Fig. 6.1
Parasternal long (a) and parasternal short-axis (a) views of the left ventricle (LV) demonstrating increased wall thickness and normal chamber size. The aortic valve is calcified. The left atrium (LA) appears mildly dilated
Fig. 6.2
Parasternal long axis view of the left ventricle (LV), left ventricular outflow tract (LVOT) and aortic valve (AV). The aortic leaflets are thickened, appear moderately calcified and demonstrate decreased excursion during this mid-systolic frame
Fig. 6.3
Doppler tracings from the patient. The pulsed wave recording (a) shows normal velocity profile with predominantly laminar flow and a peak velocity of 0.9 m/s. The aortic continuous wave recording (b) shows a peak gradient of 80 mmHg and a mean gradient of 50 mmHg. The calculate aortic valve area using Doppler measurements is 0.65 cm2. These findings are consistent with severe aortic stenosis
The patient was referred to a cardiothoracic surgeon for consideration of aortic valve replacement.
Patient Summary
This patient has a clinical presentation and echocardiographic findings of severe aortic stenosis. Each individual will behave differently in terms of his or her clinical response to the gradually progressing aortic valve obstruction [1, 2]. Despite the spectrum of disease entities and although the pathology and accompanying comorbidities may differ in terms of the response to valvular obstruction, there remain common denominators which allow for the documentation of the presence and severity of aortic stenosis.
Pathophysiology of Aortic Stenosis and Echocardiography
As the aortic valve becomes fibrotic and/or calcified, in the early stages there may be no significant change in terms of terms of hemodynamic consequences to the patient and the disease may be entirely asymptomatic [1, 3]. On occasion, the only indication that there is aortic valve pathology may be a prominent murmur detected on physical examination. As the valvular obstruction progresses, however, the afterload on the left ventricle gradually increases and the ventricle begins to adapt with concentric hypertrophy as a response to this increased resistance to outflow. During the early phases of this compensation, the systolic and diastolic function may remain preserved. However, diastolic function gradually becomes abnormal due to the added muscle mass and impaired relaxation of the hypertrophied ventricle and eventually left atrial enlargement will also ensue [4–7]. Once compensatory mechanisms have been overrun, left ventricular systolic function will become impaired. Chronic left ventricular diastolic and systolic dysfunction eventually may lead to in varying degrees of mitral insufficiency and pulmonary hypertension [8–11].
The above-described sequence of events allows us to define a collection of findings that will guide in the assessment of aortic stenosis by two-dimensional, Doppler as well as three-dimensional echocardiography.
Echocardiography and the Normal Aortic Valve
A normal aortic valve is a trileaflet structure with three thin, pliable leaflets, each having similar dimension (Fig. 6.4a, b). As valvular pathology and fibrosis progresses, the leaflets become thicker, with restricted motion and eventually demonstrate significant calcification as in (Fig. 6.5a, b).
Fig. 6.4
This echocardiographic image of a normal aortic valve is taken from the parasternal long axis during diastole (a) and systole (b). Note the thin, symmetric leaflets with a central coaptation point during diastole. As the valve opens during systole, the pliable leaflet open to their maximum excursion and are flattened against the sinuses to allow minimal obstruction to the ejection of left ventricular outflow
Fig. 6.5
Parasternal long axis (a) and short axis (b) taken during ventricular systole. As opposed to normal, thin pliable leaflets, the aortic valve depicted here is thickened with increased echodensity of the leaftlets, consistent with fibrosis and/or calcium infiltration. The excursion of the leaflets is significant restricted
In the normal pliable aortic the valve leaflets open with the onset of ventricular asystole once the left ventricular systolic pressure exceeds the central aortic pressure with only a minimal differential necessary to achieve valve opening. The valve remains open throughout left ventricular ejection until diastolic relaxation has allowed left ventricular pressure to decrease below that of the central aorta. Therefore, at any given time there will be a negligible pressure gradient between the left ventricle and the central aorta during systole.
