Left Ventricular Outflow Tract Anomalies: Echocardiographic Evaluation
Childrens Heart Centre, Kokilaben Dhirubhai Ambani Hospital, Mumbai, Maharashtra, India
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Correspondence Address: Dr. Tanuja Karande Kokilaben Dhirubhai Ambani Hospital, Mumbai, Maharashtra India
Source of Support: None, Conflict of Interest: None
The left ventricular outflow is divided into subvalvar area, the aortic valve and supra valvar region. There can occur a number of anomalies in the outflow tract, most commonly of which are obstructive lesions, which can exist in isolation or as a part of association with other defects such as ventricular septal defects or interrupted arch. The following chapter highlights on echocardiography imaging of the left ventricular outflow tract and its anomalies.
Keywords: Left ventricular outflow anomalies, left ventricular outflow imaging, left ventricle outflow obstructions
How to cite this article: Karande T. Left Ventricular Outflow Tract Anomalies: Echocardiographic Evaluation. J Indian Acad Echocardiogr Cardiovasc Imaging 2020;4:287-94
How to cite this URL: Karande T. Left Ventricular Outflow Tract Anomalies: Echocardiographic Evaluation. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2020 [cited 2021 Apr 13];4:287-94. Available from: https://www.jiaecho.org/text.asp?2020/4/3/287/303936
Anomalies of the Left Ventricular Outflow Tract
Abnormalities of the left ventricular outflow (LVOT) may occur in isolation or can commonly co-exist with ventricular septal defects (VSDs), mitral valve (MV) lesions, hypoplastic left ventricle, and discordant ventriculo-arterial connections. This chapter reviews the echocardiographic features of the LVOT tract in isolation, seen in the setting of concordant connections and normal atrioventricular valves.
Anomalies of the LVOT can occur at subvalvar, valvar, or supravalvar level or a combination thereof. They can further be grouped into obstructive lesions which accounts for the majority and miscellaneous lesions such as aortic valve prolapse and aneurysm of sinus of Valsalva.
When the ventricular septum is intact, the following morphological variations may be seen in subvalvar stenosis.
Membranous (discrete) subaortic stenosis [Figure 1] – most common type of subaortic obstruction. It appears as a fibrous shelf/ring adhered to the interventricular septum below the aortic valve. It is usually horseshoe shaped in cross-section falling short near the aortomitral region. It is has been postulated that a steeper angle between the plane of the ventricular septum and the aortic root (aorto-septal angle) [Figure 2] and an elongation of the intervalvular fibrosa between the anterior mitral leaflet (AML) and the aortic valve cause abnormal shear stress, leading to formation of a membrane. Similar theory explains its association with a restrictive perimembranous VSD and development of a jet effect leading to injury and fibrosis. Note that rarely, the membrane may involve and extend up to the AML
Tunnel or fibromuscular type [Figure 3] – it consists of a fibromuscular collar or a thick long segment muscular ring at a level lower than that of subaortic membrane in the LVOT. This form of obstruction frequently occurs early in life and is often associated with other forms of obstructions along the aorta. It is important to identify this variety as it requires extensive surgical options such as Ross–Konno operation,
Posterior deviation of outlet septum [Figure 4] and [Video 1] – this lesion typically occurs in association with a VSD and type B interruption of the aortic arch or, less commonly, coarctation of the aorta with VSD, where the aortic valve and ascending aorta may be hypoplastic. Embryologically, a posteriorly malaligned conal septum gives rise to a VSD and hypoplasia or other obstructions in the aorta due to reduction in the forward blood flow across the aortic valve
Other varieties – obstruction produced by accessory MV tissue and systolic anterior motion of MV as in cases of hypertrophic cardiomyopathy [Figure 5] and [Video 2].
Figure 1: Parasternal long-axis view showing discrete subaortic membrane seen below a slightly thickened aortic valve
Delineation of the morphology of the LVOT tract and the type of subaortic obstruction
Assessment of the severity of LVOT tract obstruction
Identifying hemodynamic effects of the LVOT obstruction such as aortic regurgitation (AR) and degree of left ventricular (LV) hypertrophy.
