Transposition of the Great Arteries

Published on 21/06/2015 by admin

Filed under Cardiovascular

Last modified 22/04/2025

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 4.5 (2 votes)

This article have been viewed 5408 times

8 Transposition of the Great Arteries

Amy H. Schultz

d-TGA: Preoperative Imaging

Background

• In this lesion, the atria and ventricles are aligned normally, but the great arteries are transposed, with the aorta (Ao) arising from the right ventricle (RV) and the pulmonary artery (PA) arising from the left ventricle (LV) (Fig. 8-1).
• The aortic valve (AV) is typically anterior and rightward of the pulmonary valve (pulmV).
• Deoxygenated systemic venous blood circulates to the right atrium (RA) to the RV to the aorta and thus back to the body.
• Oxygenated pulmonary venous blood circulates to the left atrium (LA) to the LV to the PA and thus back to the lungs.
• Mixing between the two circuits is required to sustain life. This can occur via an atrial septal defect (ASD), ventricular septal defect (VSD), or patent ductus arteriosus (PDA).
• Mixing is generally most effective and reliable at the atrial level. Thus, assessment of the size of the ASD is key for acute management.
• Major associated lesions are

• Dextro transposition of the great arteries (d-TGA) with VSD.
• d-TGA with VSD and pulmonary stenosis (PS) (typically at both the valvar and subvalvar levels).

image

Figure 8-1 Schematic diagram of the anatomy of dextro transposition of the great arteries (d-TGA). Note that the systemic venous return (blue) and pulmonary venous return (red) circulate separately unless a shunt at the atrial, ventricular, and/or ductal level permits mixing between the two circuits.

Overview of Echocardiographic Approach

• Identify the segmental anatomy (two dimensional [2D]).

• Normal atrial situs.
• Normal ventricular looping.
• d-TGA

• The aorta arises from the RV.
• The PA arises from the LV.

• The segmental anatomy can be abbreviated {S,D,D}.

• Presence or absence and size of shunts at the atrial, ventricular, and ductal levels (2D, color Doppler).
• Relative size, morphology, and function of the AV and pulmV (2D, color Doppler, pulsed wave [PW] Doppler, continuous wave [CW] Doppler).
• Coronary artery anatomy (2D, color Doppler).
• Transthoracic echocardiography (TTE) is essentially always the first study in neonates.

Anatomic Imaging

Acquisition

• Subcostal frontal (long axis [LAX]) sweep, posterior to anterior.

• Establish normal visceroatrial situs: visualize the inferior vena cava within the liver on the right, connecting to the right-sided atrium. The left-sided atrium should have some pulmonary veins (PVs) visible by 2D imaging.
• Establish normal atrioventricular alignments: two separate atrioventricular valves (AVVs) are visualized. Two ventricles of good size are visualized.
• Demonstrate transposed ventriculoarterial connections.

• The more posterior great vessel (GV) arising from the LV bifurcates, identifying it as the PA (Fig. 8-2A).
• The more anterior GV arising from the RV does not bifurcate and is thus the aorta; coronary artery origins may be seen from it.
• Note also that the GVs do not cross each other (as is the case in normally related GVs).

• Take note as to whether a coronary artery is seen running posterior to the pulmonary root as you sweep anterior from the mitral valve (MV) to the pulmV. This will be useful information to integrate with later parasternal short axis imaging of the coronary arteries.

• Subcostal short axis sweep, base to apex.

• Atrial morphology is again noted to be normal with the systemic veins entering the RA and at least some PVs visualized entering the LA. Assess the size of interatrial communication (patent foramen ovale [PFO] or ASD).
• Ventricular morphology is examined and is consistent with normal ventricular looping. The left-sided ventricle has a smooth septal surface and AVV morphology consistent with an MV (no chordal attachments to the septum). The right-sided ventricle is trabeculated, with chordal attachments of the AVV to the septum. Assess for VSD (size, location).
• GVs are noted to be transposed with the bifurcating PA arising posteriorly from the LV and the aorta arising anteriorly from the RV. Their parallel relationship is well seen (see Fig. 8-2B).

• Apical views

• Four chambers, of proportionate size, are seen. Confirm that the ventricular morphology appears usual (right-sided ventricle with moderator band, left-sided ventricle with smooth septal surface).
• Note any evidence of VSD or outflow tract obstruction.
• With anterior angulation, the bifurcating PA is seen arising from the LV. If present, a retropulmonary coronary artery may be seen by angling anteriorly from the MV to the pulmV.
• With further anterior angulation, the aorta may be seen arising from the LV.

• Parasternal long axis view

• Parallel outflow tracts and proximal GVs are seen (Fig. 8-3).
• Optimize image of each semilunar valve for measurement of annulus diameter.
• There is mitral-pulmonary fibrous continuity. If not present, consider the diagnosis of double-outlet RV.
• Note any evidence of VSD and/or outflow tract obstruction.

• Parasternal short axis view

• Note the relative orientation of the semilunar valves (Fig. 8-4). Typically, the AV is anterior and rightward of the pulmV, but can range from directly anterior to rightward and side by side.
• Note morphology of the semilunar valves. The intercoronary commissure of the AV should align with a commissure of the pulmV. If the commissures are not aligned, coronary transfer may be more difficult.
• Note any evidence of VSD. If one VSD is present, be sure to examine the remainder of the muscular septum for additional defects.
• Evaluate coronary artery anatomy: origins from the sinuses and proximal courses must be clearly visualized.

• Many variants are known, although the two most common patterns are present in about 80% of cases.
• Complex coronary artery patterns may substantially increase the risk of the arterial switch operation (ASO). Rarely, a complex coronary artery pattern precludes ASO.
• There are two major conventions for nomenclature, the Leiden convention (Fig. 8-5 and Table 8-1) and the descriptive approach advocated by cardiologists at Children’s Hospital Boston (Fig. 8-6).
• Typically, the coronary arteries arise from the two sinuses of the AV that are adjacent to or “face” the sinuses of the pulmV (see Fig. 8-4).
• In the most common pattern (1L, Cx; 2R) or “usual,” the left coronary artery (LCA) arises from the leftward facing sinus and gives rise to the anterior descending artery and circumflex artery (Cx) (Fig. 8-7). The right coronary artery (RCA) arises from the rightward facing sinus.
• In the second most common pattern (1L; 2R, Cx) or “circumflex from RCA,” the leftward facing sinus gives rise solely to the anterior descending, whereas the RCA, arising from rightward facing sinus, also gives rise to the Cx. The circumflex passes posterior to the pulmonary root to reach the left atrioventricular groove; this segment can often be visualized from the subcostal or apical four-chamber (4C) views (Fig. 8-8).
• Visualization of the coronary arteries is facilitated by decreasing the dynamic range and careful adjustment of focal zone and zoom features.
• Be alert for any evidence of intramural coronary artery (passing between the two GVs), which is rare, but can significantly complicate an ASO (Fig. 8-9). A “double border” appearance of the posterior aortic root is noted when a coronary artery passes between the aortic and pulmonary roots.

