Aortic Valve Disease: Evaluation of Stenosis. Mark A. Taylor, MD, FASE Pittsburgh, PA

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1 Aortic Valve Disease: Evaluation of Stenosis Mark A. Taylor, MD, FASE Pittsburgh, PA A careful and systematic transesophageal echocardiographic evaluation can lead to new diagnoses, altered surgical management and improved patient care. In patients undergoing aortic valve surgery for aortic stenosis, intraoperative TEE has been shown to alter the surgical plan in 13% of the cases (1). A consensus statement from the American College of Cardiology, American Heart Association and American Society of Echocardiography gave intraoperative TEE a Class I designation ( evidence and/or general agreement that a given procedure or treatment is useful and effective ) in patients undergoing surgical repair of valvular lesions (2). TEE evaluation of the aortic valve should be performed upon all patients undergoing cardiac surgery. TEE Modality 2D/3D PW CW CFD Focus of Aortic valve examination Aortic valve architecture Aortic root and LVOT architecture LV function and filling Assessment of valvular lesions and hemodynamics Assessment of valvular lesions and hemodynamics Assessment of valvular lesion Anatomy and TEE examination 4 recommended views (3) Midesophageal aortic valve short axis (ME AV SAX) Midesophageal aortic valve long axis (ME AV LAX) Transgastric long axis (TG LAX) Deep transgastric long axis (deep TG LAX) ME AV SAX Probe at mid esophageal position Rotate forward to degrees 2D and 3 D examination of aortic valve cusps and leaflets CFD examination of stenotic lesion 2D imaging with planimetry allows estimation of aortic valve opening

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3 (Figures 1 and 2. Images of 2D ME AV SAX with trileaflet aortic valve open and closed) The aortic valve apparatus is comprised of the valve cusps, sinuses of Valsalva, proximal ascending aorta, and left ventricular outflow tract. The ME AV SAX view provides detailed 2D and 3D anatomy of the leaflets and annular complex. The normal valve consists of 3 cusps suspended from an associated sinus of Valsalva. 2D and 3D examination permits evaluation of individual leaflet architecture and range of motion throughout the systolic (opening) and diastolic (closing) cycle. The degree of opening can be measured by planimetry using the trackball and the caliper and trace function. This method is simple and rapid yet poor reliability exists between observers which creates error. Overestimation of valve area by planimetry may occur in patients with pliable leaflets if the ultrasound beam intersects the leaflets below the tips. Errors are also introduced when the aortic valve is calcified and the orifice of the valve cannot be identified.

4 (Figures 3 and 4. Images of ME AV SAX with planimetry measurement of aortic valve area in a patient with aortic stenosis and AVA of 0.6 cm 2. Cartoon representation of the ME AV LAX view and how the ultrasound beam intersects the aortic valve on different planes. The possibility to overestimate AVA by planimetry exists if the echocardiographic imaging plane is not at the level of both leaflet tips.) ME AV LAX Probe at mid esophageal position Rotate forward to degrees 2D and 3 D examination of the left ventricular outflow tract, the aortic root (aortic valve, sinuses of Valsalva, and the sinotubular junction), and the proximal ascending aorta. Detailed examination of leaflet morphology, mobility, thickening, and calcification can be performed. Subaortic pathology can be identified including subaortic membranes or systolic anterior motion (SAM) of the anterior mitral valve leaflet. 2D imaging with caliper measurements allows estimation of aortic valve opening, and specific aortic root measurements. CFD examination of stenotic and/or regurgitant aortic valve lesions. CFD examination allows identification of subaortic flow disturbances. M-mode examination of leaflet mobility, thickening and tip separation.

5 (Figures 5 and 6. Images of ME AV LAX with aortic valve closed and open)

6 TG LAX Probe in transgastric position Rotate forward to degrees 2D and 3 D examination may be limited by aortic valve being in far field position CFD examination allows identification of valve position and/or associated stenotic or regurgitant lesions CW or PW Doppler examination due to parallel alignment of blood flow and Doppler beam. Allows for determination of velocities in the outflow tract and aortic valve. (Figure 7. Image of TG LAX with aortic valve closed on right side of video display) Deep TG LAX Probe started in the transgastric position, then inserted and anteflexed. Once anteflexed and inserted, the probe is slowly withdrawn 2D and 3D examination may be limited by aortic valve being in far field position CFD examination allows identification of valve position and/or associated stenotic or regurgitant lesions CW or PW Doppler examination due to parallel alignment of blood flow and Doppler beam. Allows for determination of velocities in the outflow tract and aortic valve. View provides complementary information to the TG LAX but may be utilized when the TG LAX does not provide alignment of blood flow and Doppler beam. Image acquisition improves with experience (4)

