Author + information
- Received August 3, 2000
- Revision received December 8, 2000
- Accepted December 28, 2000
- Published online April 1, 2001.
- Masood Ahmad, MD, FACCa,* (, )
- Tianrong Xie, MDa,
- Marti McCulloch, RDCSa,
- Gerard Abreo, MD, FACCa and
- Marschall Runge, MD, FACCa
- ↵*Reprint requests and correspondence:
Dr. Masood Ahmad, Division of Cardiology, University of Texas Medical Branch, 301 University Boulevard, 4.148 McCullough Building, Galveston, Texas 77555-0766
This study was designed to test the feasibility and efficacy of using real-time three-dimensional echocardiography (RT-3D) to detect ischemia during dobutamine-induced stress (DSE) and compares the results with conventional two-dimensional echocardiography (2D).
Real-time three-dimensional echocardiography, a novel imaging technique, offers rapid acquisition with multiple simultaneous views of the left ventricle (LV). These features make it attractive for application during stress.
Of 279 consecutive patients screened for image quality by 2D, 253 patients with adequate images underwent RT-3D and 2D within 30 s of each other at baseline and at peak DSE.
Real-time three-dimensional echocardiography and 2D showed good concordance in detection of abnormal LV wall motion at baseline (84%: Kappa = 0.59) and at peak DSE (88.9%: Kappa = 0.72). Left ventricular wall motion scores were similar at baseline and peak DSE using both techniques. Interobserver agreements for detection of ischemia at peak DSE were superior for RT-3D, 92.7% compared with 84.6% for 2D (p < 0.05). Mean scanning time at peak stress by RT-3D in 50 randomly selected patients was shorter, 27.4 ± 10.7 s compared with 62.4 ± 20.1 s by 2D (p < 0.0001). In 90 patients with coronary angiograms, RT-3D had a sensitivity of 87.9% in the detection of coronary artery disease (CAD) compared with 79.3% by 2D.
Real-time three-dimensional dobutamine stress echocardiography is feasible and sensitive in the detection of CAD. The procedure offers shorter scanning time, superior interobserver agreements and unique new views of the LV.
The diagnostic and prognostic values of two-dimensional dobutamine stress echocardiography (DSE) have been firmly established in numerous published studies (1–8). The two-dimensional echocardiography (2D) technique has, however, important practical limitations in its application during stress. Multiple 2D views of the left ventricle (LV) must be obtained from more than one window to completely visualize all segments. These time-consuming and operator-dependent 2D acquisitions are particularly problematic during peak stress when the time available for imaging is very brief. If the acquisitions are not completed within a critical period of time, important diagnostic information may be lost, resulting in inaccurate interpretations. The introduction of real-time three-dimensional echocardiography (RT-3D) imaging impressed us through its ability to rapidly acquire images with visualization of the entire LV in any number of different planes simply by rotating and slicing the acquired three-dimensional data set (9,10). Furthermore, a high level of operator skill is not required to obtain diagnostic quality RT-3D images at peak stress. Once a volumetric data set is acquired by RT-3D, matching views for baseline and peak stress can be aligned for a precise comparison of the same segments. Finally, the total study time needed to complete an RT-3D stress echocardiography is much shorter than the time it takes to obtain a two-dimensional stress echocardiography, making the RT-3D stress echocardiography test more cost-effective.
This study was designed to evaluate the feasibility and efficacy of RT-3D for detecting ischemia during dobutamine-induced stress and compares the results with conventional 2D. A nearly simultaneous comparison between the two methods was achieved by acquiring the 2D and RT-3D images within 30 s of each other during the same stress test.
The study was approved by the Institutional Review Board of The University of Texas Medical Branch at Galveston. All patients referred for clinically indicated DSE were eligible for the study. Patients gave informed verbal consent for recording RT-3D images. A total of 279 consecutive patients were screened for image quality by 2D before inclusion in the study; 26 patients with suboptimal images were excluded. Two-hundred fifty-three patients with adequate image quality underwent 2D and RT-3D echocardiography within 30 s of each other at baseline and at peak dobutamine-induced stress. Patient characteristics are listed in Table 1.
