVV) AV valves in the LV, the leftward and superior OC, and the outflow foramen connecting it to the LV A right hand (right upper) is required to describe the LV with the thumb into the AV valves, the fingers in the outflow and the palm against the septal wall outlet chamber A persistent outflow foramen is often large and unobstructed while more apical muscular communications are essentially always obstructed [40] The situs or organization of the left ventricle can be either solitus or D-loop, or inversus or L-loop (Figures 27.1a, 27.2b, and 27.3a) The hand rule is useful to distinguish between the types of left ventricle [43] One imagines placing the thumb in an AV valve, the palm against the septum, and the index finger in the outflow If this can be done with the left hand, the left ventricle is solitus or D-loop Alternatively, if a right hand is required, the left ventricle is inverted or L-loop (Note that the handedness of the left ventricle is opposite to that of the right ventricle where the hand rule is more frequently applied.) The importance of determining ventricular situs or loop is threefold: first, the conduction system is usually superior to the outflow foramen in an inverted or L-loop left ventricle (with atrial situs solitus) but inferior in a solitus or D-loop left ventricle [44,45]; second, the VA alignment is essentially always transposition (discordance) when the left ventricle is inverted while in 10% or so of DILV with a solitus or D-loop left ventricle the great arteries are normally related (concordance) [40]; and third, the epicardial course of the coronary arteries is determined by the arrangement of the ventricles [46] When the great arteries are transposed the pulmonary valve is typically wedged between the AV valves and is in fibrous continuity with both (Figures 27.1a and 27.3a) while the aortic valve is aligned with the outlet chamber and supported by infundibular muscle (Figures 27.1b and 27.3b) Conversely, in normally Chapter 27 Hearts with Functionally One Ventricle (a) 515 (b) Figure 27.3 A heart specimen of DILV with transposition of the great arteries {S,D,D} (a) Opened left ventricle The smooth septal wall is to the left in this image and the free wall to the right The left (LAVV) and right (RAVV) AV valves not have clear mitral or tricuspid character, except that the right AV valve attaches to the septal wall The RAVV also straddles through the outflow foramen (OF) into the outlet chamber The pulmonary valve (PV) is between the AV valves A posterior median ridge (∗) courses down the free wall between the AV valves Only a left hand (upper left) can describe this ventricle, with the thumb in the AV valves, the fingers in the PV and the palm against the septal wall (b) Opened outlet chamber The aortic valve (AoV) is supported by the infundibular muscle of the outlet chamber A small part of the RAVV straddles into the outlet chamber through the outflow foramen related great arteries it is the aorta that is wedged between the AV valves and the pulmonary artery arises from the outlet chamber The coronary artery anatomy depends on the ventricular situs or loop because the epicardial coronary pattern seems to be determined by cues from the underlying ventricle When the great arteries are malposed (transposition or double outlet), the arteries arise from the posterior, facing sinuses In an L-loop DILV, the morphologically left coronary artery is right-sided and bifurcates into a small delimiting artery (analogous to the anterior descending artery) that runs in the groove between the left ventricle and outlet chamber and a circumflex artery that passes posteriorly in the right AV groove, giving off a variable number of atrial and ventricular branches along its course The morphologically right coronary artery passes posteriorly in the left AV groove, giving off a posterior delimiting artery and a variable number of ventricular branches that run toward the apex In Dloop DILV with abnormal VA alignment, the coronary pattern is just the mirror image of that described earlier If the VA alignment is concordant (normally related great arteries), the coronary pattern is usually much like that seen in the normal heart where the ostia are in the anterior, facing sinuses The aortic arch is usually to the left and branching is usually normal The orientation of a left aortic arch is often from left-anterior to right-posterior in L-transposed arteries simply because of the location of the ascending aorta The branch pulmonary arteries are usually normal, even in hearts with subvalvar and/or valvar pulmonary stenosis The cardiotypes most frequently associated with DILV are: {S,L,L} (solitus atria, inverted L-loop left ventricle, leftward and anterior aorta) – 60%; {S,D,D} (solitus atria, solitus D-loop left ventricle, rightward and anterior aorta) – 20%; and {S,D,S} (solitus atria, solitus D-loop left ventricle, and solitus normally related great arteries) which carries the eponym Holmes heart – 15% [40] A number of associated defects can be seen with DILV A secundum atrial septal defect is occasionally present but the atrial septum is often intact Abnormalities of the AV valves including hypoplasia or stenosis or regurgitation (Figure 27.6), are seen in a significant proportion [39–41] Subvalvar obstruction of the posterior root – the one aligned with the left ventricle – can be due to posterior malalignment of the outlet septum, AV valve tissue, or a fibromuscular ridge (Figures 27.1c and 27.2c) The outflow foramen is frequently obstructed [40,41] If the great arteries are transposed this