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Computed Tomography vs. Echocardiography for Determination of Aortic Valve Area in Aortic Stenosis | Journal Scan
Study Questions:
How accurate is multidetector computed tomography (MDCT) compared to standard echocardiographic and Doppler measurements for determining the area of left ventricular outflow tract (LVOT) and for calculating aortic valve area (AVA)?
Methods:
MDCT and echo-Doppler were performed in 269 patients with isolated aortic stenosis (AS) and normal LV systolic function. AVA was calculated by echocardiography (AVAecho) using the standard continuity equation and a linear measurement of the LVOT. AVA was calculated by MDCT (AVACT) using the Doppler flow profile combined with direct measurement of the LVOT area. In an additional 135 patients, AVA was directly planimetered from MDCT (AVAPlani).
Results:
MDCT demonstrated that the geometry of the LVOT was elliptical rather than spherical in 93% of patients (>10% difference between two orthogonal LVOT diameters) with an eccentricity index of 0.79 ± 0.08 for the total cohort. For the total cohort, LVOT by echo was 3.86 ± 0.71 cm2 and 4.68 ± 1.02 cm2 by direct planimetry with MDCT. For the total cohort, AVAecho was 0.94 ± 0.32 cm2 and 1.13 ± 0.44 cm2 for AVACT. AVAecho correlated with aortic valve Δp (r = -0.65, p < 0.0001) as did AVACT (r= -0.61, p < 0.0001). The correlation between AVA and Δp was weaker in patients with an eccentricity index <0.8. Low gradient (<40 mm Hg) severe AS was seen in 66 patients (25%) and its prevalence was not different depending on the eccentricity of the LVOT. By AVAecho, 74% of patients had clinically concordant values with respect to severity of Δp and AVA, and by AVACT 70% of studies were concordant. Over a 2-year period, there were 55 deaths, and AVAecho <1 cm2 and AVACT ≤1.2 cm2 were both independent predictors of mortality with medical therapy.
Conclusions:
MDCT systematically results in a larger LVOT area compared to echo, but does not improve the correlation with transaortic gradient and provides no advantage with respect to prognosis. If MDCT is used to determine severity of AS, a larger threshold for defining severe AS of <1.2 cm2 should be used versus <1.0 cm2 for AVAecho.
Perspective:
This study was undertaken in an effort to prove or disprove the prevailing hypothesis that the well-described elliptical geometry of the LVOT, which is accurately characterized by MDCT (and potentially three-dimensional echocardiography), results in a systematic reduction in accuracy for echocardiography compared to MDCT for determining the severity of AS. While the study nicely confirmed the well-described elliptical geometry of the LVOT, the study did not confirm a decrease in the accuracy for determining the actual severity of AS or a decrease in the correlation between pressure gradient and calculated AVA for the linear measurement based on echocardiographic measurements. The study did confirm that the directly measured LVOT area by MDCT is systematically larger than the calculated area by echocardiography, which results in a different relationship between defined severe anatomical AS and pressure gradient. The authors nicely demonstrate that a slightly lower threshold for severe AS (≤1.2 cm2) should be used if CT LVOT areas are used compared to the standard definition of ≤1.0 cm2 when echocardiographic methods are used to extrapolate LVOT area. Using these thresholds, there was virtually identical prognostic information available with respect to the outcomes of medically managed severe AS. One would have expected that the more accurate direct measurement of LVOT area would have resulted in a tighter correlation with pressure gradient. The authors suggest a number of explanations for this, including the possibility that while the anatomical LVOT may be elliptical, actual flow toward the aortic valve may adhere more to a circular shape, and as such, the hypothetical calculated circular LVOT by echocardiography does not result in diminishing accuracy.
Keywords: Aortic Valve, Aortic Valve Stenosis, Echocardiography, Echocardiography, Doppler, Echocardiography, Three-Dimensional, Multidetector Computed Tomography, Systole, Ventricular Dysfunction, Left, Heart Valve Diseases, Prognosis
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