Echocardiography and the Stenotic Aortic Valve
As opposed to the normal valve (with no significant gradient between the left ventricle and the aorta during systole) (Fig. 6.6a), a stenotic aortic valve will, by definition, create a pressure differential or gradient between the left ventricle and aorta in order to achieve valve opening (Fig. 6.6b). The gradient between the left ventricle and aorta during systole will depend predominantly on the degree of the aortic valvular obstruction to flow. Other factors, however, including left ventricular systolic function, aortic valve orifice geometry, (including whether the stenotic orifice is more centrally located or eccentric), and the geometry of the ascending aorta itself will also affect the magnitude and timing of occurrence of the peak gradient.
Fig. 6.6
Graphic depiction of simultaneous pressure recordings as would be recorded in the catheterization laboratory from the left ventricle and aorta in the normal state (a) and in the case of aortic stenosis (b). Note the different values that would be recorded for the peak-to-peak gradient typically measured during catheterization (labeled “Peak-to-Peak Gradient”) and the peak instantaneous gradient (“Peak (Doppler) gradient”)
As shown in Fig. 6.6 the peak pressure differential between the left ventricle and aorta does not necessarily occur in conjunction with the peak left ventricular systolic pressure and there may be a poor correlation with the peak gradient measured with Doppler interrogation and the peak-to-peak gradient between the left ventricle and central aorta as measured during cardiac catheterization. When comparing noninvasive echocardiographic measures of aortic stenosis with invasive measurements during catheterization, this potential discrepancy must be kept in mind.
It is well known that the presence of significant valvular obstruction due to aortic stenosis when combined with symptoms will predict long-term prognosis [12]. It has also been shown by Vahanian and Otto et al. that survival is significantly lower even in those asymptomatic individuals with high gradients (Fig. 6.7) [13]. It is important to remember nonetheless that symptoms such as dyspnea, chest pain or syncope from aortic stenosis may manifestations of other diseases and therefore it is important to distinguish whether individual symptoms are truly related to aortic stenosis.
Fig. 6.7
Kaplan-Meier event free survival in asymptomatic individuals according to peak velocity of aortic stenotic jet (p < 0.0001). Note the markedly decreased survival when aortic stenotic velocity progressed from a peak of less than 3.0 m/s (dashed line) to a velocity between 3.0 and 4.0 m/s (gray line) and the higher mortality with velocity of greater than 4.0 m/s (Black line) (Adapted from Otto et al. [31], with permission)
Echocardiography has risen to the forefront and has become the “gold standard” in most institutions to screen for and assess the severity of aortic valve stenosis. Unfortunately, there are pitfalls that may be encountered along the way during these measurements. The accepted techniques and standards used in the determination of the presence of aortic stenosis as well as the potential pitfalls in this assessment will be the focus of the rest of this chapter.
Two-Dimensional Echocardiographic Evaluation of Aortic Stenosis
Aortic Valve
Two-dimensional assessment of the aortic valve should include an assessment of the valve itself as well as surrounding structures. The number and symmetry of the leaflets, the thickness as well as the mobility of each individual leaflet, the presence or absence of fused commissures and the location of any calcium deposition should be described. This will help point to an understanding of the underlying valvular pathology (i.e. bicuspid versus trileaflet valve, rheumatic vs. degenerative, etc.). The distribution of calcium and/or fibrosis generally will be asymmetric and irregular in terms of distribution within the valve and the perivalvular tissues (annulus, sinuses, sinotubular junction and mitral annulus). The degree of calcification and the location has clinical relevance when considering valve replacement and especially percutaneous valve replacement [14].
In addition to the qualitative measures noted above, the aortic valve cross-sectional area can be measured in the parasternal short axis (Fig. 6.8a, b) [15]. As is the case with measurements by planimetry used in assessing the mitral valve, often extensive fibrosis and calcification that is present in aortic stenosis will make planimetry technically challenging, however [16]. For this reason, if the visual qualitative estimation of aortic valve stenosis severity does not correlate with the Doppler measurements, the measured valve area by planimetry alone may not mitigate this discrepancy. The evaluation of concomitant aortic regurgitation is also essential. Recent data suggest that the peak trans aortic velocity is the primary prognostic determinant in patients with combined valve disease.