results in subaortic stenosis and often hypoplasia of the aorta with coarctation or even arch interruption Conversely, the obstruction is subpulmonary when the great arteries are normally related Physiology DILV results in mixing of systemic and pulmonary venous blood in the left ventricle If mixing is complete, the oxygen saturation is similar in the aorta and pulmonary artery and determined by the ratio of pulmonary to systemic blood flow (Qp/Qs) Occasionally there can be remarkable streaming that produces a marked difference between systemic and pulmonary arterial oxygen saturation Obstruction of the outflow foramen with TGA favors pulmonary blood flow and diminishes systemic cardiac output but does the opposite with normally related great arteries Left ventricular volume overload is characteristic of essentially all forms of DILV because the left ventricle must pump both systemic and pulmonary blood flow The higher the Qp/Qs 516 Part V Miscellaneous Cardiovascular Lesions (a) (b) (c) Figure 27.4 Three echocardiographic views of a heart with DILV and transposition of the great arteries {S,D,D} (a) Apical 4-chamber view showing both AV valves aligned with the left ventricle (LV) (b) Subxiphoid short-axis view showing the right (RAVV) and left (LAVV) AV valve in cross-section The pulmonary artery (PA) is aligned with the LV (c) A more apical short-axis cut showing the aorta (Ao) arising from the outflow chamber (OC) The LV can only be described using the left hand, with the thumb in the AV valves, the fingers in the outflow and the palm against the LV septal wall is, the greater the volume overload and the higher the systemic oxygen saturation Chronic volume overload results in left ventricular dilation, eccentric hypertrophy, and eventually adverse remodeling Obstruction to ventricular outflow or aortic arch obstruction causes concentric hypertrophy, myocardial fibrosis and diastolic dysfunction Concentric hypertrophy further narrows the outflow resulting in a positive feedback loop that can quickly lead to ventricular failure Excessive pulmonary blood flow, especially with pulmonary hypertension, damages the pulmonary vascular endothelium and leads to progressive pulmonary vascular obstructive disease An atrial septal defect facilitates mixing and reduces streaming AV valve regurgitation increases the ventricular volume overload The effect of AV valve stenosis depends on which valve is involved and whether an atrial septal defect is present In the absence of an atrial defect, stenosis of the left (pulmonary) AV valve causes pulmonary venous hypertension with pulmonary congestion and increased pulmonary arterial and venous smooth muscle Conversely, right (systemic) AV valve stenosis causes systemic venous hypertension with liver engorgement, peripheral edema, and serous effusion Treatment strategies Neonatal palliation is usually undertaken for systemic outflow and/or aortic arch obstruction Amalgamation of the aorta and pulmonary trunk is frequently used to bypass obstruction of the outflow foramen between the left ventricle and outlet chamber [47] Some type of systemic-pulmonary shunt is then necessary to provide pulmonary blood flow Another approach is enlargement of the outflow foramen [45] Extensive arch reconstruction may be necessary in cases of arch hypoplasia or interruption Chapter 27 Hearts with Functionally One Ventricle (a) 517 (b) Figure 27.5 Two echocardiographic views of a Holmes heart, DILV with normally related great arteries {S,D,S} (a) Parasternal long-axis view showing the left AV valve (LAVV) entering the left ventricle (LV) and the aorta (Ao) normally aligned and connected to the LV The outlet septum (▴) is small and mildly deviated posteriorly The outflow foramen connects the LV with the outlet chamber (OC) This solitus or D-loop left ventricle can only be represented by a left hand with the thumb in the LAVV, the fingers in the Ao, and the palm against the septal wall (b) Subxiphoid short-axis view through the base of the LV showing the LAVV and right AV valve (RAVV) The LAVV has a medial leaflet and a mural leaflet characteristic of a mitral valve The RAVV has three leaflets, medial, inferior, and septal, characteristic of a tricuspid valve The communication between the LV and outlet chamber (OC) extends from the outlet septum (∗), seen just posterior to the pulmonary valve (PV), behind the septal leaflet of the RAVV The size of the OC and the inferior extension of the outflow foramen suggest the presence of some RV sinus in this heart Severe pulmonary stenosis or atresia prompts creation of a systemic-pulmonary shunt Patients with unrestricted pulmonary blood flow and no systemic outflow obstruction often undergo pulmonary artery banding within the first months of life to treat heart failure and improve outlook for Fontan palliation [48] Figure 27.6 Apical 4-chamber view (left) and color flow map (right) of a heart with DILV, normally related great arteries {S,D,S}, and right AV valve hypoplasia Note the aneurysm of septum primum (▴) bulging into the left AV valve Patients with stenosis of an AV valve can benefit from creation of an atrial septal defect, either by interventional catheter procedure or surgery, to relieve systemic or pulmonary venous obstruction Conversely, severe insufficiency of an AV valve may prompt plasty or even patch closure of the valve, with creation of an atrial defect, to relieve the volume overload 518 Part V Miscellaneous Cardiovascular Lesions Figure 27.7 Opened left ventricle of a heart with DILV and double-outlet infundibulum {S,D,D} There is a