Aortic stenosis

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Aortic stenosis is a valvular heart disease resulting from the narrowing of aortic valve orifice.

Contents

Pathophysiology

The haemodynamic hallmark of aortic stenosis is the elevated gradient across the aortic valve which can be indirectly measured with doppler echocardiography. When the aortic valve orifice becomes narrower, a pressure gradient develops between left ventricle and aorta. The left ventricle progressively hypertrophies to maintain the flow across the narrowed valve.

Aetiology and echocardiographic features

Left ventricular outflow obstruction may be subvalvular, valvular and supravalvular. Valvular disease is most frequent cause of aortic stenosis in adults, while subvalular and supravalvular forms are less common. There are three main causes of valvular aortic stenosis:

Calcific aortic stenosis

Calcific (also called degenerative and senile) aortic stenosis is the most common type of aortic stenosis in adults in developed countries. It is characterized by reduced mobility of calcified, thickened leaflets, and its frequency increases with age [Video 1, Video 2, Figure 1]. Calcification and thickening of the aortic valve in the absence of obstruction of ventricular outflow (the peak jet velocity <2.6 m/s) is referred to as aortic sclerosis.

Since the biscupid valve can also calcify, in patients with severe valve calcification, it may be difficult, sometimes impossible, to distinguish between a bicuspid and tricuspid valve.

Bicuspid aortic valve

Bicuspid aortic valve is the most common form of congenital aortic stenosis. It affects 1-2% of population, more frequently male than female, and is often undiagnosed until adult life. It is sometimes associated with the coartation of the aorta and also carries an increased risk of aortic dissection. A fusion between the left and right coronary cusps is the most commonly seen type (~ 90%) of bicuspidity [Video 3]. Systolic doming during systole in parasternal long-axis view is one of the signs suggestive of a bicuspid aortic valve [Video 4]. Although transthoracic echocardiography is usually sufficient for making diagnosis of bicuspid aortic valve, in some cases, it might be difficult to discern between the bicuspid and tricuspid valve, and transesophageal echocardiography might be needed. Stenosis of bicuspid aortic valve occurs approximately two decades earlier than in tricuspid valve [Video 5 and Figure 2].


Rheumatic heart disease

In contrast to the western world where rheumatic aortic stenosis is rarely encountered, it is still a common form of aortic stenosis in developing countries. On two-dimensional echocardiography, rheumatic aortic valve stenosis is characterized by commissural fusion and thickening along leaflet edges. These patients also have some degree of aortic regurgitation and rheumatic damage to the mitral valve [Video 6, Figure 3].

Differential diagnosis

  • Fixed subvalvular obstruction
Presence of subaortic membrane
May be difficult to visualise in 2D echocardiography
Presents in early adulthood
Valve is not stenotic, but Doppler shows increased gradient.
  • Dynamic subaortic obstruction
Occurs with hypertrophic cardiomyopathy(HOCM)
Other features of HCM
- late peaking, triangular CW Doppler profile
- changes with provocative measures

Assessment by echocardiography

Transthoracic two-dimensional Doppler echocardiography permits to diagnose and estimate the severity of aortic stenosis in the majority of patients. In patients with poor echogenicity or ambiguous findings on transthoracic examination, transesophageal echocardiography should be performed. Echocardiographic evaluation comprises the assessment of valve morphology, quantification of stenosis severity and left ventricular function [Figure 4]. It should also include assessment of ascending aorta (dilation, atheroma, etc.) and coexisting valve lesions (aortic regurgitation, mitral stenosis/regurgitation).

2D echocardiography

Two-dimensional echocardiography is an excellent tool for the assessment of the aortic valve morphology. The number of aortic valve cusps (tricuspid or bicuspid), their mobility and the degree of calcification (mild, moderate, severe) can be best appreciated from a parasternal short axis view, but parasternal long-axis view and the apical 5- and 3-chamber views can also be used.

Planimetry

Planimetry of the stenotic aortic valve orifice is the tracing out of the maximal opening of the valve in a still image obtained by transthoracic or transesophageal echocardiography. This method directly measures anatomical (not functional) valve area and is often inaccurate due to low image quality (artifacts, extensive valve calcification, etc.) or anatomy of stenosis. Planimetry of aortic valve is not routinely performed for quantification of stenosis severity, but it can be useful to crosscheck Doppler-derived (calculated) aortic valve area, in patients with good image quality (e.g. on transesophageal echocardiography).

Doppler echocardiography

The easiest way to assess aortic stenosis severity is to measure peak (maximal) transaortic jet velocity by continuos-wave Doppler.

The peak signal is usually searched in the apical 5-chamber view, but other views (apical long-axis, right parasternal, suprasternal, subcostal) may also be useful [Figure 5].

The patient should be properly positioned and the transducer position optimized in each view such that the ultrasound beam is aligned to the presumed blood stream direction. Color Doppler is helpful in determining the direction of the blood stream across the valve. In case of co-existing mitral regurgitation, care should be taken to avoid confusion with the mitral regurgitation signal. In patients with arrhythmia (e.g. atrial fibrillation), an average velocity of 5-10 cardiac cycles should be reported.