Morphology – subaortic region is best assessed in parasternal long-axis view. In cases of subaortic membrane, it is important not only to assess its distance from the aortic valve but also to assess its circumferential extent, as some can involve the anterior leaflet of MV. A discrete subaortic membrane is usually resectable with preservation of the native aortic valve. Its distance from the aortic valve is a good guide for the surgeon prior to the surgery. En face view of the subaortic membrane can be obtained from the LV cavity or through the aortic valve using three-dimensional echocardiography. As mentioned, it may either appear as a horseshoe-shaped ring or rarely a complete ring. The fibromuscular or tunnel variety is seen as a muscular thick collar below the valve; if extensive, it can appear as a tunnel with a narrow lumen [Figure 3]. A posteriorly deviated septum is best visualized in apical five-chamber view, wherein its usually associated with a VSD below and some degree of aortic valve hypoplasia.
Assessing the severity – the severity of LVOT stenosis is estimated by measurement of the Doppler-derived maximum instantaneous and mean pressure drop across the aortic valve [Figure 6]. The apical five-chamber view gives the best Doppler alignment. Pulse wave Doppler can be used to localize the site of outflow obstruction. Muscular subaortic obstructions may be accompanied by an increase in the Doppler velocity in late systole leading to an asymmetric Doppler trace consistent with dynamic obstruction [Figure 7]. Severity is usually classified similar to that of valvar stenosis (mentioned below). Contractile function of the left ventricle is usually well maintained initially but may worsen with time if severe outflow tract obstruction is not relieved.
Figure 6: Continuous wave Doppler signal measuring tracing the peak and mean gradients across the left ventricular outflow in a case of discrete subaortic membrane
Assessing the health of the aortic valve is important while evaluating the subaortic stenosis. In the membranous variety, the valve may be intrinsically abnormal or be restricted in its motion. It may appear thickened due to the jet of blood which damages the valve leading to varying degrees of AR [Figure 8]. The aortic valve leaflets may “flutter” during systole as they are hit by the turbulent jet from the subaortic region. The closer the membrane is to the aortic valve, more is the restriction in the motion of the valve [Video 3]. In the tunnel variety, the LVOT and aortic annulus may appear hypoplastic. The left ventricle usually shows concentric hypertrophy and is important to differentiate it from hypertrophic cardiomyopathy which is usually asymmetric with systolic motion of the AML.
Figure 8: Parasternal long-axis view showing thickened aortic valve cusps (arrow) due to a closely placed subaortic membrane
In children, aortic valve stenosis accounts for 70%–90% of LVOT obstruction and occurs mainly due to a congenitally malformed aortic valve, as against adults which is mainly due to a degenerative process. The incidence of bicuspid aortic valve is approximately 13 per 1000 live births, whereas that of valvar aortic stenosis is approximately 0.4 per 1000 live births. Valvar aortic stenosis may occur due to (i) commissural fusion or agenesis, (ii) tricuspid thickened restricted aortic valve, and (iii) annular hypoplasia.
Imaging goals in valvar aortic stenosis
Defining the morphology
Assessing the severity of the stenosis, LV wall thickness, and LV function [Table 1]
Measuring the dimensions of the LVOT and comprehensive evaluation of all left heart structures including the MV and aortic arch
Special attention pre- and postballoon intervention of the valve.
Table 1: Classification of severity of aortic stenosis
Morphology – Aortic valve is best viewed in parasternal short-axis view. A normal aortic valve has three cusps which close along the three commissures and form a Y-shaped pattern in diastole. In systole, the leaflets open to create a wide triangular orifice [Figure 9]. The leaflets are thin and unrestricted with equal surface area in cross section. Abnormalities may arise in a number of cusps, development of commissures, and motion restriction.
Figure 9: Parasternal short-axis view showing a normal aortic valve triangular orifice during systole and corresponding right, left, and noncoronary cusps. RCC: Right coronary cusp, LCC: Left coronary cusp, NCC: Non coronary cusp
Unicuspid aortic valve is a rare variety. Has been classified into two types: i. Acommisural - Wherein there is a single membrane with a central orifice or aperture ii. Unicommisural - Single eccentric elliptical or circular opening with lateral attachment of the commisure to the aorta. Typically it is eccentric and circular in systole [Figure 10].