Patients were studied in the left lateral decubitus position after establishing an intravenous line for dobutamine infusion. Echocardiography and O2saturation were continuously monitored using a 12-lead Marquette MacVu electrocardiograph (Marquette Electronics, Inc., Milwaukee, Wisconsin) and Nellcor Symphony N-3000 oximeter (Nellcor Inc., Pleasanton, California). Two-dimensional echocardiography images were obtained with a Vingmed CFM 800 echocardiographic scanner with a 2.25 MHz annular array transducer (Vingmed Sound, Horten, Norway). All images were videotaped, and digital cineloops of parasternal long and short-axis, apical four-chamber, two-chamber and long-axis views were recorded at baseline and during low-dose, peak and recovery stages of dobutamine stress.
Real-time three-dimensional echocardiography images were recorded using a Volumetrics Medical Imaging RT-3D echocardiograph with 2.5/3.5 MHz transducers (Volumetrics, Durham, North Carolina). Real-time three-dimensional echocardiography, a matrix-based phased array imaging system, uses a new signal processing approach called “receive mode parallel processing,” resulting in a 16-fold increase in the data acquisition rate (11–14). By receiving 16 lines of image for each transmit, the system scans twenty-two 64 × 64 degree pyramidal volumes/s at a depth of 15 cm. This novel imaging technique obtains pyramidal-shaped volumetric images, which are digitized, stored and analyzed (11–14).
In this study, the RT-3D system was positioned next to the Vingmed 2D scanner for convenience in rapidly alternating 2D and RT-3D acquisitions. After transducer placement and image optimization, a cineloop of two to three cardiac cycles was acquired, and the loop was trimmed to display one representative cardiac cycle. The acquired pyramidal volume of information was recorded on an optical disc and displayed as the standard long-axis with short-axis images of the LV in the parasternal view (Fig. 1)and as intersecting orthogonal B-scans with corresponding parallel plane C-scans in the apical view (Fig. 2). Within the pyramidal volumetric data set, the various LV views were obtained by steering the viewing planes (Fig. 1 and 2). In the parasternal view, with the long-axis used as a reference, steering the elevational plane from right-to-left produced short-axis views at the various levels (Fig. 1). In the apical view, with a four-chamber view as the reference, a 10° to 15° tilt in the elevational plane produced an adjusted two-chamber view with anterior-to-inferior orientation opposite to the standard 2D, two-chamber view (Fig. 2). C-scan views parallel to the face of the transducer in the apical view were obtained as short-axis cuts by sliding the horizontal cutting plane along the long-axis of the LV (Fig. 2).
Dobutamine stress protocol
A standard dobutamine stress protocol was followed with infusions of 5, 10, 20, 30 and 40 μg/kg/min over a period of 3 min at each stage (7). Atropine (0.5 to 1 mg) was used, if and when needed, to achieve target heart rate. All image acquisitions were performed during the last minute of each stage. Real-time three-dimensional echocardiography and 2D acquisitions were performed by the same sonographer at baseline and at peak stress by quickly switching transducers between the 2D and RT-3D echocardiographs. The peak stage of stress was extended beyond 3 min, if needed, for completion of image acquisitions. The sequence of image acquisition by the two methods was uniform in all patients, that is, RT-3D images were obtained within 30 s of 2D acquisition.
LV wall motion analysis
A scheme of LV segments in the parasternal and apical views is shown in Figure 3and is essentially the 16-segment model recommended by the American Society of Echocardiography (15). Left ventricular wall motion was monitored in RT-3D images during the stress test; however, the final interpretation was performed off-line with steering and tilting of the image planes for proper alignment and to visualize C-scans at the various levels. Since all views were available from a single acquisition, the interpretation time for RT-3D stress images was usually less than the time needed to review the 2D studies. In the parasternal view, with the long-axis view as a reference, the elevational plane was tilted to view the LV at the various levels in a short-axis orientation. These views were similar to the standard 2D short-axis views. In the apical four-chamber view, lateral and septal walls were visualized at the various levels, and the corresponding C-scans were examined for the circumferential and longitudinal extent of wall motion abnormality by moving through the apical four-chamber reference view. The adjusted two-chamber view (obtained by steering the reference four-chamber view in the elevational plane) visualized the anterior and the inferior walls. All LV segments seen in the conventional 2D echocardiographs were visualized in RT-3D mode by combining the apical and parasternal data sets. Left ventricle wall motion was assessed in the C-scan images to confirm or exclude the presence of segmental wall motion changes in the orthogonal B-mode scans and was not included in the overall wall motion score. The segments recorded by RT-3D were then compared with the segments observed in the standard 2D echocardiograms following the same 16-segment model (15).