Fig. 6.8
Parasternal short axis of a severely stenotic aortic valve with mid-systolic frame without (a), and with planimetry of the estimated orifice area (b). Note that there are not always clearly defined borders to trace due to acoustic shadowing from calcification (arrows)
Left Ventricular Outflow Tract
The accurate two-dimensional measurement of left ventricular outflow tract (LVOT) is of paramount importance during the assessment of aortic stenosis, as this is the greatest potential source of error in the calculation of aortic valve area by the continuity equation (see below). This LVOT measurement itself is squared within the continuity equation and therefore small errors in LVOT measurement are magnified in the calculation. According to EAE/ASE guidelines [17], the left ventricular outflow tract should be imaged from the parasternal long axis view in a zoomed projection. A diameter measurement of the LVOT should be taken from inner edge to inner edge from the most basal aspect of the interventricular septum endocardium to the base of the anterior mitral valve leaflet during the mid-portion of systole (Fig. 6.9). The diameter of the aortic annulus, the adjoining aortic sinuses, proximal aorta and sinotubular junction should also be measured. The presence and extent of calcification in these areas may be important as well if future interventions, either surgical or percutaneous are undertaken.
Fig. 6.9
Zoomed parasternal long axis view of the left ventricular outflow tract (LVOT) demonstrating method of measurement of the LVOT diameter, which in turn is used to calculate LVOT area used in the continuity equation
Left Ventricle
As previously discussed, the increased afterload on the left ventricle during systole due to the aortic valve obstruction will cause the left equal to compensate with resulting hypertrophy of the myocardium. In general, however, the pattern and degree of left ventricular hypertrophy does not correlate well with the severity of aortic stenosis [18]. Whether surgery or percutaneous intervention is anticipated, a significant degree of left ventricular hypertrophy, especially involving the proximal interventricular septum may have implications for treatment. The wall thickness and pattern of hypertrophy should be characterized during the echocardiographic examination for aortic stenosis. Left ventricular systolic and diastolic function should be assessed in the standard manner as these parameters of left ventricular function are often abnormal with hemodynamically significant aortic stenosis, even if the patient is asymptomatic [5].
Left Atrium
As left ventricular diastolic pressures increase, there will be inevitably be a concomitant increase left atrial pressure and eventually in left atrial size. Left atrial enlargement when present is an independent indicator of prognosis in the aortic stenosis [7].
Mitral Insufficiency
Individuals with significant degenerative aortic valve stenosis may also show similar degenerative changes involving the mitral valve including mitral annular calcification and mitral leaflet thickening. Due to changes in left ventricular geometry and hemodynamics, as well as left atrial pressures, it is not uncommon for varying degrees of mitral insufficiency to be present [8, 9]. However, unless the underlying aortic valve pathology is rheumatic in origin, a primary mitral valvulopathy involving the leaflets themselves, which can be visualized on two-dimensional imaging is not often present. Patients with a more than moderate degree of mitral regurgitation and/or a structural problem with the mitral valve are unlikely to note improvement in mitral valve function following aortic valve replacement and as such require a double valve surgery.
Doppler Assessment of Aortic Stenosis
Hatle et al. [18] initially described the means of identifying and quantifying the severity of aortic stenosis by Doppler echocardiography. Since then, the use of Doppler echocardiography has been widely validated as an accurate modality to assess for the presence and severity of this entity. In the vast majority of individuals, the standard Doppler study aimed at recording the parameters of the aortic systolic velocity spectrum will determine the presence or absence of significant valvular obstruction and yield an accurate assessment of the stenosis severity. As with any other technique, Doppler echocardiography may yield conflicting data in certain situations when normal physiology is disturbed. The remainder of this chapter will focus on that situation which is most commonly encountered, i.e. that of normal flow, high gradient aortic stenosis.
As noted above, by definition with aortic stenosis, a gradient exists during systole between the left ventricle and the aorta. The modified Bernoulli (Eq. 6.1) yields an estimation of the pressure differential between two chambers separate by a stenotic valve and can be calculated using the velocity of blood flow (in this case across the aortic valve). It is important to understand the strengths and limitations of the Bernoulli equation in assessing aortic valve gradients in order to use this measurement properly.
Bernoulli’s theorem and resulting equation was initially derived to quantitate a pressure differential across a graduated narrowing in a smooth, rigid tube and is thus a model that is very different from that which represents the left ventricular outflow tract, aortic valve and proximal aorta. Even in the best of circumstances, this type of model could be expected to only partially translate to that of the intact human heart.