7 (Figure 8. Image of deep TG LAX with aortic valve closed on left side of video display) Pathphysiology Aortic Stenosis Normal aortic valve area in adults is 3-4 cm 2. Obstruction of can occur at three distinct anatomical sites: valvular, subvalvular, or supravalvular. Valvular obstructions account for the majority of cases of LVOT obstructions (5). The most common cause of aortic stenosis in the United States is calcific aortic stenosis of the elderly. This is followed by congenital abnormalities of the aortic valve including bicuspid and rarely unicuspid or quadricuspid valves. Of the aortic valve replacements performed in the US and Europe, bicuspid aortic valves account for approximately 50% and progressive calcification of a tricuspid valve aortic valve account for the remainder (6). The mechanism of aortic stenosis in the elderly and in congenital cases is distorted flow through a diseased valve, which leads to degenerative changes and progressive calcification. Rate of calcification and stenosis varies widely although elderly men with associated coronary artery disease as well as individuals with a history of smoking, hypercholesterolemia and elevated serum creatinine levels demonstrate more rapid disease progression (7-9). The development of aortic stenosis is not part of routine aging but is an active process involving chronic inflammation fueled by atherosclerotic risk factors (10).

8 Etiology Factors Features Other issues Calcific aortic stenosis Age > 65 years old Common in US and Europe. 4% of the elderly have aortic stenosis Progressive degeneration (11) Congenital Bicuspid Age years 1-2% of US population have bicuspid valve (12) Rheumatic Commissural fusion Rare in US, common worldwide Coarctation of aorta, dilation of aortic root, aortic dissection May occurs irrespective of hemodynamics and age suggesting developmental or genetic disorder (13) Patients undergoing AVR should have root replacement if greater than 4.0 or 4.5 cm (14, 24) Usually associated with mitral involvement

9 (Figure 9. Zoom image of a trileaflet aortic valve with heavy calcification and a stenotic orifice)

10 (Figure 10. Zoom image of a bicuspid aortic valve with a decreased opening consistent with aortic stenosis)

11 ECHOCARDIOGRAPHIC ASSESSMENT Modality View Echocardiographic feature 2D/3D ME AV SAX ME AV LAX Leaflet restriction, calcification, commissural fusion Planimetry # (15) Annular measurement for continuity equation * (15) sinus of Valslva, Sinotubular and ascending aorta diameter measure M-mode ME AV LAX Septal separation (16) 8 mm critical stenosis > 12 mm noncritical stenosis CFD Identify site of stenosis PWD CWD ME AV SAX ME AV LAX TG LAX Deep TG LAX TG LAX Deep TG LAX Identify associated regurgitation Measure LVOT velocity at a specific site (15) Measure highest gradient along LVOT into ascending aorta (15) o AS jet velocity * o Mean transaortic gradient* o Valve area by continuity equation using VTI* o Valve are by simplified continuity equation using peak velocities # o Velocity ratio or dimensionless index # * Level I measurement recommendation for all patients with aortic stenosis (15). # Level II methods for additional information in select patients (15).