second, more apical communication (▴) between the left ventricle and the outlet chamber in addition to the outflow foramen (OF) The long-term strategy for these patients is staging toward a Fontan procedure, with creation of a bidirectional cavopulmonary anastomosis around months of age and completion of the Fontan procedure around year of age [49] Double-inlet right ventricle DIRV is a rare heart defect (0.2% of cardiac autopsies and in 11,000 patients seen by a cardiology service [50]) in which, analogous to DILV, both AV valves are aligned and connected with a ventricle of right ventricular morphology (Figures 27.8 and 27.9; Video 27.6) Some series [51,52] have described a preponderance of cases with heterotaxy syndrome, unbalanced common AV canal, and a hypoplastic left ventricle (see Chapter 28) Embryologic development The development of DIRV is more difficult to understand because there is never a time during normal development when the AV canal is completely aligned with the developing right ventricle Even the direction of looping of the heart tube is uncertain in most cases because there is no clear septal wall of the right ventricle present and a rudimentary left ventricle is demonstrable in a minority [50] In the few cases where a small LV cavity is present, the loop appears to have been dextral in most Expansion of the AV canal appears to be normal or nearly so because it becomes normally aligned with the right atrium Further, the combined size of the two AV valves is substantially larger than either a single mitral or tricuspid valve What allows the AV canal to become situated completely above the developing right ventricle is unclear Either the LV cavity is absent or small primarily or it becomes so after losing alignment with the AV canal Atrial septation proceeds normally as in DILV resulting in division of the AV canal into two separate valves, both aligned with the right ventricle Again, it is interesting that the AV valves develop relatively normally despite absence of the muscular inflow septum The outflow then develops in broad continuity with the underlying right ventricle without interposition of an outflow foramen as seen in DILV The infundibular or outlet septum divides the outflow into aortic and pulmonary components and comes to sit above the right ventricular cavity within the outflow muscular sleeve, which is the cranial continuation of the developing right ventricle Outflow rotation appears to be abnormal in most cases resulting in double-outlet right ventricle with side-by-side or otherwise malposed great arteries On the other hand, rotation may approximate normal in hearts with a rightward and posterior aorta and anterior and leftward pulmonary artery Uneven division of the outflow, with or without deviation of the infundibular septum, likely explains the hypoplasia and obstruction of either the pulmonary or aortic outflow seen frequently in DIRV Anatomy Persistence of a left superior vena cava to coronary sinus and secundum atrial septal defect appear to be frequent findings [50] The morphology of the AV valves resembles a normal mitral or tricuspid valve less frequently than in DILV (Figures 27.8 and 27.9) Hypoplasia or stenosis of one AV valve, most frequently the left, occurs in 25% or more of cases (Figure 27.8b,c) but more than mild regurgitation is uncommon [50] A tiny hip-pocket left ventricle (Figure 27.8c) is present in up to 20–25% of patients [50], although some series have reported a higher prevalence [51] It is located posteriorly near the AV groove and usually communicates with the right ventricle through a ventricular septal defect In these cases there are clearly two ventricular sinuses although the rudimentary left ventricle can never function independently The right ventricle is often large, hypertrophied and bizarrely shaped with a few large muscle bundles There is a prominent posterior-median ridge passing from base to apex between the AV valves (Figure 27.8b and 27.9) and which often receives attachments of the AV valves This can give the appearance of a ventricular septum on clinical imaging studies and has resulted in an erroneous diagnosis of two ventricular sinuses with multiple ventricular septal defects A characteristic feature of DIRV is absence of a septal wall with a smooth basal endocardial surface (the left ventricular septal wall) as seen in DILV In addition the outflow of the heart continues broadly from the right ventricle with no constriction (outflow foramen) separating the outlet chamber from the ventricle as seen in DILV (Figures 27.8 and 27.9) The infundibular or outlet septum sits above the right ventricular cavity dividing the subarterial infundibular sleeve and does not usually join a wall of the ventricular body Uneven division of the outflow into subpulmonary and subaortic infundibula, with or without deviation of the outflow septum, is a frequent cause of obstruction, especially subpulmonary obstruction (Figures 27.8a and ... chamber and the developing left ventricle and the interventricular foramen as a constriction between the developing Echocardiography in Pediatric and Congenital Heart Disease: From Fetus to Adult, ... ventricles in SIV and the crossing of the AV valves in criss-cross heart are also best appreciated from subxiphoid views [68, 72] (Figures 27 .21 and 27 .22 ) Apical views are useful for examining and measuring... ventricles (Figure 27 .21 ) and from right-posterior to left-anterior in L-loops (Figure 27 .22 ) The crossing angle between the valves varies from 20 –100◦ [68] and is inversely proportional to the size