Background
Measurement of left ventricular outflow tract (LVOT) area for estimation of aortic valve area (AVA) using two-dimensional (2D) transthoracic echocardiography (TTE) and the continuity equation assumes a round LVOT. The aim of this study was to compare measurements of LVOT area and AVA using 2D and three-dimensional (3D) TTE and cardiac computed tomographic angiography (CCTA) in an attempt to improve the accuracy of AVA estimation using TTE.
Methods
Fifty patients were prospectively studied, 25 with aortic stenosis and 25 without aortic stenosis (group 1). LVOT area and AVA were estimated using 2D TTE, and LVOT area and diameters were measured using 256-slice CCTA and 3D TTE. AVA was also planimetered using CCTA in midsystole. LVOT area and AVA estimated by 2D TTE were correlated with measurements by 3D TTE and CCTA. Findings from group 1 were then validated in 38 additional patients with aortic stenosis (group 2).
Results
LVOTs were oval in 96% of the patients in group 1, with a mean eccentricity index (diameter 2/diameter 1) of 1.26 ± 0.09 by CCTA. Compared with CCTA, 2D TTE systematically underestimated LVOT area (and therefore AVA) by 17 ± 16%. The correlation between CCTA and 3D TTE LVOT area was only moderate ( r = 0.63), because of inadequate 3D transthoracic echocardiographic image quality. Mean AVA was 0.92 ± 0.44 cm 2 by 2D TTE and 1.14 ± 0.68 cm 2 by CCTA ( P = .0015). After correcting AVA on 2D TTE by a factor of 1.17 (accounting for LVOT area ovality), there was no difference between 2D TTE and CCTA (0.06 ± 26 cm 2 , P = .20, r = 0.86). In group 2, 2D TTE underestimated LVOT area and AVA by 16 ± 11%, similar to group 1, and AVA by TTE was 0.75 ± 0.14 cm 2 compared with 0.88 ± 0.21 cm 2 by CCTA ( P < .0001). When the correction factor was applied to the group 2 results, the corrected AVA by 2D TTE (×1.17) was 0.87 ± 0.17 cm 2 , similar to AVA by CCTA ( P = .70).
Conclusions
Three-dimensional imaging revealed oval LVOTs in most patients, resulting in underestimation of LVOT area and AVA on 2D TTE by 17%. This accounted for the difference in AVA between 2D TTE and CCTA. Current 3D TTE is inadequate to accurately measure LVOT area.
Accurate measurement of aortic valve area (AVA) using echocardiography is critical for selecting patients with aortic stenosis (AS) for aortic valve surgery and transcatheter aortic valve implantation. The measurement of AVA using two-dimensional (2D) transthoracic echocardiography (TTE) is based on the continuity equation, which has been validated in numerous clinical and in vitro studies :
AVA = LVOT area × LVOT TVI / AV TVI ,
According to current guidelines, LVOT area is calculated on the basis of LVOT diameter measured from the parasternal long-axis view, assuming a circular shape of the LVOT (LVOT area = π[LVOT diameter/2] 2 ). Preliminary studies using magnetic resonance imaging, three-dimensional (3D) echocardiography, and cardiac computed tomographic angiography (CCTA) have shown that in many cases the LVOT shape is in fact oval, which may lead to underestimation of LVOT area and hence AVA by 2D echocardiography. There are limited data, however, regarding the effect of LVOT area shape on the accuracy of LVOT area measurements and consequently on the estimation of AVA and the ways to improve its accuracy. CCTA may be especially useful to clearly delineate the LVOT in the appropriate 3D plane and to directly measure AVA by planimetry.
The aim of this study was to compare measurements of LVOT area and AVA using 2D and 3D TTE and 256-slice CCTA in an attempt to improve the accuracy of LVOT area measurement, which is included in the continuity equation when using TTE for the estimation of AVA.
Methods
Patient Population
We prospectively enrolled, from February 2009 to January 2011, 50 consecutive patients aged ≥ 18 years referred for clinically indicated CCTA (group 1). Only patients who underwent CCTA using retrospective electrocardiographic gating, enabling reconstruction of systolic images, were enrolled. The indications for CCTA were evaluation of coronary artery disease ( n = 23), AS before surgery or transcatheter aortic valve implantation ( n = 19), assessment of the ascending aorta ( n = 5), and other indications ( n = 3). In six patients with AS, the primary indication for CCTA was the evaluation of coronary artery disease ( n = 1), the ascending aorta ( n = 3), or other ( n = 2). Patients with prosthetic valves, atrial fibrillation, or poor-quality images on CCTA were excluded from the study. Patients were withdrawn from the study if they had LVOT obstructions, had inadequate-quality images on TTE, did not present for TTE, or had uninterpretable coronary computed tomographic angiographic studies ( n = 4 total). Data from group 1 were used to calculate the effect of direct LVOT area measurements (taking into account LVOT ovality) on echocardiographic AVA estimation using the continuity equation.
In the second phase of the study, to validate the findings from group 1 (AVA correction factor), we studied 38 additional consecutive patients with AS referred for CCTA for further evaluation before transcatheter aortic valve implantation (group 2), who fulfilled the following criteria: (1) underwent TTE within 2 months of CCTA and (2) did not have any of the exclusion criteria as defined for group 1.
The study protocol was approved by the local ethics committee.