12 The primary echocardiographic technique used to quantify aortic stenosis severity is Doppler echocardiography and is used to calculate pressure gradients and aortic valve area. Continuous wave Doppler (CWD) techniques are utilized to measure transvalvular blood velocity. The peak pressure gradient is then estimated from the peak velocity measurement using the simplified Bernoulli equation: Peak Aortic Valve Pressure Gradient (PG AV ) = 4 (Aortic Valve velocity) 2 P= 4 (Aortic Valve velocity) 2 The mean gradient is a derived measurement obtained by the ultrasound system by averaging the instantaneous gradients over the ejection period. Mean aortic valve pressure gradient can be estimated from the following formula: Mean Aortic Valve Pressure Gradient (MG AV ) = 2.4 (Aortic Valve Velocity) 2 When LVOT velocity is greater than 1.5 m/s, the modified Bernoulli equation should be utilized to prevent overestimation of the pressure gradient and the severity of the aortic stenosis. The modified Bernoulli equation: P= 4 (Aortic Valve Velocity 2 - LVOT Velocity 2 ) Severity of Aortic Stenosis Indicator Mild Moderate Severe Peak jet velocity (m/s) * < > 4.0 Mean gradient (mm Hg) * < > 40 Valve area (cm 2 ) * < 1.0 Dimensionless index # > < 0.25 Indexed AVA (cm 2 /m 2 ) < 0.6 * Level I measurement recommendation for all patients with aortic stenosis. # Level II methods for additional information in select patients (15). Gradients derived in the operating room can be significantly different from those obtained during preoperative echocardiographic studies or in the cardiac catheterization laboratory. Stenotic orifice gradients are flow dependent, and an increase in cardiac

13 output across the aortic valve will increase the gradient. Conditions that increase forward blood flow across the aortic valve such as hyperdynamic left ventricular function, sepsis, severe aortic insufficiency and hyperthyroidism will all increase the pressure gradient. Conversely, severe LV dysfunction, severe mitral regurgitation and a left to right shunt will all decrease the transvalvular pressure gradient. Changes in intraoperative loading conditions, heart rate and contractility may all cause significant disparities between intraoperative and preoperatively derived transvalvular gradients. Differences in gradients between the intraoperative echocardiography examination and catherization data can also occur. Peak left ventricular pressure occurs earlier in systole than the peak aortic pressure, which occurs later in systole. The measurement obtained during cardiac catheterization pull back technique is a peak-to-peak measurement. This measurement is obtained at two different times during systole. Peak left ventricular pressure to peak aortic pressure (peak to peak) is a smaller difference and occur at two different time points during systole. The echocardiographic Doppler derived pressure gradient is a reflection of the same time peak instaneous pressure difference. The Doppler peak instantaneous pressure difference will be larger than the peak to peak difference because still rising aortic pressure, which has not attained the peak position. (Figure 11. Graphic representations of simultaneous pressure recordings from the left ventricle (red) versus the aorta (purple) over time are shown. The peak pressure in the left ventricle occurs slightly before the peak pressure measured in the aorta. As such a peak to peak measurement obtained in the catheterization lab will be less than the peak instantaneous pressure difference measured by Doppler echocardiography. At the time when the peak instantaneous pressure is being measured, the left ventricular pressure is at the peak but aortic pressure is still rising and is less than the peak aorta pressure that occurs slightly later.)

14 Pressure gradients can also occur in the LVOT from subvalvular and supravalvular pathology, therefore it is important to localize the site of the pressure gradient within the flow stream. 2D imaging can localize the pathology to the aortic valve or somewhere else within the flowstreams (subvalvular or supravalvular). The ME AV LAX view obtained with 2D imaging, and supplemented with Color Flow Doppler is useful for localization of the obstruction. A color flow disturbance will be noted downstream of the obstruction and this area can be compared to the position of the aortic valve. The shape of the spectral Doppler envelope can also yield important diagnostic clues. Valvular aortic stenosis produces a rounded pattern with a midsystolic peak. Left ventricular outflow tract obstruction produces a late systolic peak and has a dagger shaped or shark s tooth spectral pattern. (Figure 12. Deep TG LAX image with CW Doppler through stenotic aortic valve. The spectral Doppler envelope has a rounded appearance with a mid systolic peak. Peak velocity approximates 4 m/s consistent with severe aortic stenosis) Given the dynamic nature of pressure gradients in relation to trans aortic blood flow, aortic valve area is considered a more constant and less dynamic measure of aortic valve stenosis. The continuity equation is utilized to calculate aortic valve area and is based upon the concept of conservation of mass and continuity of flow. The flow of blood