• Suprasternal notch views

• The usual assessment of arch sidedness and branching is performed. A right aortic arch is uncommon in d-TGA.
• Coarctation is rare unless evidence of right ventricular outflow tract (RVOT) (subaortic) obstruction (RVOTO) is present.
• A PDA is best seen in d-TGA in the sagittal view of the arch due to the parallel orientation of the GVs (Fig. 8-10). The PDA should be assessed for size, direction of shunting, and gradient.
• Short axis suprasternal notch views can sometimes be used to supplement coronary artery imaging, if the patient is positioned with a shoulder roll and significant neck extension.

• Transesophageal echocardiography (TEE)

• Given typically excellent transthoracic windows in neonates, TEE is essentially never used for initial diagnosis of d-TGA. TTE is superior for visualization of the GVs and PDA, which are key in this diagnosis.
• Depending on institutional practice, a preoperative TEE may be performed in the operating room. See Chapter 4, Intraoperative Transesophageal Echocardiography, for discussion.
• Three-dimensional (3D) imaging generally adds little information to the study, unless a complex muscular VSD is present, in which case 3D imaging can be used to evaluate its size and location.

image

Figure 8-2 A, This subcostal frontal image shows the branching pulmonary artery (PA) arising from the left ventricle (LV). The color Doppler panel shows flow into the PA (red), consistent with a patent ductus arteriosus (PDA). The left PA is not seen in this view. B, This subcostal sagittal view shows the entire course of the aorta (Ao), arising anteriorly from the right ventricle (RV). The PA parallels the aorta posteriorly. The ductus is patent with color Doppler seen from the aorta to the PA.

image

Figure 8-3 Parasternal long axis view demonstrating the parallel great vessels characteristic of d-TGA. There is fibrous continuity between the mitral valve and pulmonary valve.

image

Figure 8-4 Parasternal short axis (SAX) view showing the orientation of semilunar valves. In this case, the aortic valve (AV) is anterior and rightward, which is typical. The origins of the right and left coronary arteries are seen from the two sinuses “facing” the pulmV, although the origin of the circumflex coronary artery (Cx) is not demonstrated here.

image

Figure 8-5 A, Schematic diagram explaining the Leiden convention for numbering the sinuses of the AV to describe coronary artery anatomy. From the perspective of standing in the noncoronary sinus, looking from the AV toward the PA, the sinus on the right-hand side is 1 and that on the left-hand side is 2. Note that this convention is easily applied by the surgeon standing on the right-hand side of the patient. B, Schematic with numbering in echo view. The convention of diagnostic image display is such that cranial is behind the screen, and thus it can become confusing trying to envision oneself standing in the noncoronary sinus. It is easier to remember that sinus 1 is the more leftward and/or anterior facing sinus (depending on the relative orientation of the great vessels) and sinus 2 is the more rightward and posterior sinus. This figure shows the numbering of the sinuses from the parasternal short axis view of the two semilunar valves. RCA, right coronary artery; Cx, circumflex coronary artery; AD, anterior descending artery.

TABLE 8-1 ABBREVIATION CONVENTION USED IN THE LEIDEN CONVENTION FOR DESCRIBING CORONARY ARTERY ANATOMY

Symbols used are as follows:
1 = sinus 1
2 = sinus 2
R = right coronary artery
L = left anterior descending coronary artery; also designated by some authors AD, as it may not arise from the left.
Cx = circumflex coronary artery
Comma = major branches originate from a common vessel
Semicolon = major branches originate separately
Supplemental terms may be used to describe epicardial course and unusual origins.
The series of assigned symbols for any given anatomy are enclosed in parentheses. For example: “normal” or “usual” coronary arteries for d-TGA are designated (1L, Cx; 2R), meaning the left anterior descending and circumflex originate from a common vessel from sinus 1, whereas the right coronary artery arises separately from sinus 2.
image

Figure 8-6 The eight most common coronary artery patterns in d-TGA are shown as seen from the parasternal short axis echocardiographic view. The frequency of occurrence is shown in the corner of each panel. The Leiden convention symbols and the eponym assigned by the Boston convention are shown in each panel.

(Adapted from Figure 24.5 in Mertens LL, Vogt MO, Marek J, Cohen MS. Transposition of the great arteries. In: Lai WW, Mertens LL, Cohen MS, Geva T, eds. Echocardiography in Pediatric and Congenital Heart Disease, Hoboken, NJ: Wiley-Blackwell; 2009.)

image

Figure 8-7 The left main coronary artery and its bifurcation into the anterior descending artery and Cx are shown in this modified parasternal short axis view. Often some off-axis angulation is needed to demonstrate the course and bifurcation of the coronary arteries.

image

Figure 8-8 As demonstrated in Figure 8-6, when the Cx arises from the RCA (1L; 2R, Cx, or Cx from right coronary artery [RCA]), it reaches the left atrioventricular groove by a retropulmonary course. This may be difficult to see from the parasternal short axis view, but is usually readily evident from the subcostal frontal (shown here) or apical view.

image

Figure 8-9 Intramural coronary artery in d-TGA: this parasternal short axis view shows the RCA and left coronary artery (LCA) running between the AV and pulmV, consistent with an intramural course. Both coronary arteries originated high and close together, adjacent to the commissure that faced the pulmV.

image

Figure 8-10 Suprasternal sagittal two-dimensional (2D) image showing the PDA arising from the aorta in d-TGA.

Analysis

• Quantitative analysis consists of measuring sizes of shunts and semilunar valves.
• Atrial shunts (ASD or PFO) should be measured from the subcostal long axis or short axis views.
• VSDs should be measured in whichever view they are seen best, ideally in two orthogonal planes.
• The PDA should be measured at its narrowest point, in the suprasternal notch sagittal view of the arch.
• The pulmV and AV are measured from the parasternal long axis view in systole; compare with norms for body surface area (Z scores).

