Demonstration of left ventricular outflow tract eccentricity by 64-slice multi-detector CT

Background: Newer three-dimensional imaging technologies provide insight into cardiac shape and geometry from views previously unobtainable. Standard formulae like the continuity equation (CE) that rely on inherent assumptions about left ventricular outflow tract (LVOT) shape may need to be revisited. In the CE, small changes in LVOT diameter may significantly change calculated aortic valve area (AVA). Using 64-slice Multi-detector CT (MDCT), we performed LVOT planimetry to obviate the need for any geometric assumptions. Methods: 64-slice MDCT was performed in 30 consecutive patients. The diameter-derived LVOT area (ALVOT_diam) was calculated from a view analogous to the 2D echo parasternal long axis. Direct planimetry of the LVOT (ALVOT_plan) was performed just beneath the aortic valve in a plane perpendicular to the LVOT long axis. Further, assuming an ellipsoid outflow tract shape, LVOT area (ALVOT_ellip) was calculated using πab from the long and short diameters of the planimetered LVOT view. Eccentricity index (EI) was estimated by subtracting the ratio of shortest and longest LVOT diameters from one. Results: ALVOT_diam always measured smaller than ALVOT_plan (mean 3.7 ± 1.2 cm² vs. 4.1 ± 1.3 cm², respectively). The median EI was 0.18 (95% CI = 0.16-0.2; P = 0.0001). ALVOT_ellip more closely agreed with ALVOT_plan (correlation = 0.96; P < 0.0001) than did ALVOT_diam (correlation = 0.87; P < 0.0001). Conclusion: Using MDCT, the LVOT was shown to be elliptical in most patients. Applying the CE which assumes roundness of the LVOT consistently underestimated the LVOT area which may affect estimated AVA. Planimetry of the LVOT utilizing three-dimensional imaging modalities such as 3-D echocardiography, MRI, or MDCT may render a more precise AVA.