These are clips from a perioperative TEE study performed in a patient undergoing aortic valve replacement surgery for severe aortic stenosis. The key findings are summarised below. This is followed by a tutorial on determining AS severity.
The commonest causes for aortic stenosis are calcific stenosis of a tricuspid valve, calcification of a bicuspid valve and rheumatic valve disease. Senile calcification of tricuspid valves are more prevelant in ages > 75years. Bicuspid aortic valve calcification and stenosis presents at ages <65 years.
2D evaluation begins with the mid esophageal aortic valve short and long axis views. These are shown above & below. The short axis view is obtained with the multiplane angle between 30-60 degrees, whilst the long axis view is obtained between 120-140 degrees. The non-coronary (NCC), right and left coronary (RCC, LCC) are labelled.
Short and long axis views are used for anatomical evaluation. This includes valve morphology to determine number of cusps, leaflet thickness, mobility and degree of calcification. 2D imaging is additionally useful for the determination of the level of stenosis – subvalvular, supravalvular or valvular.
Bicuspid valve – 80% are due to fusion of the left and right coronary cusp, leading to the formation of a larger anterior cusp and smaller posterior cusp. Both coronary arteries arise from the anterior cusp. 20% are due to fusion of the right and non coronary cusp. The right cusp is then larger than the left. Fusion of the left and non-coronary cusp is extremely rare (true bicuspid). Diagnosis is made in the short axis view, where 2 commisures are seen with an eliptical valve orrifice in systole. In diastole, the valve may appear tricuspid if a raphe is present. The long axis view may reveal leaflet doming or prolapse.
Calcification – Tricuspid valve calcification normally involves the basal and central parts of each cusp. The orrifice appears stellate in shape. Bicuspid valve calcification is more asymetric. The degree of calcification can be classified into:
1. Mild – Few areas of dense echogenicity with little acoutic shadowing
2. Moderate – Multiple larger areas of dense echogenicity
3. Severe – Extensive thickening, increased echogenicity and significant acoutic shadowing.
Rheumatic Disease is characterised by calcification of the edges of the cusps and commisural fusion, giving rise to a triangular orrifice. Rheumatic valvular disease involving the aortic valve is almost always avccompanied by mitral valve involvement.
Supravalvular Stenosis – This uncommon and may be seen in congenitally in syndromes such as William’s syndrome.
Subvalvular Stenosis – Stenosis may be fixed secondary to a muscular band or membrane. Dynamic subaortic obstruction refers to stenosis that changes in severity during left ventricular systole. Examples include hypertrophic cardiomyopathy. Obstruction occurs in mid to late systole and is also dependent on loading conditions and ventricular contractility.
3 main hemodynamic parameters are recommended for AS severity evaluation. These are :
1. AS peak jet velocity
2. Mean transvalvular gradient
3. Aortic Valve Area (AVA) by continuity equation
The transgastric TEE views used for doppler evaluation of the aortic valve are shown. The Deep TG 5 Chamber view is shown above. The blood flow through the aortic valve is in good alignment for doppler interogation in these views. The TG LAX view is shown below.
This is obtained with continuous wave doppler. Multiple measurements in multiple windows should be used to ensure accuracy. TTE – Right parasternal, apical and suprasternal windows. TEE – Transgastric and deep transgastric windows.
AS peak jet velocity is defined as the highest velocity signal obtained from any window after a careful examination; lower values from other views are not reported. The acoustic window that provides the highest aortic jet velocity is noted in the report and usually remains constant on sequential studies.
A cut off value of 4.0m/s is used to indicate severe aortic stenosis. 3 or more beats should be averaged for sinus rhythm, whilst 5 beats is used for irregular rythms.
Continuous wave doppler interrogation of the aortic valve is shown above. The peak velocity is greater than 2.4m/s. The shape of the CWD envelope is useful for distinguishing the level and severity of stenosis. With incresing severity of stenosis, the peak velocity occurs later in systole. With milder degrees of stenosis, the peak velocity occurs earlier in systole with a triangular shaped velocity profile. The velocity profile also helps to distinguish dynamic from fixed stenosis. In dynamic stenosis, a characteristic late peaking velocity curve is seen, with a concave shape in early systole.
The simplified Bernoulli equation is used to calculate pressure gradient from velocity (v).
Pmax = 4v2max:
The mean gradient is calculated by averaging the instantaneous gradients over the ejection period, a function included in currently available clinical instrument measurement packages using the traced velocity curve.
Misalignment of the ultrasound beam with the AS jet results in significant underestimation of jet velocity and an even greater underestimation of pressure gradient, owing to the squared relationship between velocity and pressure difference. This highlights the importance of using multiple acoustic windows (as detailed above) for the CWD assessment of AS.
AVA is calculated by using the continuity-equation (Figure 4) which is based on the concept that the stroke volume (SV) ejected through the LV outflow tract (LVOT) all passes through the stenotic orifice (AVA) and thus SV at valve orifice level is equal to the LVOT SV.
Standard calculation of continuity-equation valve area requires three measurements:
1. AVvti: AS jet velocity by CWD.
2. LVOTcsa: LVOT diameter for calculation of the cross-sectional area (CSA).
3. LVOTvti: LVOT velocity recorded with pulsed Doppler.
AVA x AVvti = LVOTcsa x LVOTvti
therefore, AVA = (LVOTcsa x LVOTvti) / AVvti
Measurements required for the continuity equation are shown. LVOT diameter is measured in the ME AV LAX view (above). PWD is used to measure LVOT velocity as shown below. The sample volume is place in the LVOT proximal to the aortic valve (below).
Indicators of severe aortic stenosis:
1. Peak velocity > 4.0m/s
2. Mean gradient > 40mmHg
3. AVA < 1.0cm2
1. Messika-Zeitoun & Lloyd. Aortic valve stenosis: evaluation and management of patients with discordant grading. ESC e-Journal of Cardiology Practice.
2. Saikrishnan et al. Accurate assessment of aortic stenosis. A review of diagnostic modalities and hemodynamics. Circulation 2014.
3. Baumgartner et al. Recommendations on the Echocardiographic Assessment of Aortic Valve Stenosis: A Focused Update from the European Association of Cardiovascular Imaging and the American Society of Echocardiography. JASE 2017.
