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Characterization and Normal Measurements of the Left Ventricular Outflow Tract by ECG-gated Cardiac CT

Posted Sep 30, 2012 Updated Apr 10, 2023
By Ethan J. Halpern MD , Shiva Gupta MD , David J. Halpern BA , David H. Wiener MD , Alyson N. Owen MD
10 min read

Rationale and Objectives

Studies suggest that electrocardiographically gated coronary computed tomographic angiography provides a clear definition of the left ventricular outflow tract (LVOT), and normal LVOT morphology may not be round, as assumed when the continuity equation is applied during echocardiography. The aims of this study were to demonstrate the morphology of the LVOT on coronary computed tomographic angiography and to establish normal values for LVOT measurements.

Materials and Methods

Two independent readers retrospectively measured anterior-posterior (AP) and transverse diameters of the LVOT and performed LVOT planimetry on coronary computed tomographic angiographic studies of 106 consecutive patients with normal aortic valves.

Results

Excellent interobserver agreement was observed for all measurements ( r = 0.78–0.94). The LVOT was ovoid, with a larger transverse diameter than AP diameter during diastole and systole ( P < .001). However, the ratio of AP diameter to transverse diameter was closer to 1.0 during systole ( P < .001). Mean indexed LVOT area was minimally larger in systole than in diastole ( P = .01–.04) and was larger in men than in women during diastole ( P ≤ .001) and systole ( P ≤ .01). Mean LVOT area indexed to body surface area was 2.3 ± 0.5 cm 2 /m 2 in women and 2.6 ± 0.7 cm 2 /m 2 in men. LVOT area demonstrated significant correlation with aortic root diameter.

Conclusions

The normal LVOT is ovoid in shape. LVOT is more circular during systole, but the AP diameter remains smaller than the transverse diameter throughout the cardiac cycle. The oval shape of the LVOT has important implications when LVOT area is calculated from LVOT diameters. Normal LVOT area values established in this study should facilitate diagnosis of the fixed component of LVOT obstruction.

A diagnosis of left ventricular outflow tract (LVOT) obstruction is suggested by an elevated gradient in the LVOT in the setting of a normal aortic valve. LVOT obstruction is classified into two categories, dynamic and fixed, which may coexist. Dynamic LVOT obstruction is classically associated with primary hypertrophic cardiomyopathy, a genetic disorder of sarcomeric proteins . Many of these patients demonstrate asymmetric hypertrophy of the ventricular septum and dynamic left ventricular contraction, which draw the anterior mitral valve leaflet or its chordal apparatus into the LVOT during mid to late systole . Fixed LVOT obstruction is a congenital abnormality in which reduced outflow tract area may be related to fibromuscular narrowing, a discrete fibrous ring, or a membrane. A dynamic component that changes in severity during ventricular ejection can coexist with fixed obstruction, complicating the assessment and treatment of LVOT obstruction .

Two-dimensional transthoracic Doppler echocardiography (TTE) is widely used for the diagnosis of LVOT obstruction by measuring Doppler-derived pressure gradients . Decisions regarding the timing of surgery for fixed LVOT obstruction are based on Doppler-derived gradient measures . However, this method has inherent limitations, such as variation in gradients with changes in cardiac output and the challenge of separating fixed from dynamic components of gradients measured in the LVOT. Accurate depiction of the shape and size of the LVOT is critical in this decision process. Thus, the ability to produce high-resolution static and dynamic images of the LVOT throughout the cardiac cycle with electrocardiographically (ECG) gated computed tomographic (CT) angiography provides a unique opportunity for the evaluation of LVOT obstruction.