15 through the left ventricular outflow tract must equal flow through the aortic valve into the ascending aorta. SV AV = SV LVOT Stroke Volume = Area x Flow rate SV AV = CSA AV x VTI AV SV LVOT = CSA LVOT x VTI LVOT Where SV = stoke volume and CSA is cross sectional area. Thus, CSA AV x VTI AV = CSA LVOT x VTI LVOT Aortic valve area (CSA AV ) = CSA LVOT x VTI LVOT VTI AV Assuming the LVOT is circular: CSA LVOT = π x (d/2) 2 CSA LVOT = π /4 x (d) 2 CSA LVOT = x (d) 2 To obtain the three measurements required to solve the continuity equation for aortic valve area, one must utilize three different echocardiographic techniques. Technique Aortic valve velocity by CWD LVOT velocity by PWD or CWD LVOT diameter by 2D with calipers Tomographic View TG LAX or deep TG LAX TG LAX or deep TG LAX ME AV LAX The velocity time integral of the left ventricular outflow tract is obtained with either the deep TG LAX or the TG LAX view, and requires parallel alignment of the Doppler beam through the left ventricular outflow tract. Pulse wave Doppler is utilized for sampling LVOT velocities given the need for range specificity. The sample volume is placed within the LVOT and records sample velocities at a specific location in the LVOT. The sample volume is slowly moved closer to the aortic valve and records corresponding sample velocities. As the sample velocities increase and begin to alias because of flow acceleration, the sample volume is withdrawn slightly back into the LVOT and the spectral velocity is recorded and saved at this portion of the LVOT. The aliasing should be eliminated and the spectral profile should demonstrate laminar flow and have a hollow spectral envelope. Normal peak velocities in the LVOT will range from 0.8 m/s to 1/5 m/s. To have an accurate measure of stroke volume, the LVOT diameter should be measured at the same point in the LVOT where the LVOT spectral display was recorded.

16 The ME AV LAX often affords the best image for measuring the LVOT diameter. A midsystolic frame should be selected and the LVOT diameter measured proximal to the aortic annulus corresponding to the same anatomic location as the pulsed wave Doppler recording of the LVOT velocity. Once 2D imaging and Color Flow Doppler has verified the aortic valve as the source of the stenosis, continuous wave Doppler (CWD) is applied to either the TG LAX or the deep TG LAX view. Continuous wave Doppler must be utilized for sampling the stenotic aortic valves because velocities are usually greater than 2 m/s. Proper alignment of the Doppler stream to flow across the aortic valve is essential. Color flow Doppler and audible signaling may help the operator align the Doppler beam, especially as the valve orifice becomes smaller. The spectral envelope with continuous wave Doppler of the stenotic aortic valve is solid in character (non laminar flow) and demonstrates a high velocity that peaks in mid systole. Planimetry is then utilized to trace the spectral envelope and obtain the aortic valve velocity time integral (VTI) and peak velocity. Although some consensus statements suggest aortic valve area can be calculated utilizing either the velocity time integrals or the peak velocities in both the LVOT and the aortic valve (17), recent EAE/ASE statement has express concerns in using the peak velocities secondary to higher variability when compared to using the VTI (15). Accurate measurement of the LVOT diameter is essential given that this variable is squared when solving the continuity equation for aortic valve area. Any underestimation of LVOT diameter will cause a resultant significant underestimation of the true aortic valve area and an overestimation of aortic stenosis severity. Another potential source of error is the inability to align the Doppler beam to be parallel with flow. Ensuring that axial alignment deviates no more than 20 degrees from parallel will minimize this error. In up to 6% of patients the inability to obtain adequate tomographic views limits the TEE evaluation of aortic stenosis(18,19). In these cases, epicardial echocardiography can overcome this limitation in 100% of patients and demonstrates excellent correlation with TEE, TTE, and cardiac catheterization-derived measures (20). Ideally the pulse wave calculation and the continuous wave calculation would occur with the same heartbeat and therefore the same stroke volume. The variability in patients in normal sinus rhythm in stroke volume is negliable. Intraoperative patients with sudden changes in preload, heart rate and contractility can have an affect on the nonsimultaneous measure of flow across the LVOT and the aortic valve. Marked variability is also seen in patients with irregular heartbeats, especially atrial fibrillation. It is recommended in patients with atrial fibrillation that six consecutive beats are analyzed and averaged to obtain the velocity time integral in both the LVOT and the aortic valve. Velocity Ratio or Dimensionless Index CWD can be utilized to calculate both the aortic valve and the left ventricular outflow tract velocity time integrals from the same beat. This eliminates the beat-to-beat variability and both the higher velocity related to the aortic valve and a lower denser

