Aortic Stenosis
Standard 2D and Doppler transthoracic echocardiography remains the
primary diagnostic tool for the diagnosis of aortic stenosis using
pressure gradients and the continuity equation method. It is important
to understand that both methods are flow dependent and indirect estimate
of the aortic valve area. There are several limitations to the use of
continuity equation to calculate the aortic valve area because it
assumes that the LVOT is circular in geometry, which is not true in many
cases. In fact, the coronal measurement of the LVOT is the smaller of
the LVOT diameters compared to the sagittal. A narrow LVOT also
overestimates the severity of aortic stenosis when using the continuity
equation. In addition, obtaining the peak Doppler velocity across the
aortic valve can be challenging, which may lead to underestimation of
the aortic stenosis, whereas overestimation can occur in cases of
significant pressure recovery phenomenon. The use of 3D echocardiography
either from a transthoracic or transesophageal approach can overcome
these limitations and has several advantages over 2D imaging. For
instance, the ability of 3D echocardiography to visualize the aortic
valve and annulus from different angles allows for accurate planimetry
of both the aortic valve and LVOT. Using multiplanar reconstruction
mode, 3D datasets of the aortic valve allows cropping to the tips of the
aortic valve leaflets for precise planimetry and calculate the aortic
valve area (Movie 2).11 It also can assess
individualized leaflets motion and localize the site of obstruction in
patients with subvalvular or supravalvular obstruction.
In addition, 3D echocardiography can be very valuable in the assessment
of aortic valve stenosis in certain clinical scenarios when Doppler
measurements are inaccurate due to overestimated or underestimated
Doppler velocities across the aortic valve. For example, in patients
with hypertrophic obstructive cardiomyopathy and high LVOT gradient and
in patients with suspected severe low-flow low-gradient aortic stenosis
(stage D disease) undergoing low dose Dobutamine challenge, 3D TEE can
supplement 2D echocardiography by accurate planimetry of the aortic
valve area and precise sizing of the LVOT regardless of the Doppler
velocities. Accurate sizing of the LVOT is not only important for
quantifying the severity of aortic stenosis, but also for proper annulus
sizing for patients undergoing TAVI. However, it is important to
understand that 3D aortic valve planimetry can be limited by three main
factors, very low stroke volume which may fail to open the aortic valve
fully leading to overestimation of the degree of stenosis, lower
temporal resolution that may miss the maximal systolic opening of the
aortic leaflets, and reverberations artifact of heavily calcified aortic
valve leaflets making accurate planimetry of the orifice area difficult
to trace. 3DTTE is also useful to assess patients with aortic stenosis
who also have another lesion in tandem such as subvalvularor
supravalvular stenosis by separate planimetry of the two lesions and
determining stenosis at each level. In fact, subvalvular membranes are
particularly well imaged by 3D echocardiography. Standard continuous
wave Doppler using 2D echocardiography will give the maximum velocity
across both lesions with no differentiation of severity at individual
levels (Figure 3A-C).13