Pitfalls

• The coronary arteries are the most difficult aspect of d-TGA to image.
• The most common pitfall is identifying an RCA and LCA from the two facing sinuses and assuming that the coronary pattern is (1L, Cx; 2R) or “usual,” when in fact the Cx has not been imaged. The second most common pattern, (1L; 2R, Cx) or “circumflex from RCA,” has a retropulmonary Cx, which can generally be well imaged from the subcostal long axis or the apical view. A slow, careful sweep in one of these views from the plane of the MV to the plane of the pulmV should identify this structure. However, the clinical consequence of this mistake is generally not important because both of the two most common patterns are readily amenable to ASO.
• Failure to identify a more complex coronary artery pattern, in particular, one involving an intramural coronary artery, will result in inaccurate counseling as to the risk of the operation. Many surgeons now consider all coronary patterns to be “switchable,” but the risk of the operation is certainly higher with a complex coronary artery pattern. Some surgeons will alter their operative approach due to an extremely complex coronary artery pattern.
• Also note any evidence of high or juxtacommissural origin.
• Avoid these pitfalls by careful 2D imaging with optimization of focal zone, zoom, and dynamic range settings with confirmation of structures believed to be coronary arteries with color flow imaging. Follow-up imaging may be needed if the pattern is not determined on the first study. Inspect the area between the two semilunar valves carefully for evidence of a “double border” of the posterior aortic root, which actually represents the coronary passing between the aortic and pulmonary roots. Keep an open mind as to what the coronary artery pattern might be rather than assuming it to be “usual.”
• Multiple muscular VSDs, which occur uncommonly with d-TGA, can be easy to miss in the presence of equal ventricular pressures due to either a large primary VSD or a large PDA. Without a pressure gradient across the ventricular septum (VS), color Doppler across such a defect will not produce an aliased jet. Careful inspection of 2D sweeps of the muscular septum from the subcostal short axis, apical, and parasternal short axis views can identify muscular defects, which can then be confirmed by color Doppler imaging.

Physiologic Data

Acquisition

• Subcostal views

• Perform frontal or short axis color Doppler sweeps of the atrial septum to identify shunts. The mean gradient across the atrial septum is determined by spectral Doppler.
• Short axis color Doppler sweeps of the VS are used to identify any VSD present and determine direction of shunting. Carefully assess any area that the 2D imaging suggests may have a defect (see Pitfalls). Shunting will not be aliased if ventricular pressures are equal due to a large VSD or PDA.
• Acquire peak velocity/peak instantaneous pressure gradient across any VSD in the view that most aligns the Doppler angle with the jet.
• RVOT and left ventricular outflow tract (LVOT) and the semilunar valves may be evaluated for evidence of stenosis or regurgitation by color and spectral Doppler from the subcostal views.

• Apical views

• Assess the apical muscular septum by color Doppler for evidence of VSD.
• Assess the RVOT, LVOT, and semilunar valve function by color Doppler and spectral Doppler.

• Parasternal short axis view

• Color sweep of the VS is used to identify any VSD present and determine the direction of shunting. Carefully assess any area that the 2D imaging suggests may have a defect (see Pitfalls). Shunting will not be aliased if ventricular pressures are equal due to a large VSD or PDA.
• Acquire peak velocity/peak instantaneous pressure gradient across any VSD in the view that most aligns the Doppler angle with the jet.
• Use color Doppler to confirm the presence and direction of blood flow in the coronary arteries (Fig. 8-11).

• Suprasternal notch view

• Use color Doppler and PW Doppler to determine direction of shunting in the PDA. Use CW Doppler to determine peak velocity and peak instantaneous pressure gradient (PIPG) across the PDA.
• Assess the flow in the aortic arch carefully by color Doppler and spectral Doppler if there is evidence of RVOTO.

image

Figure 8-11 Single LCA (1L, R, Cx). A, A single LCA seen from the parasternal short axis view with 2D imaging. B, Color imaging of the single LCA. C, 2D and color Doppler imaging showing the RCA branch coursing anterior to the aortic root.

Pitfalls

• Color Doppler in a PDA is frequently interpreted as being left to right when it is in fact bidirectional because the eye tends to focus on the red jet in diastole. Use of PW Doppler in the PDA easily identifies the directionality of shunting.

Alternate Approaches

• If TTE is unable to define the coronary artery pattern, cardiac catheterization is recommended.

Key Points

• Establish the diagnosis of d-TGA by demonstrating the segmental anatomy: normal atrial situs, normal ventricular looping, and d-TGA.
• Mixing between the systemic and pulmonary circuits is required to sustain life. This can occur via an ASD, VSD, or PDA, but is most effective at the atrial level.
• Thus, assessment of the size of the ASD is key for acute management.
• Major associated lesions are VSDs and PS.
• Coronary artery anatomy can be highly variable and can influence surgical management, although the two most common patterns account for 83% of cases of d-TGA.
• Intramural coronary arteries are most problematic and should not be missed.

Transposition of the Great Arteries: Status Post Arterial Switch Operation

Background

• The ASO became the standard operative approach for management of d-TGA in the late 1980s.
• In this operation:

• The great arteries are transected above the sinotubular junctions.
• The coronary arteries are transferred from the native aortic root to the “neo-aortic” (native pulmonary) root.
• The ascending aorta is anastomosed to the neo-aortic (native pulmonary) root.
• The branch PAs are brought anterior to the ascending aorta (LeCompte maneuver).
• The main PA (MPA) is anastomosed to the neo-pulmonary (native aortic) root.
• Any associated ASD or VSD is closed.
• The resulting anatomy is depicted in Figure 8-12.

image

Figure 8-12 Schematic depicting the anatomy of d-TGA after arterial switch operation (ASO). Note the suture lines in each GV.

Overview of Echocardiographic Approach

• Evaluate for complications related to the repair.

• Residual ASD or VSD (2D, color Doppler, PW Doppler, CW Doppler).
• Supravalvar PS at the anastomosis (color Doppler, PW Doppler, CW Doppler).
• Branch PA stenosis due to the branch PAs being stretched across the ascending aorta (color Doppler, PW Doppler, CW Doppler).
• Supravalvar aortic stenosis at the anastomosis (color Doppler, PW Doppler, CW Doppler).
• Coronary artery patency and secondary effects of coronary artery occlusion (regional wall motion abnormalities [RWMAs]) (2D).

