We appreciate the comments of Drs. Hahn and Pibarot regarding left ventricular outflow tract (LVOT) measurement in our focused update of the recommendations on the echocardiographic assessment of severe aortic stenosis. They advocate that LVOT measurement should be performed at the aortic annulus and state that LVOT measurement performed 0.5 to 1.0 cm within the aortic annulus should be discouraged.
Ideally, LVOT diameter measurement should be performed at the same location as LVOT velocity recording. When a laminar LVOT velocity profile can be obtained at the aortic annulus, measurement of the LVOT diameter at the level of the aortic annulus is preferred. However, a laminar LVOT velocity Doppler profile is not always present at the aortic annulus, as often flow acceleration and turbulence have already developed at the level of the aortic annulus and even more proximally, especially in patients with calcific aortic stenosis. The LVOT diameter can be accurately measured within 0.5 to 1 cm of the aortic annulus, as this location best reflects the same anatomic location of the laminar LVOT velocity profile, especially in LVOT geometries with a uniform shape. As stated in our recommendations document, there are a number of limitations with this LVOT diameter measurement method, including nonuniform LVOT shapes such as occur with discrete upper septal hypertrophy or the presence of LVOT calcification (p. 381).
Hahn and Pibarot highlight the elliptical shape of the LVOT as a source of underestimation of aortic valve area calculation. Our recommendations document also acknowledges this. However, measurement of LVOT area incorporating the elliptical shape is difficult in practice and has not been widely performed in clinical studies. In addition, a circular geometric assumption for LVOT area has been used extensively in a number of studies and shown to correlate strongly with clinical outcomes.
There was considerable discussion regarding the best method to measure LVOT diameter, and there was no definitive consensus based on the writing committee members’ expert opinion and practice experience as well as a review of the literature. We in fact address and acknowledge this specifically in the recommendations document, stating “However, there is no general consensus and many laboratories measure the diameter routinely at the annulus level whereas others measure more apically in the LVOT, depending on the flow pattern in each patient” (p. 381).
We thank Hahn and Pibarot for the points raised regarding the most accurate method to measure LVOT diameter, but ultimately, what is most important is to maintain consistency in the method of LVOT measurement and to be aware of the advantages and disadvantages of both methods for LVOT measurement. Because of the complexity of aortic stenosis, none of the proposed methods can be expected to be 100% precise or accurate. As strongly recommended in our document, the echocardiographic assessment of severe aortic stenosis should be an integrated approach in clinical practice, incorporating multiple criteria and methods and importantly considering these in context of the patient.