Aortic Stenosis - Diagnosis
The diagnosis of aortic stenosis is made mostly on physical examination and by echocardiography.
The ECG in patients with aortic stenosis frequently shows left ventricular hypertrophy with strain and left atrial enlargement; however, these findings are non-specific for aortic stenosis. The chest radiography may reveal a normal cardiac size since the hypertrophy in aortic stenosis is concentric. However, once LV systolic failure occurs, cardiomegaly will be seen. Calcification of the aortic valve, pulmonary congestion and post-stenotic dilation of the aorta are other non-specific findings.
The severity of aortic stenosis is determined by measuring the aortic valve area (AVA) and calculating the pressure gradient between the LV and the aorta on echocardiography. Aortic stenosis is described as mild, moderate or severe based on these measurements.
|Mean gradient (mmHg)||Aortic Valve Area (cm2)|
The echocardiogram can both confirm the diagnosis of aortic stenosis and quantify the severity. Two-dimensional echocardiography can demonstrate a thickened aortic valve, reduced leaflet mobility and concentric left ventricular hypertrophy. Doppler is used to quantify the severity of aortic stenosis by measuring the pressure gradient across the aortic valve and by calculating the aortic valve area (AVA). The velocity of blood flow across the aortic valve, as determined by continuous-wave Doppler, is used to calculate the transaortic pressure gradient using the modified Bernoulli equation:
Pressure gradient = 4v2
v = velocity
The AVA is calculated using the continuity equation:
A1 X V1 = A2 X V2
A2 = (A1 X V1) / V2
Where A1 is the area of the left ventricular outflow tract, V1 is the velocity of flow at the left ventricular outflow tract, A2 is the area of the aortic valve and V2 is the velocity of flow at the aortic valve. All of the above, except A2, can be seen directly by 2D echocardiography (LV outflow tract area) or measured using continuous wave (CW) Doppler. The AVA is then calculated as shown above.
Cardiac catheterization is indicated when the angina of aortic stenosis may be due to coexistent coronary disease or when aortic valve replacement is indicated. Rarely, catheterization with crossing of the aortic valve for hemodynamic measurements may be needed if echocardiography is unable to determine if severe aortic stenosis is present. During cardiac catheterization, the cardiac output and pressure gradient are directly measured and used to calculate the AVA using the Gorlin equation below:
The pressure gradient is found simply by using the catheter to measure the pressure in the aorta, then advancing it into the LV and taking another pressure reading. The difference between these two pressures is the pressure gradient. The mean transaortic valve pressure gradient is used in the Gorlin equation to calculate the AVA, not the peak gradient. The cardiac output is calculated using either the Fick principle or the indicator-dilution principle. It is important to note that the Gorlin formula was originally derived using patients with mitral stenosis and not aortic stenosis. The Gorlin equation is also flow dependent, so if the patient has a significantly decreased ejection fraction, the AVA may be underestimated.