• In addition, the abnormal natural history of the native pulmV and root functioning as the systemic outflow can result in:

• Neo-AV regurgitation (color Doppler).
• Neo-aortic root dilation (2D).

• TTE is the standard modality for assessment. TEE may be used in the case of poor transthoracic windows or pre- and postoperatively.

Anatomic Imaging

Acquisition

• Subcostal views

• Assess for residual ASD or VSD.
• In addition, in patients with good windows, subcostal short axis imaging provides excellent views of the RVOT and supravalvar pulmonary region.

• Apical views

• Assess the regional wall motion and evidence of supravalvar aortic stenosis in the five-chamber (5C) view.

• Parasternal views

• Long axis view: visualize neo-AV and root; neo-pulmV and root; supravalvar aortic and pulmonary anastomoses; assess regional wall motion.
• Short axis view: assess for residual ASD or VSDs; assess regional wall motion; evaluate proximal branch PAs, which now straddle the ascending aorta (may be better seen in the suprasternal notch view). If the windows are good, the reimplanted coronary arteries may be imaged, typically arising from the two sinuses of the neo-AV that “face” the neo-pulmV (native aortic).

• Suprasternal notch views

• Short axis view: visualize the branch PAs straddling the ascending aorta (Fig. 8-13).
• Sagittal view of the arch: evaluate supravalvar aortic anastomosis.

• TEE: intraoperative TEE is often used to assess the repair. In patients with poor transthoracic windows, TEE can be used for later follow-up as well.

• Assess for evidence of residual ASD or VSD (use two views).
• Assess the regional wall motion from standard views given coronary transfer. Keep in mind original coronary artery pattern.
• The reimplanted coronary artery buttons can be visualized in the short axis view of the neo-AV.
• Assess the supravalvar aortic and pulmonary anastomoses in long axis mid-esophageal views (generally between 90 and 120 degrees).
• Visualize the branch PAs straddling the ascending aorta. This can generally be done from the midesophageal (ME) long axis view of the RVOT, withdrawing slightly and then rotating right and left to see the respective branch PAs and also by withdrawing from the short axis view of the neo-AV.

• 3D imaging: generally not a standard part of a routine evaluation, but could be used to further assess the neo-AV and root or left ventricular (LV) systolic function.
image

Figure 8-13 LeCompte maneuver: the branch PAs are seen straddling the ascending aorta from this suprasternal notch image.

Analysis

• Measure any residual ASD or VSD noted on 2D imaging in whichever view it is best visualized, ideally two orthogonal planes.
• Measure neo-aortic root in the parasternal long axis view. Measure supravalvar aortic and pulmonary anastomoses in the parasternal long axis view if any concern for narrowing. Pediatric norms for these measurements are taken in systole (valve maximally open).
• Measure LV chamber dimensions (LV end-diastolic dimension, LV end-systolic dimension) and parameters of LV systolic function (ejection fraction [EF], shortening fraction).

Pitfalls

• Branch PAs can be difficult to visualize. Try the suprasternal notch view in addition to the parasternal short axis view.

Physiologic Data

Acquisition

• Subcostal views

• Use color Doppler to assess for residual ASDs or VSDs. Obtain peak velocity/PIPG across any residual VSD.
• Subcostal short axis views of the RVOT, neo-pulmV, and supravalvar pulmonary region can be very useful in identifying and localizing obstruction. Use color Doppler to screen for aliasing of flow or significant regurgitation. Use PW Doppler to “march” through these levels of pulmonary outflow to identify the site of flow acceleration, most commonly in the supravalvar pulmonary region at the anastomosis. Use CW Doppler to identify the highest velocity, once the site of flow acceleration has been identified.
• The LVOT, neo-AV, and supravalvar aortic region can be similarly evaluated from the subcostal views, but supravalvar aortic stenosis is less common than supravalvar PS.

• Apical views

• Assess for tricuspid regurgitation (TR) or mitral regurgitation (MR) with color Doppler; further quantify as appropriate.
• Assess the LVOT, neo-AV, and supravalvar aortic region in the 5C view, using color Doppler to screen for aliasing of flow or significant regurgitation. Use PW Doppler to “march” through these levels of outflow to identify the site of acceleration. Use CW Doppler to identify the highest velocity.

• Parasternal views

• Assess the RVOT, neo-pulmV, supravalvar pulmonary region, and branch PAs with a combination of color Doppler, PW Doppler, and CW Doppler as above to identify any obstruction or significant valve regurgitation.
• Perform a short axis color Doppler sweep to assess for residual ASD or VSD. Obtain peak velocity/PIPG across any residual VSD.

• Suprasternal notch view

• Assess branch PAs with color Doppler (see Fig. 8-13), PW Doppler, and CW Doppler for degree of stenosis.

Alternate Approaches

• Magnetic resonance imaging (MRI) may be used to evaluate aspects of the anatomy, particularly branch PAs, if windows are poor.
• Cardiac catheterization and angiography can be used to evaluate coronary artery patency as well as severity of obstruction to RV or LV outflow.
• Nuclear medicine pulmonary perfusion scans can be used to quantify the percentage of blood flow to each lung, in the setting of suspected asymmetric branch PA stenosis.

Key Points

• Understand how the operation is performed (see text) to guide the postoperative study.
• The most notable difference from normal anatomy is that the branch PAs are translocated anterior to the ascending aorta (LeCompte maneuver).
• Assess for major complications related to the repair, including residual shunts, supravalvar pulmonary or aortic stenosis, branch PA stenosis, and coronary stenosis or occlusion.
• Evaluate the abnormal natural history of the native pulmV and root in the systemic circulation: neo-AV regurgitation and neo-aortic root dilation.

Transposition of the Great Arteries: Status Post Atrial Switch Operation (Senning/Mustard)

Background

• The Senning and Mustard operations baffle systemic and pulmonary venous return within the atria such that the systemic venous return is directed to the left-sided tricuspid valve (TV) and RV. Pulmonary venous return is directed to the right-sided MV and LV (Fig. 8-14).
• The correction is physiologic, not anatomic.
• The RV remains the systemic ventricle.
• In the Mustard operation, the atrial septum is excised and the baffle is constructed with other tissue, typically pericardium.
• In the Senning operation, atrial septal and free wall flaps are used to construct the baffle.
image

Figure 8-14 Schematic depicting the anatomy of d-TGA after atrial switch operation (Mustard or Senning).