Peak and mean pressure gradients

Transaortic peak and mean gradients can be calculated from transaortic velocities using the simplified Bernoulli equation (ΔP = 4v2). This equation assumes that the velocity proximal to the stenosis is lower than 1 m/s and can be ignored, while transvalvular flow is at least 4 times the proximal flow. The peak gradient is calculated from peak velocity (ΔP max = 4v2 max*). The mean gradient is calculated by integrating the gradient over the entire systole (ΔP mean = Σ 4v2 / N). When the proximal velocity is >1.5 m/s, or the aortic velocity is <3 m/s, the proximal velocity should be included in the Bernoulli equation (ΔP max = 4 (v2max - v2 proximal)). All commercially available ultrasound machines provide the peak and mean gradients after manual tracing of transaortic continuous-wave Doppler spectrum. Transaortic jet velocities and pressure gradient are flow-dependent measures of aortic stenosis severity. In low cardiac output states (e.g. severe left ventricular systolic dysfunction), transaortic velocities and gradients may be relatively low despite the presence of severe aortic stenosis (see low-flow, low-gradient aortic stenosis).

The continuity equation

The continuity equation states that the flow in one area must equal the flow in a second area if there are no shunts in between the two areas [Figure 6]. In practical terms, the flow through the left ventricular outflow tract (LVOT) equals the flow through the (stenotic) aortic valve. The aortic valve area is calculated using the continuity equation after other three variables (cross-sectional area of LVOT and velocity time integrals (VTI) in LVOT and across the aortic valve) are obtained by Doppler echocardiography.

The flow through the LVOT (i.e. stroke volume at the level of LVOT), is calculated by multiplying cross-sectional area by the VTI of LVOT.

Cross-sectional area of LVOT is calculated as follow: CSA (LVOT) = π (LVOT diameter/2)2.

LVOT diameter is measured in a parasternal long-axis view (ideally in a zoom mode), as an inner diameter just below the aortic valve [Figure 7]. As this diameter is squared in the continuity equation, potential measurement error is also squared, leading to imprecise aortic valve area calculation. LVOT VTI is measured from an apical 5-chamber view, with pulsed-wave Doppler at the same level where the LVOT diameter measurement was made. VTI of transvalvular aortic flow (AV VTI) is measured from continuous-wave Doppler recordings in an apical 5- or 3-chamber view [Figure 7].

From these, it is easy to calculate the aortic valve area (AVA) by dividing the LV stroke volume by the AV VTI:

AVA [cm2] = (LVOT CSA x LVOT VTI) / AV VTI

Velocity ratio (Dimensionless velocity index)

One method to avoid potential error is to remove the LVOT cross sectional area from the continuity equation. It can be expressed as a simple ratio of velocities (or velocity-time integrals) in left ventricular outflow track and across the valve.

Velocity ratio = VTILVOT/VTIAoV

Velocity ratio = VelocityLVOT/VelocityAoV

Velocity ratio <=0.25 is associated with severe aortic stenosis [Figure 8].

Velocity ratio is particularly useful in assessment of prosthetic valve dysfunction even when it is difficult to measure the LVOT diameter accurately and the valve size is not known. Velocity ratio is not altered by conditions producing high flow across the valve.

Transoesophageal echocardiography

In patients in whom transthoracic echocardiography is unsatisfactory or not feasible (poor echogenicity, ambiguous findings, mechanically ventilated patients, etc.), multiplane transeshophegal Doppler echocardiography offers an alternative approach for assessing the severity of aortic stenosis [Video 7 and 8].

Left ventricular function assessment

In order to maintain the normal wall stress despite chronic pressure overload, the left ventricle in patients with aortic stenosis usually hypertrophies in a concentric manner. Nevertheless, in some patients with severe aortic stenosis, no or only mild left ventricular hypertrophy can be seen.

Global left ventricular systolic function (i.e. ejection fraction) may remain normal until the later stages of the disease, while the diastolic function is usually disturbed early in the disease course. As a consequence, patients with severe aortic stenosis may have signs and symptoms of heart failure, despite preserved ejection fraction (Video 9 and Figure 9).

Myocardial deformation-derived parameters, such as global longitudinal strain by speckle tracking imaging, might be useful to detect subtle changes in left ventricular function in patients with aortic stenosis and preserved ejection fraction, but the added value of this approach has not yet been proven.


Severity summary

Guideline-proposed echocardiographic parameters for classification of the aortic stenosis severity are summarized in Table 1 and Figure 10. Nevertheless, in some cases, echocardiographic indices may not be consistent due to errors in measurements or the presence of associated disorders and conditions (mitral or aortic regurgitation, left ventricular systolic dysfunction, low cardiac output states, atrial fibrillation, etc.).