(6.1)
The modified Bernoulli equation itself and a more simplified version (Eq. 6.2), also involve assumptions that may or may not be proper in an individual patient. The modified Bernoulli equation itself arbitrarily eliminates the factors involved with viscosity and potential energy that are usually (although not always) relatively small as compared to the velocity factor in the equation. As may be seen with instances of significant pressure recovery, (covered in a subsequent chapter) elimination of factors that may impact on the calculation of gradients and create a significant error in the estimation of the true pressure gradient when using velocities across the valve.
In addition to the above considerations, changes in flow that are present as a result of normal variations in physiology in the individual heart will yield differences in the calculation of valve gradients using the Bernoulli equation. Increases in flow across the aortic valve as seen in high output states including anemia, hyperthyroidism and in other entities that increase flow such as significant aortic regurgitation may falsely overestimate the severity of aortic stenosis. Likewise, significant pathology that decreases flow rate such as intravascular volume depletion and mitral insufficiency will have the opposite effect. This is the case where there is left ventricular dysfunction from any cause resulting in decreased forward stroke-volume and therefore decreased flow across the aortic valve. In these situations one must resort to the use of other modalities such as dobutamine echocardiography to help in the assessment of the aortic valve gradient for this assessment. Dobutamine stress echocardiography is discussed subsequently in this text in the assessment of low flow, low gradient aortic stenosis.
(6.2)
Despite the above limitations and potential errors, the assessment of aortic stenosis severity by Doppler echocardiography remains the mainstay of noninvasive assessment in this disease entity [19, 20]. Subsequent discussion will discuss standard techniques for Doppler echocardiography in aortic stenosis.
Proper Doppler Flow Measurement
As described in previous chapters, generation of a tans-aortic valve gradient depends on the law of conservation of energy. The primary modality used in the assessment for the presence of, and quantification of aortic stenosis severity remains the continuous wave Doppler recording of the peak and mean gradients ΔPpeak and ΔPmean across the aortic valve during systole. Multiple windows are used to record the highest velocities from aortic outflow during systole. The velocities recorded are then used to calculate the peak instantaneous gradient (using the peak velocity) and the mean instantaneous gradient (using the mean velocity) in the simplified Bernoulli equation where v equals the maximum jet velocity (in m/s). The assumptions inherent in this simplified Bernoulli formula are several, including the assumption that the velocity prior to flow through a narrowing is much less than the velocity in the narrowing (stenosis) itself and thus the velocity proximal to the stenosis is negligible. In certain situations of high flow the velocity proximal to the stenosis is not negligible, and one must estimate the pressure drop across the aortic valve using the non-simplified version (Eq. 6.1) where vmax equals the maximal aortic jet velocity and vprox equals the peak velocity of the LVOT jet just proximal to the aortic valve. This equation is appropriate in those conditions with increased stroke volume such as in moderate to severe aortic insufficiency, high cardiac output states due to sepsis, thyrotoxicosis and anemia, or when there is a subvalvular gradient.
Depending on valvular anatomy and anatomy of the thorax, the systolic jet of aortic stenosis may be oriented in any of a number of different three-dimensional orientations. As is well known, Doppler assessment of the aortic valve gradient is dependent upon achieving a Doppler insonation angle as close to the true get orientation is possible. If the angle of insonation increases above 20 or 30° beyond the true jet orientation, the discrepancy between the true velocity and the measured velocity increases dramatically. Although the apical window will yield the maximum jet velocities in aortic stenosis most of the time, all windows including the apical, right parasternal, suprasternal and atypical windows should be imaged in the continuous wave (CW) mode and using a dedicated Doppler (Pedoff) transducer. Vmax is located outside the apical window in > 60 % of patients, and neglecting the nonapical windows results in the misclassification of AS severity in > 20 % of patients. The left ventricular to aortic root angle as measured in the parasternal long window influences the location of Vmax modestly, being far less likely in the apical window (< 20 %) if the angle is acute. The less standard left parasternal or subcostal windows may be necessary in certain individuals. Despite the best intentions and meticulous technique, angulation of the jet may lead to an underestimation of the velocity of the aortic stenotic jet and therefore in a normal rhythm, one should always use the highest measured jet (Fig. 6.10).