Figure 10: Parasternal short-axis view showing unicuspid valve with a small eccentric circular opening
Bicuspid aortic valve is the most common type of aortic valve malformation. It may arise from lack of formation of three cusps (developmentally bicuspid) or fusion of commissures between two unequal cusps producing a fish mouth opening oriented vertically or transversely, with apparent two cusps and a fused raphe. Various classifications are available for its description and the following [Figure 11] is most commonly followed. Fusion of left and right coronary cusps (RCCs) (70%) results in a bicuspid valve with two cusps positioned anteroposteriorly and a transverse opening [Figure 12]. Fusion of the noncoronary cusp (NCC) and left coronary commissure or NCC and RCC can result in a vertically positioned opening slit [Figure 13]. A doming aortic valve is best seen in parasternal long axis view [Figure 14]. Dilation of the ascending aorta has been increasingly recognized as an accompaniment, which is linked to the structural abnormalities of the aortic wall similar to that seen in cystic medial necrosis associated with Marfan syndrome and other connective tissue disorders [Figure 15] and [Video 4]. Detailed measurements of the annulus, aortic root, ST junction, and ascending aorta need to be taken at each follow-up. Thus, every case of bicuspid aortic valve needs to be evaluated for associated aortopathy with serial measurements of the aortic root [Figure 16]. Congenitally stenotic tricuspid aortic valve will have three cusps with thickened and rolled edges and varying degrees of asymmetry in the size of cusps. Quadricuspid valve is rare and occurs when an extra commissure is present and such valves are mostly associated with AR rather than stenosis.
Figure 11: Classification of morphology of bicuspid aortic valve with incidences mentioned in percentages. RCA: Right coronary artery, LCA: Left coronary artery, RC: Right cusp, LC: Left cusp, NC: Non coronary cusp
Assessing the severity of aortic stenosis – normal aortic valve blood flow velocity usually measures up to 1.5 m/s. Classification of severity is based on the highest continuous wave Doppler signal obtained from any window. Only well-defined envelopes are utilized with the best alignment. Overestimations can occur if one interrogates mitral regurgitation jet. Underestimation of aortic stenosis can occur if the cursor is improperly aligned and in cases of LV systolic dysfunction with low cardiac output. Patients with reduced LV ejection fraction will falsely show “low gradients” on interrogating the stenotic aortic valve. Dobutamine stress echocardiography examines for increase in effective orifice area and gradients; however, this is usually employed in adults and older children. Color Doppler flow mapping is extremely helpful to observe the direction of the flow jet, which may be eccentric, and allows optimal alignment of the Doppler cursor with the jet. Imaging from the suprasternal notch often has the best alignment with the direction of flow in cases with eccentric jet [Figure 17].
Figure 17: Alignment of the continuous wave Doppler from the suprasternal notch with cursor placed across the eccentric jet of the aortic stenosis
A comprehensive assessment of left heart structures is essential when aortic stenosis is diagnosed. In infancy, critical aortic stenosis is frequently accompanied by hypoplasia of left heart structures including the MV, left ventricle, and aorta. There are scoring systems available such as Rhodes score, CHSS, and discriminant score, which allow the clinician to decide the adequacy of left heart size and plan management (univentricular vs. biventricular). As early as 1991, Rhodes et al. put forward a biventricular repair predictive equation in critical aortic stenosis: score = 14.0 (BSA) + 0.943 (ROOTi) + 4.78 (LAR) + 0.157 (MVAi) – 12.03. Among which, BSA = body surface area, ROOTi = aortic root dimension indexed to BSA, LAR = ratio of the long-axis dimension of LV to long-axis dimension of heart, and MVAi = MV area indexed to BSA, with a discriminating score of <−0.35 predictive of death after a biventricular repair. In cases of critical aortic stenosis, endocardial fibro elastosis may be observed, sometimes in its mildest form involving only the papillary muscle [Figure 18] to its most severe form lining the entire endocardial borders of the LV. It is important to look for supramitral ring, mitral stenosis, and coarctation of aorta in older children presenting with valvar aortic stenosis.
Assessment prior to and following balloon aortic valvotomy – an accurate measurement of the aortic valve at the hinge points of the aortic valve leaflets is essential for selection of the appropriate size of balloon for valvuloplasty [Figure 17]. Preprocedure assessment of the morphology, number of cusps, and identifying fused commissures is necessary. Following the procedure, it is necessary to evaluate for residual gradients and occurrence of AR. Quantum of AR may be underestimated immediately after the procedure due to the presence of poor LV function and high LV end diastolic pressures [Video 5].