17 pattern consistent with the left ventricular outflow tract are present on spectral display (21). Comparison of the LVOT velocity time integral to the Aortic valve velocity time integral is called the dimensionless index as the LVOT area has been eliminated from the continuity equation. This index can be utilized for quantification of the severity of aortic stenosis. In normal valves, the continuous wave VTI for the aortic valve will equal the VTI of the left ventricular outflow tract (DI=1). With disease progression and worsening severity of stenosis, the aortic valve velocity increases while the left ventricular outflow tract velocity will remain unchanged. Severe aortic stenosis is present when the dimensionless index is less than 0.25 (22). (Figure 13. Deep TG LAX image with CW Doppler through LVOT and aortic valve. A double envelope spectral pattern is displayed with the denser spectral display flow through the LVOT represented by the X trace. The fine feathery display is flow through the stenotic aortic valve and is represented by the + symbol. The TEE machine measures velocities through the aortic valve (3.6 m/s) as well as the LVOT (0.974 m/s). The dimensionless index in this case is 0.974/3.6 and equals 0.27 which does not qualify for severe aortic stenosis as it is greater than 0.25.) Associated Echocardiographic Findings Other diagnostic findings will be present in patients with aortic stenosis. As an adaptive mechanism to chronic pressure overload, the left ventricle will secondarily hypertrophy. This concentric increase in wall thickness reduces wall stress by distributing the pressure

18 overload over a greater myocardial mass. Posterior wall thickness greater than 1.0 cm in women and 1.1 cm in men is considered abnormal. Left ventricular compliance is altered markedly by the secondary hypertrophy and traditional estimates of left ventricular volume based upon left ventricular filling pressures with pulmonary artery catheter data are unreliable. Transesophageal echocardiography examination can yield direct estimates of left ventricular volume status and ejection fraction. Systolic and diastolic dysfunction is also likely to be present in patients with aortic stenosis. Septal hypertrophy can lead to the development of systolic anterior motion (SAM) of the mitral valve. This leads to a fall in the cardiac output secondary to left ventricular outflow tract obstruction (LVOTO) by the anterior mitral leaflet during systole. The anterior mitral valve leaflet is drawn into to LVOT during systole causing an abrupt decrease in forward flow through the aortic valve. In addition to LVOTO there is associated mitral regurgitation (MR) because the anterior mitral leaflet is in the LVOT during systole and the mitral valve is therefore rendered incompetent. SAM and LVOTO may occur after an AVR. This postoperative complication may occur after an aortic valve replacement because of the low preload, a low afterload and an increased LV contractility that is common in post bypass patients. Left ventricular outflow tract obstruction (LVOTO) secondary to SAM is diagnosed with TEE and effectively treated with volume administration, elevation of afterload with vasoconstrictors and the discontinuation of inotropic and chronotropic medications. The diagnosis LVOTO and SAM should be excluded in all patients postoperatively. Echocardiographic evaluation of the mitral valve and LVOT can lead to predictions about the likelihood of this occurring based upon anatomic factors (23). Many patients with aortic stenosis will have associated aortic insufficiency. Mitral regurgitation is common in patients with aortic stenosis and is related to either LV pressure overload or intrinsic mitral disease. In the majority of patients without intrinsic mitral disease, mitral regurgitation improves after aortic valve replacement. Dilation of the ascending aorta may be a consequence of aortic stenosis from an adaptive mechanism, or may be due to intrinsic disease within the aortic wall. Evaluation of the ascending aorta, including epiaortic scanning, is recommended to determine the need for surgical repair and to guide cannulation and perfusion strategies. References: 1. Nowrangi SK, Connolly HM, Freeman WK, Click RL. Impact of intraoperative transesophageal echocardiography among patients undergoing aortic valve replacement for aortic stenosis. J Am Soc Echocardiogr 2001;14: Cheitlin MD, et al. ACC/AHA/ASE 2003 Guideline update for the clinical application of echocardiography: Summary article. JACC 2003;42: Shanewise JS, et al. ASE/SCA Guidelines for performing a comprehensive Intraoperative multiplane transesophageal echocardiographic examination: Recommendations of the American Society of Echocardiography Council for Intraoperative Echocardiography and the Society of Cardiovascular Anesthesiologist Task Force for certification in perioperative transesophageal echocardiography. Anest Analg 1999;89: Stoddard MF, Hammons RT, Longaker RA. Doppler transesophageal

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