Overview of Echocardiographic Approach

• Evaluate for complications related to the repair.

• Residual shunts: baffle leaks or residual VSD (2D, color Doppler, PW Doppler, CW Doppler).
• SVC or IVC stenosis or occlusion (contrast, color Doppler, PW Doppler, CW Doppler).
• Baffle stenosis (color Doppler, PW Doppler, CW Doppler).

• In addition, the abnormal natural history of the right ventricle (RV) functioning in the systemic circulation can result in:

• TR (color Doppler).
• RV dysfunction (2D).
• Left ventricular outflow tract obstruction (LVOTO) due to compression (2D, color Doppler, PW Doppler, CW Doppler).

• TTE is the standard modality for assessment. TEE may be used in the case of poor transthoracic windows or pre- and postoperatively.

Anatomic Imaging

Acquisition

• Subcostal view

• Given the predominantly adult age of this patient population, windows may be limited. However, in cases with good windows, the SVC and IVC and their entry into the baffle should be imaged to the full extent possible.

• Apical 4C view

• Assess RV and LV size and function.
• Focus on the atria and use careful posterior and anterior angulation to demonstrate the systemic venous baffle (the portion in the LA is best seen) and the course of the PVs to the TV, respectively (Figs. 8-15 and 8-16).

• Parasternal views

• Long axis

• Compression of the LV outflow by the enlarged RV may be seen.
• The SVC is seen along its course to the MV by angling somewhat toward the RV inflow.

• Short axis

• Assess RV and LV size and function.
• Evaluate IVC limb of the baffle (Fig. 8-17).
• Evaluate pulmonary venous pathway.

• Suprasternal notch view

• Visualize SVC.
• If occlusion of the SVC is suspected, use saline solution contrast injection in an upper extremity while imaging the IVC and SVC in turn to evaluate.
• IVC complications are less common, but an analogous technique can be used.
• Saline solution contrast injections can also be used to identify baffle leaks.
• TEE is particularly useful in better assessing atrial anatomy. It may also be used to guide procedures such as stenting of stenosis or baffle leak closure.

• Assess for baffle leak along the entirety of the interatrial baffle, with both 2D and color Doppler.
• Stenosis of the SVC or IVC limb may be seen, particularly if pacemaker or implantable cardioverter-defibrillator leads traverse the SVC limb.
• The pulmonary venous pathway may be assessed for obstruction (Fig. 8-18).
• For patients with atrial arrhythmias requiring TEE cardioversion, both atria and the baffle should be assessed for thrombus or baffle leak that would serve as a source of potential paradoxical emboli.
• The mechanism of TR and pulmonary or subpulmonary stenosis and location of residual VSD may all be identified.

• 3D imaging

• With appropriate applications, 3D imaging may develop a role in quantification of systemic RV function.
• 3D imaging could be used by a skilled operator to assess baffle anatomy.

image

Figure 8-15 Apical view of the systemic venous pathway coursing toward the MV after a Mustard operation.

image

Figure 8-16 Apical view of the pulmonary venous pathway coursing toward the pulmV after a Mustard operation.

image

Figure 8-17 Parasternal short axis view demonstrating the course of the inferior vena cava to the MV after a Mustard operation.

image

Figure 8-18 Transesophageal echocardiography with color flow Doppler demonstrates unobstructed flow through the pulmonary venous pathway toward the tricuspid valve.

Analysis

• Measure any residual VSD or baffle leak in whichever view it is best seen (ideally two orthogonal views).
• Although RV EF and other standard measures of systolic function are not as accurate in the assessment of the LV, area of fractional shortening, RV end-diastolic diameter and end-systolic dimension, and RV EF may be useful measures, particularly when comparing with previous studies in the same patient.

Pitfalls

• The main challenge is usually visualization of complex baffle anatomy in patients with poor windows due to older age and history of cardiac surgery.
• Learn to visualize the baffles from as many windows as possible so that the sonographer can take advantage of whatever window is best in an individual patient.

Physiologic Data

Acquisition

• Subcostal view

• Assess the SVC and IVC with color Doppler and spectral Doppler.
• Follow the cavae through the baffle to the MV as permitted by the windows.
• Identify any aliasing or flow acceleration suggestive of stenosis.
• PW Doppler is best for localizing obstruction.

• Apical views

• Assess TR and MR; quantify as appropriate.
• Angle posteriorly to assess the systemic venous baffle for evidence of leak or obstruction by color Doppler (see Fig. 8-15). PW Doppler throughout course of cavae to MV to identify obstruction.
• Angle anteriorly to visualize course of PVs to TV. Use color Doppler and PW Doppler to identify any obstruction (see Fig. 8-16).
• Apical 5C: assess for LVOTO with color Doppler and spectral Doppler.

• Parasternal views

• Long axis

• Assess for MR and TR.
• Assess the SVC along its course to the MV with CF Doppler and PW Doppler by angling somewhat toward the RV inflow.

• Short axis

• Evaluate the IVC limb of the baffle with color Doppler (see Fig. 8-17) and PW Doppler.
• Assess the course of PVs to the TV with color Doppler and PW Doppler.
• Assess for any residual VSD with color Doppler; if present, obtain peak velocity/PIPG.

• Suprasternal notch view

• Assess the SVC flow pattern with spectral Doppler, which should appear normal.
• Little flow at all may suggest occlusion.
• Accelerated, nonphasic pattern may suggest significant SVC stenosis.

Alternate Approaches

• MRI may be used for anatomic and functional assessment in patients with poor windows. RV function and TR can be quantified.
• Cardiac catheterization can be used to assess hemodynamics and anatomy. Interventions can be performed to close baffle leaks and to dilate and/or stent a stenotic SVC or IVC.

Key Points

• The Senning and Mustard operations restore normal physiology by redirecting blood flow at the atrial level: systemic venous return to the LV and pulmonary venous return to the RV.
• Anatomically, connections remain abnormal.
• The RV functions as the systemic ventricle and the TV as the systemic AVV.
• Assess for complications related to the repair, including residual shunts (baffle leaks or residual VSD), SVC or IVC stenosis/occlusion, and baffle obstruction.
• Use multiple views to visualize the systemic and pulmonary venous pathways, as well as careful color Doppler and PW Doppler analysis.
• Saline solution contrast injection can be very helpful in identifying SVC or IVC stenosis or occlusion and baffle leaks.
• The abnormal natural history of the RV functioning in the systemic circulation can result in RV dysfunction, TR, and LVOTO due to compression by the RV.