Supravalvar Aortic Stenosis
Supravalvar aortic narrowing constitutes the least common of the LVOT obstructive lesions with a reported incidence of 0.05 per 1000. It typically occurs at the ST junction and is usually characterized by discrete narrowing. Membranous form has been mentioned in literature; however, this type is rarely seen. Thickening of the aortic media at the sinotubular junction produces the narrowing and defects in the elastin gene have been implicated. In addition, the anomaly can involve the aortic valve, producing tethering and restriction, it can be associated with hypoplasia of the ascending aorta and also involve the coronary Ostia.
Imaging in supra valvar aortic stenosis
Delineating the site and morphology of the lesion
Special attention to ascending aorta dimension, head-and-neck vessels, coronary flow, and motion of aortic valve
Assessing the severity of the stenosis, LV wall thickness, and LV function.
Morphology of the lesion can be best assessed in parasternal long-axis view of the LV [Figure 19], though obtaining good images of the ascending aorta can be difficult and further imaging in the form of computed tomography may be required for profiling the ascending aorta and head-and-neck vessels. Narrowing is mostly observed at the ST junction. Various types of morphologies have been described such as hourglass variety and membranous form. Imaging from the suprasternal notch may be helpful in aligning the Doppler for accurate assessment of severity. Assessment of severity in supravalvar aortic stenosis will follow the same principles as mentioned above in the section of aortic valve stenosis. The aortic valve may show tethering, and motion of the aortic valve needs to be evaluated in detail in patients with supravalvar narrowing.
Figure 19: Parasternal long-axis view showing discrete narrowing and thickening at the ST junction. LV: Left ventricle, RV: Right ventricle, LA: Left atrium, AS: Aortic stenosis
Ventricular Septal Defect with Coronary Cusp Prolapse
Coronary cusp prolapsing into a closely situated VSD (perimembranous or doubly committed) is commonly seen in children with VSD. It has been postulated that the high-velocity jet across a restrictive VSD produces a Venturi effect, drawing the aortic cusp into the VSD and restricting it further. Most commonly, it is the RCC which prolapses through the VSD due to its proximity to the perimembranous area. Distortion of the coaptation line occurs as a result of the prolapsing leaflet leading to the development of AR, which if more than mild, requires surgical intervention., Surgical repair involves suspension of the affected aortic valve cusp and closure of the VSD. The parasternal long-axis view helps in identifying this clearly [Figure 20].
Figure 20: Parasternal long-axis (a) and short-axis views (b) showing prolapse of the right coronary cusp into the perimembranous ventricular septal defect
Aortic sinus is defined as the area between the aortic valve attachment and ST junction. Aneurysm arising from this area may occur in isolation or in association with VSD and other lesions and occurs more frequently in males and in patients of Asian origin. Any of the aortic sinus may be involved; however, the right coronary sinus is most commonly involved (73%) followed by noncoronary and left coronary sinus. It is thought to arise due to deficiency of elastic tissue in the wall of sinus. Enlargement of the aneurysm occurs in the direction of least resistance; hence, it commonly bulges into right sided structures. The most common type is aneurysm arising from the RCC and rupturing into the RV [Figure 21] followed by the right atrial. The most common type is aneurysm arising from the RCC and rupturing into the right ventricular, followed by the right atrial. With rupture of the aneurysm into the right-sided structures, there will be dilatation of the right-sided chambers with a large left to right shunt. Unruptured aneurysm can bulge into the surrounding structures causing obstruction. Loss of support of the aortic sinus frequently leads to AR.
Figure 21: Parasternal long- and short-axis views demonstrating aneurysm of right coronary sinus protruding into the right ventricular outflow
The aim of the echocardiographic study will be to determine which sinus of Valsalva and aortic valve leaflet are affected and if ruptured, to determine the site of rupture. As with other LVOT structures, aneurysm of the sinus can be best imaged in the parasternal long-axis view and show axis view. Apical four-chamber view can guide in tracing the course and site of rupture. Rupture into low-pressure chambers such as the right atrium, left atrium, or pulmonary artery will result in a continuous flow into that chamber, whereas rupture into LV will show flow only during diastole. Transesophageal echocardiography allows better imaging and should be employed in patients with suboptimal windows.
I would like to acknowledge the contribution of our echocardiography technicians, Mrs. Asavari Tawade and Mrs. Sayli Kadu, for their work in image preparation.
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