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Background: Evaluation of aortic stenosis (AS) requires calculation of aortic valve area (AVA), which relies on the assumption of a circular-shaped left ventricular outflow tract (LVOT). However, the LVOT is often elliptical, and the circular assumption underestimates the true LVOT area (LVOTA). Biplane imaging using transthoracic echocardiography allows direct planimetry of LVOTA. The aim of this study was to assess the feasibility of obtaining LVOTA using this technique and its impact on the discordance between AVA and gradient criteria in AS grading., Methods: We prospectively studied 134 patients (median age, 80 years; interquartile range, 73-87 years; 39% women) with AS, including 82 (61%) with severe AS and 52 (39%) with mild or moderate AS. LVOTA was traced using direct planimetry (LVOTA biplane ) and compared with LVOTA calculated using the circular assumption (LVOTA circ ). In a subset of patients who underwent cardiac computed tomography, direct planimetry of LVOTA was used as a reference standard., Results: LVOTA biplane was significantly larger than LVOTA circ (4.20 cm 2 [interquartile range, 3.66-4.90 cm 2 ] vs 3.73 cm 2 [interquartile range, 3.14-4.15 cm 2 ], P < .001). Among 30 patients who underwent cardiac computed tomography, LVOTA biplane had better agreement with LVOTA by direct planimetry than LVOTA circ (mean bias, -0.45 ± 0.63 vs -1.02 ± 0.63 cm 2 ; P < .0001). Of 82 patients with severe AS (AVA ≤ 1 cm 2 using LVOTA circ ), 40 (49%) had discordant mean gradient (<40 mm Hg). By using LVOTA biplane , patients with discordant AVA and mean gradient decreased from 49% to 27% (P = .004), and 29% of patients with severe AS were reclassified with moderate AS, with the highest percentage of reclassification in the group with low-gradient AS with preserved left ventricular ejection fraction., Conclusions: Direct planimetry using biplane imaging avoids the inherent underestimation of LVOTA using the circular assumption. LVOTA obtained by biplane planimetry can lead to better concordance between AVA and mean gradient and classification of AS severity., (Copyright © 2020 American Society of Echocardiography. Published by Elsevier Inc. All rights reserved.)