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Materials and methods

Study Population

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CT Scan Protocol

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CT Imaging Analysis

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Figure 1, Measurement of the normal left ventricular outflow tract (LVOT) from electrocardiographically gated computed tomographic (CT) angiography. (a) Coronary CT angiographic image in the long axis of the left ventricle and ascending aorta. The plane of the LVOT is defined just below the level of the aortic valve. (b) Coronary CT angiographic image at 90° to the image in (a) . The plane of the LVOT is again defined parallel to and just below the aortic valve. (c) Short-axis coronary CT angiographic image immediately below the aortic valve/annulus defined by the planes in (a) and (b) , showing the LVOT in cross-section. Note the oval shape of the LVOT. The transverse diameter is longer when the left atrium lies posterior to the LVOT. (d) Diameter measurements. Anterior-posterior diameter of the LVOT corresponds to the minor axis, and transverse diameter of the LVOT corresponds to major axis of the LVOT. (e) Planimetry of the LVOT was obtained by a freehand tracing of the LVOT.

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Statistical Analysis

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Results

Study Population

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Interobserver Agreement

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Figure 2, Bland-Altman plots for (a) anterior-posterior (AP) diameter, (b) transverse diameter, and (c) planimetry of the left ventricular outflow tract measured at end-systole. Plots show the mean of the two readers' measurements on the x axis and the difference between the two readers' measurements on the y axis.

Table 1

Correlation Coefficients for Interobserver Agreement

Correlation AP Diameter of LVOT TRV Diameter of LVOT LVOT Area by Planimetry Maximum Aortic Root Diameter End-diastole Pearson 0.88 0.79 0.94 0.91 Intraclass 0.88 0.79 0.93 0.90 End-systole Pearson 0.81 0.84 0.91 — Intraclass 0.78 0.82 0.90

AP, anterior-posterior; LVOT, left ventricular outflow tract; TRV, transverse.

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AP and TRV LVOT Measurements

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Table 2

AP Diameter/TRV Diameter Ratios of LVOT

AP Diameter (mm) TRV Diameter (mm) AP Diameter/TRV Diameter Ratio End-diastole Reader 1 20.77 ± 2.93 28.33 ± 3.57 0.74 ± 0.07 Reader 2 20.89 ± 3.24 28.39 ± 3.77 0.73 ± 0.09 End-systole Reader 1 22.68 ± 3.09 27.02 ± 3.50 0.84 ± 0.08 Reader 2 21.93 ± 3.54 27.73 ± 3.80 0.79 ± 0.09

AP, anterior-posterior; LVOT, left ventricular outflow tract; TRV, transverse.

Data are expressed as mean ± standard deviation.

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LVOT Planimetry Area

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Table 3

Left Ventricular Outflow Tract Planimetry Area (cm 2 )

Overall ( n = 106) Gender Ethnicity Women ( n = 52) Men ( n = 54) African Americans ( n = 27) Caucasians ( n = 74) End-diastole Reader 1 4.81 ± 1.24 4.08 ± 0.81 5.52 ± 1.18 4.43 ± 0.96 4.95 ± 1.33 Reader 2 4.86 ± 1.33 4.11 ± 0.84 5.57 ± 1.34 4.48 ± 1.09 4.99 ± 1.42 End-systole Reader 1 4.88 ± 1.25 4.18 ± 0.86 5.55 ± 1.20 4.52 ± 1.00 5.01 ± 1.32 Reader 2 4.98 ± 1.41 4.28 ± 0.97 5.65 ± 1.45 4.47 ± 1.13 5.17 ± 1.47

Data are expressed as mean ± standard deviation.

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Table 4

Left Ventricular Outflow Tract Planimetry Area Indexed to Body Surface Area (cm 2 /m 2 )

Overall ( n = 106) Gender Ethnicity Women ( n = 52) Men ( n = 54) African Americans ( n = 27) Caucasians ( n = 74) End-diastole Reader 1 2.38 ± 0.50 2.20 ± 0.39 2.55 ± 0.53 2.27 ± 0.40 2.42 ± 0.54 Reader 2 2.40 ± 0.58 2.22 ± 0.46 2.58 ± 0.63 2.29 ± 0.47 2.44 ± 0.63 End-systole Reader 1 2.41 ± 0.52 2.25 ± 0.41 2.57 ± 0.56 2.32 ± 0.44 2.45 ± 0.56 Reader 2 2.47 ± 0.63 2.31 ± 0.50 2.62 ± 0.70 2.29 ± 0.52 2.54 ± 0.67

Data are expressed as mean ± standard deviation.