Ao sclerosis Mild AoS Moderate AoS Severe AoS
Peak jet velocity (m/s) < 2.6 2.6 - 3 3 - 4 > 4
Mean gradient (mmHg) - < 30 (25*) 30 - 50 (25 - 40*) > 50 (40*)
AVA (cm2) - > 1.5 1 -1.5 < 1
Indexed AVA (cm2/m2 BSA) - > 0.9 0.6 - 0.9 < 0.6
Velocity ratio - > 0.5 0.25 - 0.50 < 0.25

(*) ACC/AHA Guidelines

Therefore, if different indices for quantification of the stenosis severity do not fit in the same category, an echocardiographer should crosscheck the accuracy of the measurements (e.g. LVOT diameter) and also take into account potential coexisting pathological conditions or physiological states.

Low-flow, low-gradient aortic stenosis

According to the latest ESC guidelines, as long as mean transaortic pressure gradient is > 40 mmHg, there is virtually no lower EF limit for surgery[1]. On the other hand, the management of patients with low-flow, low- gradient AS (AVA <1 cm2, EF <40% and mean pressure gradient of <40 mmHg) is controversial, due to high operative mortality and in the uncertainty about the severity of the valve stenosis [Video 10 and Figure 11].

From the echocardiographic parameters at rest, it is not clear whether the reduced AVA indeed reflects severe AS rather than an effect of low flow (i.e. stroke volume index <=35 ml/m2) on a moderately stenosed valve. Furthermore, it is difficult to predict functional recovery following the surgery since the reason for depressed ejection fraction can be twofold: either it is caused by excessive afterload or is a consequence of scarring due to extensive myocardial infarction. Assessment by dobutamine stress echo testing is essential to distinguish between true (fixed) and pseudostenosis and to verify the presence of the contractile reserve[2]. The latter is defined by an increase of stroke volume for at least 20% during dobutamine infusion [Figure 12].

It should be noted that although the absence of contractile reserve is related to high operative mortality (30% vs. 6% with contractile reserve), it cannnot predict the absence of LVEF recovery in patients surviving AVR[3] [Figure 13]. Aortic valve replacement is recommended in patients with low-gradient low-flow aortic stenosis with contractile reserve, since it carries an acceptable operative risk and improves long-term outcome in most survivors. In patients without contractile reserve, a trend towards better survival after AVR is compromised by a high operative mortality that reaches 50%.

Nevertheless, the prognosis is also grim if these patients were treated medically – a 2-year survival rate in only 15% – and, therefore AVR should not be contraindicated on the sole basis of exhausted contractile reserve[3].

Paradoxical low-flow aortic stenosis

Similarly to low-flow, low-gradient aortic stenosis, patients with paradoxical low-flow aortic stenosis also have have small calculated AVA, transaortic pressure gradients corresponding to non-severe aortic stenosis, and low stroke volume index (<=35 ml/m2). In contrast to low-flow, low-gradient aortic stenosis, patients with paradoxical low-flow aortic stenosis have normal left ventricular ejection fraction. Provided that both LVOT measurement is correct and peak jet velocity is not underestimated, this scenario may be seen in elderly patients with a history of hypertension, small LV dimensions and LV hypertrophy[4]. Paradoxical low-flow aortic stenosis may often be misdiagnosed, leading to an underestimation of symptoms and an inappropriate delay of aortic valve replacement surgery, without which the patients' prognosis is poor [5].

References

  1. Vahanian A, Baumgartner H, Bax J, Butchart E, Dion R, Filippatos G, Flachskampf F, Hall R, Iung B, Kasprzak J, Nataf P, Tornos P, Torracca L, Wenink A; Task Force on the Management of Valvular Hearth Disease of the European Society of Cardiology; ESC Committee for Practice Guidelines. Guidelines on the management of valvular heart disease: The Task Force on the Management of Valvular Heart Disease of the European Society of Cardiology. Eur Heart J. 2007;28(2):230-68.
  2. Monin JL, Quéré JP, Monchi M, Petit H, Baleynaud S, Chauvel C, Pop C, Ohlmann P, Lelguen C, Dehant P, Tribouilloy C, Guéret P. Low-gradient aortic stenosis: operative risk stratification and predictors for long-term outcome: a multicenter study using dobutamine stress hemodynamics. Circulation. 2003;108(3):319-24.
  3. 3.0 3.1 Quere JP, Monin JL, Levy F, Petit H, Baleynaud S, Chauvel C, Pop C, Ohlmann P, Lelguen C, Dehant P, Gueret P, Tribouilloy C. Influence of preoperative left ventricular contractile reserve on postoperative ejection fraction in low-gradient aortic stenosis. Circulation. 2006;113(14):1738-44.
  4. Hachicha Z, Dumesnil JG, Bogaty P, Pibarot P. Paradoxical low-flow, low-gradient severe aortic stenosis despite preserved ejection fraction is associated with higher afterload and reduced survival. Circulation. 2007;115(22):2856-64.

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