Transposition of the Great Arteries, Ventricular Septal Defect, and Pulmonary Stenosis: Status Post Rastelli or Nikaidoh Procedure

Background

• If the LVOT or pulmV is sufficiently obstructed to preclude ASO, in the presence of a VSD, two operative techniques are available for repair.
• Rastelli procedure: the long-standing surgical approach to this lesion (Fig. 8-19).

• The VSD is closed such that LV outflow is baffled to the AV.
• The PA is transected and the pulmV is oversewn.
• Continuity between the RV and the PAs is established with a conduit.
• Nikaidoh procedure (aortic translocation): newer, more complex surgical technique; argued to produce a more anatomically normal result, no conduit.

• The MPA is divided, and the pulmV leaflets are excised.
• The conal septum separating the aortic and pulmV is divided.
• The AV and root are mobilized from the RV and translocated posteriorly into the pulmV annulus. The coronary arteries may or may not be harvested and reimplanted.
• The VSD is closed with a patch.
• The aorta is transected, and the branch PAs are brought anterior to the aorta (LeCompte maneuver).
• A segment of ascending aorta is excised to reduce its length, and then the ascending aorta is reanastomosed.
• The MPA is sutured posteriorly to the opening in the RV and then augmented anteriorly with a patch.

image

Figure 8-19 Schematic depicting Rastelli repair of d-TGA with VSD and pulmonary stenosis (PS). Note that continuity between the RV and PAs is achieved with the insertion of a conduit.

Overview of Echocardiographic Approach

• Evaluate for complications related to the repair.

• Residual ASD or VSD (2D, color Doppler, PW Doppler, CW Doppler).
• Rastelli procedure

• LVOTO (2D, color Doppler, PW Doppler, CW Doppler).
• Conduit stenosis or insufficiency (color Doppler, PW Doppler, CW Doppler).
• Right ventricular dilation and dysfunction secondary to ventriculotomy and chronic conduit dysfunction (2D).

• Nikaidoh procedure

• LVOTO: theoretically less likely than in Rastelli procedure (2D, color Doppler, PW Doppler, CW Doppler).
• RVOTO or branch PA stenosis (2D, color Doppler, PW Doppler, CW Doppler).
• Coronary complications as manifested by RWMAs (2D).
• Supravalvar aortic stenosis (2D, color Doppler, PW Doppler, CW Doppler).
• RV dilation and dysfunction secondary to free pulmonary insufficiency (2D).

• TTE is the standard modality for assessment. TEE may be used in the case of poor transthoracic windows or pre- and postoperatively.

Anatomic Imaging

Acquisition

• Subcostal views

• Assess for residual ASD or VSD.
• In addition, in patients with good windows, subcostal short axis imaging provides excellent views of the RVOT and proximal MPA or conduit.

• Apical views

• Assess the regional wall motion.
• Assess for any LVOTO.
• Nikaidoh procedure: assess for evidence of supravalvar aortic stenosis in the 5C view.

• Parasternal views

• Long axis

• Assess the LVOT (Fig. 8-20); evaluate for residual VSD; visualize the AV and root; assess for supravalvar aortic anastomosis (Nikaidoh procedure); assess the RV-to-PA conduit (Rastelli procedure); assess regional wall motion.

• Short axis

• Assess for residual ASD or VSD; assess regional wall motion; assess RV size and function; evaluate conduit (Rastelli procedure); evaluate the proximal branch PAs (Nikaidoh procedure: branches now straddle the ascending aorta, may be better seen from the suprasternal notch view).

• Suprasternal notch views

• Visualize the branch PAs in a short axis view (for Nikaidoh procedure: straddling the ascending aorta).
• Nikaidoh procedure: evaluate supravalvar aortic anastomosis.

image

Figure 8-20 Left ventricular outflow tract obstruction after baffling of left ventricular (LV) outflow through a VSD to the AV (Rastelli). A, 2D parasternal long axis view shows the long pathway from the LV to the AV through the VSD. Not well seen on this image is a subaortic membrane within the tunnel. B, Parasternal long axis color Doppler imaging shows aliasing within the tunnel, at the level of the membrane, proximal to the AV. Spectral Doppler interrogation from other planes demonstrated a peak velocity of 4.9 m/s, mean gradient 49 mm Hg.

Analysis

• Measure any residual ASD or VSD noted on 2D imaging in whichever view it is best visualized, ideally two orthogonal planes.
• Measure LV chamber dimensions (LV end-diastolic dimension, LV end-systolic dimension) and parameters of LV systolic function (EF, shortening fraction).

Physiologic Data

Acquisition

• Subcostal views

• Use color Doppler to assess for residual ASD or VSD. Obtain peak velocity/PIPG across any residual VSD.
• Subcostal short axis views of the RVOT and proximal conduit/PA: use color Doppler and spectral Doppler to evaluate for obstruction or significant regurgitation.
• The LVOT, AV, and supravalvar aortic region can be evaluated in the subcostal views; use color Doppler and spectral Doppler to document obstruction.

• Apical views

• Assess for TR or MR with color Doppler; further quantify as appropriate.
• Assess the RV pressure estimate by TR velocity given the potential for conduit or PA stenosis.
• Assess the LVOT, AV, and supravalvar aortic region from the 5C view, using color Doppler to screen for aliasing of flow or significant regurgitation. Use PW Doppler to “march” through these levels of outflow to identify the site of acceleration. Use CW Doppler to identify the highest velocity.

• Parasternal views

• Assess the LVOT with color Doppler (see Fig. 8-20).
• Assess the RVOT, conduit or PA anastomosis, and branch PAs with a combination of color Doppler, PW Doppler, and CW Doppler as above to identify any obstruction or significant conduit regurgitation. Mean gradients are likely to be a more accurate assessment of true gradients in a conduit.
• Use color Doppler to assess for residual ASD or VSD. Obtain peak velocity/PIPG across any residual VSD.
• Assess the RV pressure estimate by TR velocity as above.

• Suprasternal notch views

• Assess the branch PAs with color Doppler, PW Doppler, and CW Doppler for the degree of stenosis.
• Nikaidoh procedure: assess the ascending aortic velocity to rule out anastomotic narrowing.

