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