CCTA
CCTA was performed using a 256-slice scanner (Brilliance iCT; Philips Medical Systems, Cleveland, OH) with retrospective electrocardiographic gating. Oral and/or intravenous β-blockers were used to lower the heart rate to <70 beats/min when possible. Sublingual nitroglycerin was used only in patients without AS. Contrast-enhanced scans were performed using a bolus of 80 to 100 mL contrast medium (Ultravist 370 mg I/mL; Schering AG, Berlin, Germany) injected into an antecubital vein at a flow rate of 5 to 6 mL/sec, followed by 30 to 40 mL of mixed 50% contrast and 50% normal saline, followed by a saline chaser bolus. Scanning was performed with 128 × 0.625 mm collimation (with dual focal sampling allowing 256-slice acquisition), gantry rotation time of 0.27 to 0.33 sec, and pitch of 0.18 at a tube voltage of 120 kV and an effective tube current of 600 to 1,000 mA, on the basis of patient size. The average heart rate during acquisition was 63 ± 9 beats/min. A dose modulation protocol (DoseRight; Philips Medical Systems) was used to decrease radiation whenever possible, with a mean effective radiation dose < 12 mSv.
Systolic and diastolic phases were reconstructed at 0%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, and 75% of the R-R interval.
Coronary computed tomographic angiographic images were analyzed on a dedicated computed tomographic workstation (Extended Brilliance Workspace; Philips Medical Systems). All studies in group 1 were analyzed by two investigators (T.G. and R.S.), experienced in interpreting CCTA and blinded to all echocardiographic data and to each other. Each data point was an average of the two measurements. All studies in group 2 were analyzed by R.S.
LVOT area was measured manually at midsystole (20%–40% of the R-R interval depending on the heart rate), in a double oblique cross-sectional image perpendicular to the aortic root, below the level of the aortic valve ( Figure 1 ). LVOT minimal and maximal diameters were also measured at the same level, to calculate the eccentricity index (maximal LVOT diameter/minimal LVOT diameter).
AVA on CCTA was measured manually at maximal aortic valve opening (20%–40% of the R-R interval), using a double oblique plane parallel to the aortic root. By scrolling through the images, the level of minimal orifice, toward the tip of the aortic valve cusps, was found, and AVA was traced at the inside borders of the coronary cusps ( Figure 2 ).
Left ventricular ejection fraction was calculated from measurements of end-diastolic and end-systolic volumes using dedicated software (Comprehensive Cardiac; Philips Medical Systems).
To assess intraobserver agreement, LVOT measurements were repeated (>1-month interval) in 10 patients with AS and 10 without AS. AVA measurements were repeated in all 25 patients with AS.
Echocardiography
TTE was performed within 1 day of CCTA (typically on the same day) in group 1 and within 2 months (mean, 15 ± 23 days) in group 2, using a commercially available echocardiographic system (iE33; Philips Medical Systems), with 1-MHz to 3-MHz broadband-array transducers: an S3 for 2D echocardiography and an X3-1 xMATRIX for 3D echocardiography. A complete echocardiographic study was performed using standard views and techniques according to established guidelines, and data were digitally recorded for offline analysis.
Two-Dimensional Echocardiography
Two-dimensional AVA was calculated using the continuity equation according to published guidelines. The LVOT was imaged from the parasternal long-axis view. Special care was taken to maximize LVOT diameter and to optimize the image for best LVOT border delineation. Magnified cine loops of LVOT area were digitally recorded for offline analysis. LVOT diameter was measured at midsystole just below the insertion of the aortic valve leaflets.
In patients with AS (maximal aortic valve flow ≥ 2.5 m/sec), LVOT TVI was measured from the apical view using pulsed-wave Doppler, with the sample volume located just proximal to the aortic valve, taking care to obtain a laminar flow curve. Maximal aortic velocity, time-velocity integral, and maximal and mean aortic gradients were measured using continuous-wave Doppler from the apical, suprasternal, and right sternal views, and data from the view with the maximal velocity were recorded. Each data point was averaged from measurements of three cardiac cycles.
Three-Dimensional Echocardiography
Full-volume data sets of the LVOT reconstructed from four gated cardiac cycles were acquired during breath holding from the parasternal long-axis view and digitally recorded for offline analysis. Data sets were analyzed using a QLAB workstation (Philips Medical Systems). The true short axis of the LVOT was identified just below the aortic valve after reorientation of the three imaging planes of the 3D data set in midsystole: the idealized parasternal long-axis view, the plane perpendicular to the long-axis view, and the LVOT short-axis plane ( Figure 3 ). LVOT area and the short-axis and long-axis minimal and maximal LVOT diameters were then measured.
All measurements were performed in a blinded fashion (2D vs 3D vs CCTA, as well as comparisons with previous measurements by the same modality) by an experienced echocardiographer (I.A.). To assess intraobserver and interobserver agreement, measurements were repeated (>1 month apart) in 10 patients with AS and 10 without AS (I.A. and M.G.).
Statistical Analysis
Continuous variables are presented as mean ± SD and categorical variables as absolute numbers and percentages. LVOT and AVA data were compared using paired or unpaired t tests as appropriate. LVOT area and AVA derived from CCTA and TTE were compared using Spearman’s rank correlation and Bland-Altman plots.
Intraobserver and interobserver variability were calculated as mean absolute difference divided by the mean of the two repeated measurements to result in percentage error. Differences were considered statistically significant at a two-sided P value < .05. All statistical analyses were performed using Statistix version 8.0 (Analytical Software, Tallahassee, FL).
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