All 2D and RT-3D echocardiograms were reviewed by two observers without knowledge of the results from the other test. Regional wall motion was evaluated at rest and at peak stress in 16 LV segments using a scoring system of 1 through 5, as recommended by the American Society of Echocardiography (15). Normal or increased LV wall motion during stress was considered to be a normal response to stress. Development of new or worsening of resting LV wall motion abnormality during stress was considered to be a sign of ischemia (1–8). A side-by-side comparison of apical RT-3D acquisitions at baseline and at peak stress in a patient with inducible ischemia is shown in Figure 4. Coronary angiograms were obtained in 90 patients within three months based on the recommendations of the referring physicians. A luminal narrowing of >50% was considered significant.
Student paired ttest was used for comparison of continuous variables and the McNemar test for comparison of proportions. A p < 0.05 was considered significant. The agreements between the test results obtained by the 2D and the RT-3D methods were tested by measuring the coefficient of variation, Kappa. A Kappa value of 0.45 or greater was considered to be good agreement, and a Kappa of greater than 0.75 was considered to be excellent agreement.
The two methods of acquisition, 2D and RT-3D, were successfully completed in all patients. Mean ± SD heart rate (beats/min) and systolic blood pressure (mm Hg) were: 69.8 ± 12.9, 142.9 ± 25.1 at baseline and 138.7 ± 16.5, 149.7 ± 35.2 at peak stress (p < 0.0001, p < 0.05). Mean RT-3D scanning time at peak stress, measured in 50 randomly selected patients, was 14.6 ± 6.2 s from the apical window and 12.8 ± 5.0 s from the parasternal window. The total mean scanning time at peak stress was 27.4 ± 10.7 s by RT-3D compared with a mean scanning time of 62.4 ± 20.1 s for a complete 2D acquisition (p < 0.0001).
The interpretations of the LV wall motion obtained by the two techniques were compared. At baseline, depending on whether LV wall motion was normal or abnormal, there was 84.2% agreement (Kappa = 0.59; Fig. 5), and at peak stress, based on whether there was evidence of inducible ischemia, there was 88.9% agreement (Kappa = 0.72; Fig. 5). In a segment-to-segment comparison for LV wall motion score, of a total of 3,728 segments that could be satisfactorily visualized by both techniques, at baseline there was 83.8% agreement (Kappa = 0.58), and at peak stress there was 81% agreement (Kappa = 0.59). The mean coefficient of agreement (Kappa) at peak stress ranged from 0.56 in inferoseptal segments to 0.71 in lateral segments. There were no significant differences between RT-3D and 2D in visualization of segments by region of interest. Comparisons of the overall LV wall motion scores at baseline and at peak stress in the total group of 253 patients were not significantly different (1.09 and 1.07 at baseline and 1.14 and 1.17 at peak stress). Similarly, in 54 patients with evidence of ischemia by both techniques, LV wall motion scores revealed no significant differences between RT-3D and 2D (1.18 and 1.14 at baseline and 1.38 and 1.47 at peak stress). In 78 patients with evidence of ischemia by RT-3D, there were no significant differences in LV wall motion scores at baseline (1.13 by RT-3D and 1.16 by 2D). However, a significantly higher LV wall motion score was observed by RT-3D at peak stress when compared with 2D (1.47 by RT-3D and 1.3 by 2D, p < 0.05). Five patients with nondiagnostic studies by 2D (no detectable new LV wall motion abnormality at submaximal heart rate) had evidence of ischemia by RT-3D.
The studies were evaluated by two different observers for the presence or absence of segmental LV wall motion abnormality at baseline and for the presence or absence of inducible ischemia at peak stress. The interobserver agreements, based on a patient-by-patient analysis, were significantly higher for RT-3D both at baseline and at peak stress, (88.1%, Kappa = 0.67 and 92%, Kappa = 0.81 for RT-3D compared with 81.4%:,Kappa = 0.49 and 84.6%, Kappa = 0.49 for 2D, p < 0.05).
