Background
Although left atrial volume (LAV) by two-dimensional (2D) echocardiography provides prognostic information, the misalignment of the 2D cutting plane of the left atrium could make the measurements inaccurate. The aim of this study was to test the hypothesis that LAV measurement from three-dimensional (3D) echocardiographic data sets using the biplane Simpson’s technique is a more reliable approach for measuring LAV.
Methods
The accuracy of 3D echocardiographic LAV measurements was retrospectively determined in 20 patients using multidetector computed tomography as a reference. LAV indexed to body surface area (LAVI) was measured using 2D and 3D echocardiography in 200 other subjects. LAV determination by 2D echocardiography was performed using the biplane Simpson’s method. A 3D determination of LAV was performed using quantitative software and the biplane Simpson’s method using the anterior-posterior and medial-lateral 2D views extracted from the 3D data sets.
Results
Although LAV using the 3D volumetric method (mean, 98 ± 24 mL) was slightly but significantly lower than LAV on multidetector computed tomography (mean, 103 ± 23 mL), a significant correlation between the two methods ( r = 0.97, P < .001) with acceptable limits of agreement was noted. The left atrial short-axis image extracted from the 3D data sets revealed an ellipsoid shape. Although a good correlation for LAVI was noted between the 2D biplane Simpson’s method and the 3D volumetric method ( r = 0.96, P < .001), the mean value of 2D echocardiographic LAVI was significantly greater compared with 3D echocardiographic LAVI, with a mean bias of 4.7 mL/m 2 . An excellent correlation was noted between the 3D biplane Simpson’s and 3D volumetric methods ( r = 0.99, P < .001), with a lower bias (0.54 mL/m 2 ) and limits of agreement of ±5.8 mL/m 2 . The time required for LAV analysis was significantly shorter with the 2D (mean, 82 ± 7 sec) and 3D (mean, 94 ± 11 sec) biplane Simpson’s methods ( P < .01 vs 2D biplane Simpson’s method) compared with the 3D volumetric methods (mean, 135 ± 24 sec) ( P < .01 vs 2D and 3D biplane Simpson’s methods).
Conclusions
The 2D biplane Simpson’s method overestimates LAV because of the misalignment of the 2D cutting plane, and the 3D biplane Simpson’s method is a practical and more reliable way to accurately determine LAV.
Left atrial volume (LAV) has been shown to provide powerful prognostic information in patients with various cardiovascular diseases. The current American Society of Echocardiography guidelines recommend two-dimensional (2D) biplane measurement of LAV using either Simpson’s method or the area-length method using the apical four-chamber and two-chamber views. The normal cutoff value for maximum LAV indexed to body surface area (LAVImax) is 28 mL/m 2 . However, we occasionally encounter abnormal values of LAVImax even in healthy subjects. In addition to inaccurate LA border tracing, this unexpected finding might be related to the misalignment of the 2D cutting plane of the left atrium. Specifically, the shape of the LA cross-sectional view perpendicular to the LA long-axis is not circular but ellipsoid, and the biplane 2D cutting plane does not always dissect both major and minor axes of the LA short-axis view, resulting in the inaccurate calculation of LAV. Because LAV is a continuous variable, and the intervals for the definition of LA size categories are narrow, with 5 mL/m 2 differences between normal size and mild, moderate, and severe dilatation, a practical and accurate method for measuring LAV is quite important.
Although three-dimensional (3D) echocardiography has been increasingly used to more accurately measure LAV because of the lack of geometric assumptions, it requires specific quantitative software and considerable time for analysis, which limit its routine use in busy echocardiography laboratories. If the shape of the LA short-axis view is ellipsoid, we hypothesized that the biplane Simpson’s method using the anterior-posterior (minor axis) and medial-lateral (major axis) views extracted from 3D data sets is a practical and accurate approach for measuring LAV. Accordingly, this study had two aims. In protocol 1, we aimed to determine the accuracy of the current 3D volumetric method for measuring LAV compared with multidetector computed tomographic (MDCT) imaging, which was used as a reference. In protocol 2, we aimed to determine the accuracy of LAV measurements using the 2D biplane Simpson’s method and the biplane Simpson’s method using two 2D orthogonal views extracted from 3D data sets compared with the 3D volumetric method (validated against MDCT imaging in protocol 1) as a reference standard.
Methods
Study Subjects
Protocol 1
Twenty patients who had undergone both 3D echocardiography and MDCT imaging within 1 month were retrospectively selected in this protocol (mean age, 68 ± 13 years; 12 men). The indication for MDCT imaging was solely to detect coronary artery disease, not to evaluate LAV. The exclusion criteria included atrial fibrillation, chronic renal failure, and other well-known contraindications to iodinated contrast media. The institutional review board of the hospital approved the study, and all subjects provided written informed consent before participation.
MDCT Imaging
The MDCT examinations were performed using a 64-slice MDCT scanner (Aquilion 64; Toshiba Medical Systems, Tokyo, Japan). Patients had taken an oral β-blocker (atenolol 25 mg) in the morning on the day of the MDCT examination. Nonionic iodinated contrast (Iopamiron 300; Bayer Healthcare, Osaka, Japan) was injected into an antecubital vein (1 mL/kg at 4 mL/sec), followed by a 30-mL saline bolus. Image acquisition was triggered by the appearance of contrast in the aortic root. The imaging parameters included a 350-msec gantry rotation time with 5 mm per rotation, tube voltage of 120kV, and tube current of 500 mA. Scan data were then reconstructed at a 0.5-mm slice thickness and 0.5-mm slice resolution using retrospective electrocardiographic gating from end-diastole (0% of the RR interval) to late diastole (90% of the RR interval) in 10% steps. For the LAV measurements, the data sets showing the largest LA size, which are close to end-systole, were selected. Using multiplanar reconstruction mode, the LA cross-sectional image, which is perpendicular to the LA long axis, was obtained. A total of 15 equidistant, cross-sectional slices from the roof of the left atrium to the mitral annulus were extracted. In each slice, the LA wall was traced, excluding the LA appendage and the confluence of the pulmonary veins. The LA volume was determined by the disk summation method.
Protocol 2
A total of 200 subjects were retrospectively selected from our 2D and 3D echocardiography database, which consisted of 272 examinations from healthy subjects and 970 examinations in patients with any cardiovascular diseases. In this database, healthy subjects were recruited from hospital employees and their relatives. None of the healthy subjects had hypertension (blood pressure > 140/85 mm Hg) at the time of the echocardiographic examination or histories of hypertension, diabetes, hypercholesterolemia, and/or cardiovascular disease. Additionally, they were also not taking cardiac medications. To determine the distribution of maximum LAV in normal subjects, we selected 77 healthy subjects and 123 patients who had various cardiovascular diseases.
2D Echocardiography
2D echocardiographic examinations were performed with subjects in the left decubitus position using a commercially available ultrasound machine and transducer (iE33 and S5-1 transducer; Philips Medical Systems, Andover, MA). Three consecutive beats in the apical four-chamber and two-chamber views including the entire left atrium were acquired, with specific attention directed to ensure that the long axis of the left atrium was maximally delineated and the difference between the two imaging planes was <5 mm. Maximum LAV was determined using the biplane Simpson’s method on the apical four-chamber and two-chamber end-systolic frames just before mitral valve opening. In each view, the LA wall was traced excluding the LA appendage and pulmonary veins. LAV was calculated using the following formula on the Xcelera workstation (Philips Medical Systems):
LAV ( mL ) = π / 4 ∑ i = 1 20 A i × B i × L / 20 ,
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