From: Contemporary Cardiology: Essential Echocardiography: A Practical Handbook With DVD Edited by: S. D. Solomon © Humana Press, Totowa, NJ
209 P
ITFALLSC
ONCLUSIONS
UGGESTEDR
EADINGINTRODUCTION
Transthoracic echocardiography has largely replaced cardiac catheterization as the primary modality for the hemodynamic assessment of valvular heart disease. A comprehensive evaluation of valve structure, function, and hemodynamics is possible through a carefully per- formed transthoracic study. In the case of aortic stenosis, echocardiography is used to define the initial severity of disease, etiology, and monitor its progression through serial follow up studies. Evaluation of aortic stenosis would be incomplete without a comprehensive examina- tion of overall left ventricular function and estimation of pulmonary artery pressures. Transthoracic echocardio- graphy, therefore, can provide important information about the initial diagnosis, management, and follow-up of adult patients with native aortic valve stenosis.
ETIOLOGY
When considering the etiology of aortic stenosis, it is important to consider the age and the demographics of the patient (Fig. 1). Rheumatic valvular disease, once the most common form of aortic stenosis, is less common today in developed countries. Commissural fusion is a hallmark of this disease and often presents
with concomitant mitral stenosis (Fig. 2; please see companion DVD for corresponding video). Less commonly, the commissures may fuse eccentrically, producing a de facto bicuspid valve. In this case, dif- ferentiation from a congenitally bicuspid valve can be difficult.
Calcific degeneration is currently the most common form of aortic stenosis in the developed world. It gener- ally presents later in adult life and is sometimes called senile calcific degeneration (Fig. 3). Commissural fusion can occur, but is not a hallmark of this disease process.
Calcification is visualized as “echo bright” reflectivity of the valve leaflets and the aortic root.
Congenital bicuspid aortic valves may calcify and
thicken over time, eventually producing stenotic lesions
that present earlier than other common causes of adult
aortic valvular stenosis (Fig. 1). When observed in the
parasternal short-axis view, bicuspid aortic valves exhibit
either a vertical or a horizontal commissural orientation
in the parasternal short-axis view. In the vertical orienta-
tion, commissures are in the anterior and posterior posi-
tion with a right and left cusp. In the horizontal
orientation, commissures are in the right and left
positions, with anterior and posterior cusps. Raphes are
commonly seen in both of these orientations (Fig. 4).
210 Bermudez
The appearance of the valve in systole is characteristic of this etiology, and has been referred to as a fish-mouth appearance, rather than the triangular appearance of normal valves (Fig. 5; please see companion DVD for corresponding video).
TWO-DIMENSIONAL ASSESSMENT Important morphological information can be gained from initial two-dimensional (2D) views of the aortic valve. Leaflet restriction in the setting of adequate cardiac
output is the hallmark of aortic stenosis, which can be readily discerned in the parasternal long-axis view.
Failure of aortic leaflets to open fully with ventricular ejection often signifies some degree of aortic stenosis (Fig. 6; please see companion DVD for corresponding video). Eccentric valve opening visualized in the parasternal long views is a further clue to the presence of a bicuspid aortic valve. Stenotic aortic valves are generally thickened or calcified. This is easily seen on either 2D or M-mode assessment of the valve (Fig. 7).
Fig. 1. The etiology of aortic stenosis by age group. (Reproduced with permission from Passik CS, Ackermann DM, Pluth JR, Edwards WD. Temporal changes in the causes of aortic stenosis: a surgical pathologic study of 646 cases. Mayo Clin Proc 1987;62:119–123.)
Fig. 2. (See legend, facing page)
Therefore, the 2D characteristics of the aortic valve can provide important clues to the etiology of aortic stenosis (Table 1).
DOPPLER ASSESSMENT
Doppler measurements across the aortic valve are essential to the determination of the severity of aortic
stenosis. As the aortic valve narrows, the velocity of blood flow across the valve will generally increase. This velocity is dependent on the pressure gradient across the valve. Continuous-wave (CW) Doppler measurements across the aortic valve should be performed where the ultrasound beam is most parallel to the flow of blood across the valve. Therefore, measurements are best
Fig. 2. (A–C) Aortic stenosis showing thickened valve leaflets. (C) Commissural fusion (arrows) is a hallmark feature of rheumatic valvular heart disease. (Please see companion DVD for corresponding video.)Fig. 3. Calcific (“senile”) degeneration of aortic valve showing “echo-bright” reflection (arrows).