Alternate Approaches

• MRI is used to quantify RV size and function and pulmonary regurgitation and is instrumental in determining appropriate timing of pulmV replacement. MRI may also be used for anatomic assessment if TTE windows are poor.
• Cardiac catheterization and angiography can be used to evaluate coronary artery patency (Nikaidoh procedure) as well as severity of obstruction to RV or LV outflow.
• Nuclear medicine pulmonary perfusion scans can be used to quantify percentage of blood flow to each lung, in the setting of suspected asymmetrical branch PA stenosis.

Key Points

• Two different surgical techniques can be used for repair of this defect; the complications are somewhat different.
• The Rastelli operation baffles the LV outflow through the VSD to the AV and uses an RV-to-PA conduit. Assess for evidence of LVOTO, residual VSD, conduit stenosis, or regurgitation and their effect on RV size and function, and branch PA anatomy.
• The Nikaidoh operation translocates the aorta into the pulmonary annulus before closing the VSD, and thus LVOTO is thought to be less likely. Continuity is established between the RV and PAs without a conduit but with obligate free pulmonary insufficiency. A LeCompte maneuver is performed, bringing the branch PAs anterior to the aorta. Assess for evidence of LVOTO, residual VSD, RVOTO, branch PA stenosis, RV size and function, and coronary complications.

L-TGA (Congenitally Corrected TGA)

Background

• This lesion is also known as congenitally corrected TGA.
• Demonstration of the segmental anatomy is key (Fig. 8-21).

• Visceroatrial situs is normal.
• Ventricular looping is abnormal, with the morphologic LV being right sided and aligned with the morphologic RA. The morphologic RV is left sided and aligned with the morphologic LA.
• The PA arises from the right-sided morphologic LV, and the aorta arises from the left-sided morphologic RV. However, the GVs have an abnormal orientation to each other, with the aorta being leftward and anterior of the PA.
• The blood flow is normal, with systemic venous return passing through the right side of the heart to the lungs and pulmonary venous return passing through the left side of the heart to the body.
• The morphologic RV functions as the systemic ventricle, and thus patients are at risk of systemic ventricular dysfunction.
• The AVVs are derived, embryologically, from the ventricles. Thus, the left-sided AVV in this case is a TV and the right-sided AVV is a MV.
• Frequently associated lesions include

• Ebstein’s anomaly of the left-sided morphologic TV.
• VSD.
• PS.
• Various degrees of heart block.

• Whether any surgery is required depends on the associated lesions.
image

Figure 8-21 Schematic depicting the anatomy of L-TGA, also known as congenitally corrected TGA. Note that although the anatomy is abnormal, the physiology is normal.

Overview of Echocardiographic Approach

• Identify the segmental anatomy (2D).

• Normal atrial situs.
• Levo (L)-looped ventricles (ventricular inversion).

• The morphologic LV and MV are on the right, receiving inflow from the RA.
• The morphologic RV and TV are on the left, receiving inflow from the LA.
• A morphologic RV has a moderator band, coarse trabeculations, chordal attachments to the septum, and a morphologic TV. A morphologic LV has a smooth septal surface, lack of chordal attachments to the septum, and a morphologic MV (2D).

• L-transposed GVs

• The aorta arises anterior and leftward from the left-sided morphologic RV.
• The PA arises posterior and rightward from the right-sided morphologic LV.

• Segmental anatomy can be abbreviated {S,L,L}.

• Presence or absence and size of shunts at the atrial and ventricular levels (2D, color Doppler).
• Morphology and function of the AVVs; in particular, assess the left-sided TV for evidence of Ebstein’s anomaly and regurgitation (2D, color Doppler, PW Doppler, CW Doppler).
• Evaluate for outflow obstruction; in particular, assess for PS or subpulmonary stenosis (2D, color Doppler, PW Doppler, CW Doppler).

Anatomic Imaging

Acquisition

• Subcostal frontal (long axis) sweep, posterior to anterior.
• Establish normal visceroatrial situs: visualize the IVC within the liver on the right, connecting to the right-sided atrium. The left-sided atrium should have some PVs visible by 2D imaging.
• Demonstrate abnormal ventricular looping: in this view, the main clue is that the VS has a smooth septal surface on the right side.
• Demonstrate L-transposed ventriculoarterial connections.

• The more posterior GV, arising from the right-sided (morphologic left) ventricle, bifurcates, identifying it as the PA.
• The more anterior GV arising from the left-sided (morphologic right) ventricle does not bifurcate; coronary artery origins may be seen from it.
• Note any evidence of outflow tract obstruction.
• Note also that the GVs do not cross each other (as is the case in normally related GVs).

• Subcostal short axis sweep, base to apex.

• Atrial morphology is again noted to be normal with the GVs entering the RA and at least some PVs are visualized entering the LA. Assess for evidence of ASD.
• Ventricular morphology is examined and is consistent with L-looping (ventricular inversion). The right-sided ventricle has a smooth septal surface and AVV morphology consistent with an MV (no chordal attachments to the septum). The left-sided ventricle has coarse trabeculations, with chordal attachments of the AVV to the septum (Fig. 8-22). Assess for VSD (size, location).
• GVs are noted to be L-transposed with the bifurcating PA arising posteriorly from the right-sided (morphologic left) ventricle and the aorta arising anteriorly from the left-sided (morphologic right) ventricle. Note any evidence of outflow tract obstruction. Their parallel relationship is well seen.

• Apical views

• Ventricular morphology is inverted; the moderator band is seen within the left-sided (morphologic right) ventricle. The morphologic RV is dilated and hypertrophied compared with a normal RV (Fig. 8-23). Assess systolic function.
• Inspect the left-sided TV for evidence of Ebstein’s anomaly.
• Note any evidence of VSD or outflow tract obstruction.
• With anterior angulation, the PA is seen arising from the right-sided (morphologic left) ventricle.
• With further anterior angulation, the aorta may be seen arising from the left-sided (morphologic right) ventricle.

• Parasternal long axis view

• The parasternal long axis view can be confusing in L-TGA.
• The ventricles are located more side by side than usual.
• The long axis view is more parallel to the long axis of the body.
• If the transducer is aligned more conventionally, it is possible to align the pulmV with the left-sided TV.
• The AV is not seen in the long axis view of the TV and RV.
• Other windows are recommended for understanding the spatial relationships.