Of 90 patients with coronary angiographic data, 21 patients had one-vessel disease, 16 patients had two-vessel disease, and 21 patients had three-vessel disease. Of the remaining 32 patients, 16 patients had normal coronary arteries, and 16 patients had noncritical disease (<50% lesions). The overall sensitivity of RT-3D in detecting coronary artery disease (CAD) in 58 patients with angiographically significant lesions, taking into account both fixed and reversible wall motion abnormalities, was 87.9% compared with 79.3% by 2D. The sensitivity of RT-3D in detecting ischemia was 77.5% compared with 58.6% for 2D. There were no significant differences in the detection of single or multivessel disease between RT-3D and 2D. A vessel-by-vessel comparison showed similar detection rate by the two techniques. Of 32 patients without significant CAD, four patients were observed to have ischemia by RT-3D compared with six patients by 2D. In the total group of 90 patients with coronary angiograms, RT-3D showed ischemia in 57.7% compared with 44.4% by 2D.
Comparison of RT-3D with 2D
This is the first study demonstrating the use of RT-3D echocardiography during dobutamine-induced stress. Recent studies utilizing RT-3D have shown its efficacy in assessing LV wall motion (9,10). All myocardial segments seen in the conventional 2D images can be satisfactorily displayed in orthogonal B-mode scans (9). In another study, an excellent concordance was noted between conventional 2D and RT-3D in the assessment of segmental LV wall motion (10). Unlike the previously reported nonreal-time three-dimensional methods (16–18), RT-3D provides a comprehensive evaluation of LV wall motion by completing the entire imaging sequence in a single heart beat (9,10). Our study showed a good correlation of segmental LV wall motion assessment between the RT-3D and conventional 2D techniques both at baseline and at peak stress. The LV wall motion scores, ascertained by the two techniques, were similar in the overall group and in patients with evidence of ischemia by both techniques. A higher LV wall motion score in patients with inducible ischemia by RT-3D and a superior interobserver agreement in interpretation of RT-3D images were due to a more complete and simultaneous visualization of all segments in multiple views. In a number of patients, uncertainty regarding the presence or absence of LV wall motion abnormality was resolved by reviewing the simultaneously available parallel or C-scans.
Real-time three-dimensional echocardiography technique is relatively easy to learn and can be mastered quickly by sonographers trained in routine 2D. The ability to obtain multiple views from a single acquisition makes the technique less demanding at peak stress. Real-time three-dimensional echocardiography is an evolving technology, which has some limitations in its current form. The resolution of images is not as high as those obtained with conventional 2D scanners. However, with the development of second harmonic imaging and the addition of contrast for better definition of the LV cavity, there is potential for further advancement.
RT-3D in patients with coronary angiograms
Coronary angiograms were performed selectively based on the recommendations of the referring physicians; however, in this subgroup of patients, RT-3D appeared equally sensitive for detecting stress-induced ischemia. Because coronary angiograms were not performed in all patients, we recognize the limitations of our data in ascertaining the actual sensitivity of RT-3D. The reported range of sensitivity for detection of CAD by two-dimensional DSE varies from a low of 54% to a high of 95% (1–8). Our sensitivity levels of 79.3% for 2D and 87.9% for RT-3D, when resting LV motion abnormality was included, and of 58.6% and 77.5%, when inducible ischemia was present, are in the reported range. However, the incidence of inducible ischemia assessed by both methods in our series of patients was somewhat low and may be related to patient selection. The majority of patients with normal image results did not undergo coronary angiography; therefore, the specificity of the two techniques could not be evaluated.
Our study demonstrates that RT-3D stress echocardiography is feasible and offers a number of advantages including rapid acquisition, simultaneous visualization of the same segments in different planes, superior interobserver agreement in assessing LV wall motion and increased patient throughput by reducing the total study time. The shorter study time should allow greater efficiency in performing stress studies. Based on our experience, the short learning curve and the ease of use of this technology should facilitate its application in the stress laboratory. Real-time three-dimensional echocardiography offers an entirely new approach in the evaluation of patients with CAD. Larger clinical trials involving comparisons with 2D and coronary angiography are needed to further establish its efficacy in the detection of ischemia.
- coronary artery disease
- dobutamine stress echocardiography
- left ventricle
- real-time three-dimensional echocardiography
- two-dimensional echocardiography
- Received August 3, 2000.
- Revision received December 8, 2000.
- Accepted December 28, 2000.
- American College of Cardiology
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