212 Bermudez
acquired in the apical five-chamber, right parasternal long axis, or suprasternal views to obtain the highest velocities across the valve. The maximum velocity obtained is highly dependent on accurate transducer positioning, and will be underestimated when Doppler beam angle devi- ates from that of blood flowing through the stenotic valve (Fig. 8). A nonimaging probe can be used if inadequate envelopes are found with the duplex imaging transducer.
The calculation of transvalvular pressure gradients is based upon the Bernoulli principle. The modified Bernoulli principle relates the transaortic pressure gradient to the square of the maximal velocity found on CW Doppler: P = 4V
2, and is used when the velocity of blood proximal to the stenosis is negligible. In sig- nificant aortic stenosis, high gradients across the aortic valve can be seen. The relationship is accurate
Fig. 4. Sketch showing bicuspid aortic valve variants with different commissural orientations (parasternal short-axis views [PLAX]).Fig. 5. (See legend, facing page)
Fig. 5. Bicuspid aortic valve during diastole (A) and systole (B) in parasternal short-axis (PSAX) view. Note horizontal commissure (arrow) in diastole and “fish mouth” appearance during systole. (C,D) Systolic doming of bicuspid aortic valve during systole (parasternal long-axis view [PLAX]). (Please see companion DVD for corresponding video.)
Fig. 6. Aortic stenosis showing restricted leaflet opening (arrow) during systole (parasternal long-axis view [PLAX]). Left ventricular ejec- tion fraction was reduced, hence “low stroke volume-low gradient” aortic stenosis. (Please see companion DVD for corresponding video.)
when the velocity proximal to the stenosis is less than 1.5 m/s. Should the velocity in the left ventricular out- flow tract (LVOT) exceed 1.5 m/s, then the long version of the Bernoulli equation should be used to calculate the transaortic gradient. Therefore:
Transaortic pressure gradient = 4 (V
max2– V
LVOT2) when V
LVOT≥ 1.5 m/sec
In most cases, the modified Bernoulli equation is an accurate and simple measure of the peak instantaneous transaortic pressure gradient.
Peak transaortic gradient measured by Doppler should be differentiated from the peak-to-peak gradient obtained at cardiac catheterization. Hemodynamically,
the peak-to-peak gradient measures the difference between the peak left ventricular pressure and the peak aortic pressure—which are not measured simultaneously.
Therefore, peak-to-peak gradients are not physiologi- cal, and no Doppler measurement exactly corresponds to this measurement in the cathetherization laboratory.
However, mean gradients across the aortic valve are similar when measured by Doppler and by cardiac catheterization.
The Doppler derived mean gradient is the average
gradient over the systolic ejection period. This is usu-
ally calculated by tracing the CW envelope obtained
across the aortic valve, and the numerical values are
automatically calculated by the machine’s software
214 Bermudez
package (Fig. 9). It is important to ensure that it is actu- ally the aortic envelope that is traced, as a mitral or tri- cuspid regurgitant jet may mimic an aortic stenotic envelope (Fig. 10).
CALCULATION OF AORTIC VALVE AREA The calculation of aortic valve area (AVA) is based on the continuity equation (Fig. 11). This equation relates the flow proximal to the stenosis and flow through the valve.
Area
LVOT× V
LVOT= AVA × V
maxWhere the AVA is the aortic valve area and V
maxis the maximal velocity obtained by CW Doppler. V
LVOTis the velocity in the LVOT obtained by pulse wave Doppler usually in the apical five-chamber view. Area
LVOTis the area of the LVOT and is calculated by multiplying π by the square of the radius of the LVOT. The equation thus becomes:
AVA = π(radius of LVOT)
2× V
LVOT/V
maxTherefore, the AVA can be easily calculated using velocities. Alternatively, the velocity time integral (VTI) can also be used instead of velocities to obtain the AVA.
ASSESSMENT OF STENOSIS SEVERITY The normal aortic valve orifice area in adults ranges from 2 to 4 cm
2. In general terms, severe stenosis is not seen with an AVA of more than 1.0 cm
2. According to American College of Cardiology/American Heart Association guidelines, severe aortic stenosis is seen when the AVA is less than 1 cm
2(Table 2). Transvalvular gradients should not be ignored when assessing severity of aortic stenosis. Overall, when the mean transvalvular gradient exceeds 50 mmHg, severe stenosis is usually present.