• Parasternal short axis view

• Relative orientation of the semilunar valves is seen.
• Typically, the AV is anterior and leftward of the pulmV (Fig. 8-24).
• Coronary arteries arise from the aortic root most commonly, as depicted in Figure 8-25.
• Note morphology and relative sizes of the semilunar valves.
• Sweeping toward the head, the posterior PA can be noted to branch.
• Note any evidence of VSD.
• Assess ventricular function.
• The coronaries can be visualized; most commonly, the RV coronary arises by itself from the leftward posterior sinus, and the anterior descending and Cx arteries arise from a common trunk from the rightward sinus. As in d-TGA, the coronary pattern can be variable.
• The aortic arch, which runs more anteroposterior than usual, is often better imaged in a high left parasternal window than from the suprasternal notch.

image

Figure 8-22 Subcostal short axis view at the level of the ventricles in a patient with L-TGA. Note the right-sided morphologic LV with its smooth septal surface. There are no attachments of the MV to the septum. In contrast, note the chordal attachments of the left-sided tricuspid valve within the RV. This patient also has a VSD, seen here, and PS.

image

Figure 8-23 Apical 2D view of L-TGA. Note the presence of the moderator band in the left-sided morphologic RV. This patient does not have Ebstein’s anomaly of the left-sided TV, although that is a common association.

image

Figure 8-24 Parasternal short axis view showing the typical orientation of GVs in L-TGA. The AV is anterior and leftward of the pulmV.

image

Figure 8-25 Schematic showing typical anatomy of the coronary arteries in L-TGA, parasternal short axis echo view.

Analysis

• Quantitative analysis consists of measuring sizes of shunts and semilunar valves.
• Atrial shunts (ASD or PFO) should be measured from the subcostal long axis or short axis view.
• VSDs should be measured in whichever view they are seen best, ideally in two orthogonal planes.
• The pulmV and AV are measured from the parasternal long axis view.

Pitfalls

• As noted above, the parasternal long axis views can be particularly confusing. Align the plane of sound more parallel to the long axis of the body. Use other views to understand anatomic relationships.

Physiologic Data

Acquisition

• Subcostal views

• From the subcostal frontal or short axis view, color Doppler sweeps of the atrial septum identify shunts.
• Short axis color Doppler sweeps of the VS are used to identify any VSD present and determine the direction of shunting. Carefully assess any area that the 2D imaging suggests may have a defect.
• Acquire peak velocity/PIPG across any VSD in the view that most aligns the Doppler angle with the jet.
• The RVOT, LVOT, and the semilunar valves may be evaluated for evidence of stenosis or regurgitation by color Doppler and spectral Doppler (PW Doppler and CW Doppler) from the subcostal views. In particular, there is an association between L-TGA and PS, either at the valvar or subvalvar level.

• Apical views

• Assess AVV function, in particular, the left-sided TV, which has a high likelihood of regurgitation due to its association with Ebstein’s anomaly and function in the systemic ventricle.
• Assess the apical muscular septum by color Doppler for evidence of VSD.
• Assess the RVOT, LVOT, and semilunar valve function by color Doppler and spectral Doppler.

• Parasternal views

• Color sweep of the VS is used to identify any VSD present and determine the direction of shunting. Carefully assess any area that the 2D imaging suggests may have a defect.
• Acquire the peak velocity/PIPG across any VSD in the view that most aligns the Doppler angle with the jet.
• Further assess valve function with color Doppler.

Alternate Approaches

• MRI is used for quantification of RV function and TR and can also be used for anatomic definition in patients with poor TTE windows.

Key Points

• Establish the diagnosis of L-TGA by demonstrating the segmental anatomy: normal atrial situs, abnormal (L) ventricular looping, and L-transposed GVs.
• The morphologic RV and TV are on the left; the morphologic LV and MV are on the right.
• A morphologic RV has a moderator band, chordal attachments to the septum, and a morphologic TV. A morphologic LV has a smooth septal surface, lack of chordal attachments to the septum, and a morphologic MV.
• Although the anatomy is abnormal, physiology is normal.
• Frequently associated lesions are Ebstein’s anomaly of the left-sided morphologic TV, VSD, PS, and various degrees of heart block.
• The segmental anatomy is best understood from subcostal and apical views. Parasternal long axis views, in particular, can be confusing. The ventricles are more side by side than usual, and the AV is separated by conus from the TV.
• On long-term follow-up, systemic RV and TV function are of greatest importance.

Suggested Reading

1 Earing MG, Ayers NA, Cetta F. Congenitally corrected transposition of the great arteries. In: Eidem BW, Cetta F, O’Leary PW, editors. Echocardiography in Pediatric and Adult Congenital Heart Disease. Philadelphia: Lippincott Williams & Wilkins, 2010.

2 Mahle WT, Marx GR, Anderson RH. Anatomy and echocardiography of discordant atrioventricular connections. Cardiol Young. 2006;16(suppl 3):65-71.

3 Mertens LL, Vogt MO, Marek J, Cohen MS. Transposition of the great arteries. In: Lai WW, Mertens LL, Cohen MS, Geva T, editors. Echocardiography in Pediatric and Congenital Heart Disease. Hoboken, NJ: Wiley-Blackwell, 2009.

4 Miller-Hance WC, Silverman NH. Transesophageal echocardiography in congenital heart disease with focus on the adult. Cardiol Clin. 2000;18(4):861-892.

5 Oechslin E. Physiologically “corrected” transposition of the great arteries. In: Lai WW, Mertens LL, Cohen MS, Geva T, editors. Echocardiography in Pediatric and Congenital Heart Disease. Hoboken, NJ: Wiley-Blackwell, 2009.

6 Pasquini L, Parness IA, Colan SD, et al. Diagnosis of intramural coronary artery in transposition of the great arteries using two-dimensional echocardiography. Circulation. 1993;88:1136-1141.

7 Schultz AH, Lewin MB. D-Transposition of the great arteries. In: Eidem BW, Cetta F, O’Leary PW, editors. Echocardiography in Pediatric and Adult Congenital Heart Disease. Philadelphia: Lippincott Williams & Wilkins, 2010.

Share this:

Related posts:

The Fetal Echocardiogram Echocardiography in the Cardiac Catheterization Laboratory Atrioventricular Septal Defect: Echocardiographic Assessment Echocardiographic Assessment After Heart Transplantation Myocardial Pathology Thromboembolic Phenomena and Vegetations