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Maximum Aortic Root Diameter and LVOT Planimetry Area

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LVOT Planimetry Area Compared to Area Assuming a Circular LVOT (π r 2 )

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Discussion

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Figure 3, A 40-year-old man (body surface area, 2.14 m 2 ) with an unclear history of repaired congenital aortic stenosis. Transesophageal echocardiography demonstrated an elevated velocity of 1.9 m/s through the left ventricular outflow tract (LVOT) at rest despite a normal opening of the aortic valve (2.7 cm 2 by planimetry). (a) Coronary computed tomographic angiographic image in the three-chamber view shows normal systolic opening of the aortic valve, with no evidence of systolic anterior motion of the mitral valve. (b) Short-axis view shows the LVOT in cross-section with anterior-posterior diameter (minor axis) of 1.8 cm and transverse diameter (major axis) of 2.6 cm. (c) Short-axis view with freehand tracing of the LVOT for planimetry demonstrates an LVOT area of 3.59 cm 2 (indexed area, 3.59 cm 22.14 m 2 = 1.68 cm 2m 2 ), which is approximately 1.5 standard deviations below the expected mean male LVOT area (see Table 5 ).

Figure 4, A 61-year-old man (body surface area, 2.28 m 2 ) with normal left ventricular systolic function. A fixed left ventricular outflow tract (LVOT) gradient of 55 mm Hg as well as a second dynamic gradient of 54 mm Hg were measured by transthoracic echocardiography. The aortic valve was mildly thickened, with normal motion. (a) Coronary computed tomographic (CT) angiographic image in the three-chamber view demonstrates coaptation of the mitral valve. The coapted leaflet tips are tilted toward the interventricular septum; cine images (not shown) were interpreted as demonstrating systolic anterior motion of the associated chordal structures. (b) Short-axis coronary CT angiographic image shows the LVOT in cross-section with anterior-posterior diameter (minor axis) of 1.6 cm and transverse diameter (major axis) of 2.1 cm. (c) Short-axis coronary CT angiographic image shows freehand tracing of the LVOT in cross-section with planimetry measure of 2.41 cm 2 (indexed area = 2.41 cm 22.28 m 2 = 1.06 cm 2m 2 ), which is approximately 2.5 standard deviations below the expected mean male LVOT area (see Table 5 ). CT angiography was helpful in demonstrating both the dynamic and fixed components of the LVOT obstruction.

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Table 5

Lookup Table for LVOT Area

Height (cm) Weight (kg) BSA (m 2 ) LVOT Area (cm 2 ) Women Men Mean SD Mean SD 150 40 1.29 2.90 0.57 3.33 0.78 160 40 1.33 2.99 0.59 3.44 0.81 170 40 1.37 3.09 0.60 3.55 0.83 180 40 1.41 3.17 0.62 3.65 0.86 150 60 1.58 3.55 0.70 4.08 0.96 160 60 1.63 3.67 0.72 4.21 0.99 170 60 1.68 3.78 0.74 4.34 1.02 180 60 1.73 3.89 0.76 4.47 1.05 150 80 1.83 4.10 0.80 4.71 1.10 160 80 1.89 4.23 0.83 4.86 1.14 170 80 1.94 4.36 0.86 5.01 1.18 180 80 2.00 4.49 0.88 5.16 1.21 150 100 2.04 4.58 0.90 5.27 1.23 160 100 2.11 4.73 0.93 5.44 1.28 170 100 2.17 4.88 0.96 5.61 1.31 180 100 2.24 5.02 0.98 5.77 1.35 150 120 2.24 5.02 0.98 5.77 1.35 160 120 2.31 5.18 1.02 5.96 1.40 170 120 2.38 5.34 1.05 6.14 1.44 180 120 2.45 5.50 1.08 6.32 1.48

BSA, body surface area; LVOT, left ventricular outflow tract; SD, standard deviation.

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Conclusions

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Academic Radiology, Volume 19, Issue 10
Journals General Radiology
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