Alternative methods of assessing aortic stenosis severity exist (Table 3). The Doppler Velocity index or Dimensionless index is a measure of the ratio of the LVOT velocity to the AV velocity using CW Doppler
Fig. 7. M-mode through aortic valve showing characteristic thickening of valve leaflets in aortic stenosis (compare Chapter 3, Fig. 14A).Table 1
Two-Dimensional Characteristics of Classic Aortic Stenosis According to Disease Etiology
Calcific
degenerative Rheumatic Bicuspid
Eccentric closure line No No Yes
in the parasternal long-axis
Leaflet doming No Yes Yes
in systole
Commissural fusion No Yes No
Associated aortic No No Yes
coarctation
Fish-mouth appearance No No Yes
of valve in systole
across the aortic valve (Fig.12). It is normally higher than 0.28 but decreases with significant aortic stenosis.
An index of less than 0.25 generally correlates with severe aortic stenosis. VTI may be used instead of velocities. In addition, because peak transvalvular gra- dients are linearly correlated with mean gradients, a peak gradient greater than 4.5 m/s is also indicative of severe stenosis.
Therefore, a combination of methods may be used to assess aortic stenosis severity. Nonetheless, symp- toms are the major guide to therapeutic decisions, and no single measurement is used to guide referral for surgical intervention. Thus, echocardiographic meas- urements of severe stenosis in conjunction with the wider clinical picture are the benchmarks for referral for aortic valve replacement.
Fig. 8. (A) Continuous-wave (CW) Doppler of maximum velocity across stenotic aortic valve. (B) Optimal Doppler alignment for assessment of peak velocities in aortic stenosis.
216 Bermudez
LOW-GRADIENT AORTIC STENOSIS In some situations, patients present with severe aor- tic stenosis (calculated AVA <1.0 cm
2) and low trans- valvular gradients (<30 mmHg). These patients have low ejection fractions and low cardiac output (Fig. 6).
In this situation, the calculated AVA may be small sec- ondary to the low flow state, and should be interpreted with caution. These patients may benefit from further testing to differentiate true severe aortic stenosis from apparent severe aortic stenosis owing to a functionally stenotic valve because of the low output state.
Dobutamine echocardiography can help to differenti- ate these two conditions. With the infusion of low-dose dobutamine, stroke volume increases and the calculated AVA increases in those with functionally stenotic valves.
However, in those with true severe aortic stenosis, the calculated AVA remains unchanged. Low-dose dobuta- mine challenge therefore can assess the “contractile reserve”—a predictor of which patients are more likely to benefit from aortic valve replacement.
Even patients with critical stenoses (valve area
<0.75 cm
2), low gradients (<30 mmHg), and low ejection fraction (<35%) may benefit from aortic valve replacement if they exhibit contractile reserve. Those who do not exhibit augmentation in cardiac output are prob- lematic and prognosis is generally poor. The quantitative assessment of severity with low-dose dobutamine
challenge should follow the same guidelines as in other situations (AVA, transvalvular gradients, and dimen- sionless index).
Although planimetry may be used to measure the orifice area by transesophageal echocardiography, it is not commonly implemented in clinical scenarios, as transvalvular gradients and AVA can be easily applied in most situations.
PITFALLS
The accurate assessment of aortic stenosis severity is contingent on factors related to the subject, image acquisition and analysis, and the interpretation of the data (Table 4). Potential pitfalls related to the patient cannot be helped, but the importance of a meticulous transthoracic examination by a skilled operator cannot be overstated. It is essential to acquire the maximum velocity using multiple windows to avoid underestima- tion of the peak velocity measurement and hence over- estimation of AVA (Fig. 8). In addition, because the LVOT radius is squared in the continuity equation, care- ful measurement of this parameter is crucial for accurate calculations.
The modified Bernoulli equation (P = 4V
2) should
not be used when the velocity proximal to the stenosis
(V
LVOT) is greater than 1.5 cm
2. The long version of the
Bernoulli equation should be used to calculate a more
Fig. 9. Calculation of trans-aortic gradients by continuous-wave Doppler.accurate peak transaortic gradient. In this situation, the proximal velocity cannot be ignored, as the modified Bernoulli is based on a negligible value for the proxi- mal velocity.
It is essential to acquire adequate transaortic envelopes via CW Doppler. The aortic envelope must be
distinguished from that of mitral regurgitation and tricus-
pid regurgitation. These envelopes are generally longer in
duration as valve regurgitation usually encompasses iso-
volumetric contraction and relaxation. In addition,
inspection of diastole often shows the presence of an
aortic regurgitation signal that is not seen with mitral
Fig. 10. Distinguishing aortic stenosis from mitral regurgitation on Doppler examination. Both jets both occur in systole and appear as downward spectral velocity shifts when recorded from the apex. The onset of ventricular systole and mitral regurgitation occurs prior to aortic valve opening. In addition, mitral regurgitation is longer in duration.218 Bermudez
regurgitation. Mitral regurgitant velocities generally exceed 4–5 m/s, and usually exceed the transaortic velocity when found in the same patient.
A subaortic membrane should be considered when high velocities are observed on CW Doppler in the setting of normal aortic valve leaflet excursion on 2D examination. Local flow acceleration during color flow Doppler examination can often be seen in the LVOT even when no membrane is seen on transtho- racic imaging (Fig. 13; please see companion DVD for corresponding video). In this situation, trans- esophageal echocardiography may confirm the pres- ence of a subaortic membrane. Early systolic closure of the aortic valve leaflets on M-mode imaging may be seen in this condition.
A complete assessment of aortic stenosis incorpo- rates other aspects of the transthoracic echocardiogram, as well as other factors that predict progression in indi- vidual patients. Aortic insufficiency frequently accom- panies aortic stenosis and should be taken into account when considering surgery. The degree of left ventricu- lar hypertrophy and LV function reflects the hemody- namic impact of the stenosis. Pulmonary hypertension in the presence of severe aortic stenosis may be an omi- nous sign. The presence of a bicuspid aortic valve should always prompt a search for coarctation of the aorta, because there is an established association between the two conditions.
CONCLUSION
Transthoracic echocardiography is the standard for evaluation of aortic stensosis severity. If a careful exam- ination is performed, cardiac catheterization is rarely needed for confirmation. Many institutions only employ cardiac catheterization merely to assess coronary arteries’
status prior to aortic valve replacement. Transthoracic echocardiography remains a comprehensive tool in the initial diagnosis, follow up, and management of patients with aortic valvular disease.
Table 2
Severity of Aortic Stenosis by Valve Area
Severity Calculated valve area
Mild >1.5 cm2
Moderate 1–1.5 cm2
Severe <1 cm2
Fig. 11. The continuity equation and assumptions for measuring aortic valve area (AVA) in aortic stenosis.
Fig. 12. The Doppler velocity index or dimensionless index by CW Doppler across aortic valve. The continuous-wave Doppler enve- lope shows a peak velocity of 3.8 m/s across aortic valve. The velocity proximal to the valve (depicted by the bright lower veloci- ties highlighted by the arrow) reached 87 cm/s. The ratio of the latter to the former is less than 0.25. This Doppler velocity index, also known as dimensionless index, is normally more than 0.25, but decreases with significant valve stenosis.
Maximum aortic Maximum velocity A5C, A3C, right CW Doppler
jet velocity PLAX, SSN
Planimetry 2D planimetry Midesophageal views TEE Planimetry by TEE,
when transthoracic study suboptimal A3C, apical three-chamber view; A5C, apical five-chamber view; AF, atrial fibrillation; CW, continuous wave; 2D, two-dimensional;
LVOT, left ventricular outflow tract; PLAX, parasternal long axis; SSN-suprasternal notch; TEE, transesophageal echocardiography; VTI, velocity time integral; PVC, premature ventricular contraction.
220 Bermudez
Fig. 13. Subvalvular aortic stenosis (arrows) with fixed membrane seen in apical long-axis view (A). (Please see companion DVD for corresponding video.)
Table 4
Pitfalls in the Echocardiographic Assessment of Aortic Stenosis Pitfalls related to subject Body habitus and anatomy
General status: postoperative, acute illness, chest disorders—chronic obstructive pulmonary disease, and so on.
Physiology: rate, rhythm Pitfalls related to image Operator skill and experience
acquisition Mistaking MR or TR for AS
Pitfalls re: continuity equation acquisition
Underestimation of AVA if highest VTI or velocity not recorded Difficulty measuring LVOT diameter, e.g., heavy calcification Subaortic obstruction leading to difficulty measuring LVOT or VTI Inaccurate PW sampling—leading to over or under-estimation
If patient not in sinus rhythm: 8–10 quality beats needed to accurately assess parameters, otherwise use CW, and measure Doppler velocity (dimensionless) index
Pitfalls related to analysis Inter- and intra-observer error in assessment and interpretation Learning curve
Pitfalls related to method Flow dependence: affects most methods, especially the continuity equation of assessment Low-dose dobutamine challenge may be needed to assess contractile reserve
AF, atrial fibrillation; AS, aortic stenosis; AVA, aortic valve area; CW, continuous wave Doppler; LVOT, left ventricular outflow tract;
MR, mitral regurgitation; PVC, premature ventricular contraction; PW, pulsed wave; TR, tricuspid regurgitation; VTI, velocity time integral.
severe valvular aortic stenosis in patients with depressed left Philadelphia: Lea and Fibiger, 1994.
