Stressing the Limits of Strain Echocardiography

The clinical utility of stress echocardiography has been established over the more than three decades of its existence, with numerous studies documenting the diagnostic and prognostic value of the technique. Throughout this entire period of time, visual ‘‘eyeball’’ assessment of regional wall motion and thickening has been the standard method for detection of abnormalities in contractile function related to ischemia. The limitations of visual assessment were acknowledged 20 years ago, in two widely referenced studies showing reduced interinstitutional reproducibility and reduced accuracy for less than expert interpreters. 1,2 With the expanding popularity of stress echocardiography in the 1990’s, emphasis was placed on training expert interpreters, which was shown to improve the accuracy and reproducibility of wall motion analysis. As the technique matured into an established modality, the emphasis on becoming an expert interpreter waned. There are currently two major approaches to improving our ability to assess abnormalities in regional function. The first, the use of contrast agents for endocardial border detection, has improved the clinical utility of stress echocardiography by increasing the number of studies that can be assessed by visual analysis. The second has been the development of quantitative techniques for the analysis of regional and global ventricular function, including automated endocardial border detection and pulsed and color tissue Doppler. Tissue Doppler was particularly appealing because it enabled the assessment of displacement, velocity, strain (shortening), and strain rate (rate of change of shortening) with stress. A modest number of studies showed that tissue Doppler–based methods provided quantitative information that could both increase the accuracy of nonexpert readers and yield similar accuracy to expert visual assessment of wall motion. 3,4 However, tissue Doppler has technique-specific limitations, including labor- and time-intensive analysis, angle dependence, base-to-apex gradients in velocity and displacement, and noisy signals. As the limitations of Doppler-derived methods of quantitation became apparent, assessment of strain based on two-dimensional speckle-tracking was introduced as another potentially useful quantitative method. The major advantages of speckle-tracking over tissue Doppler methodology include the lack of angle dependency, favorable signal-to-noise ratio, and less time-intensive analysis. An ever increasing number of studies have demonstrated the clinical value of speckle-tracking for the assessment of left ventricular longitudinal strain in the resting state. The enthusiasm generated by the results of these studies has raised our hopes that speckle-tracking will have equal clinical value when applied with stress testing. Abnormal longitudinal strain may be a sensitive marker of stress-induced ischemia because of the predominance of longitudinal fibers in the subendocardium. In an animal model of coronary stenosis, reduction in longitudinal strain was readily detected by speckle-tracking during dobutamine stress. 5 In this context, in their study reported in this issue of JASE, Joyce et al. 6 compared the value of two-dimensional speckle-tracking and conventional visual assessment for the detection of significant ($70% stenosis) coronary artery disease (CAD) in subjects with previous ST-segment elevation infarction treated with percutaneous intervention. After the exclusion of 20 subjects (15%) because of suboptimal image quality, the investigators enrolled 105 subjects who underwent dobutamine echocardiography 3 months after infarction and angiography within 1 year of their events. Angiography revealed significant CAD in 38 subjects. Those with significant disease had greater worsening of global longitudinal end-systolic strain from rest to peak dose (16.8 6 0.5% to 12.6 6 0.5%) compared with subjects with no disease (16.6 6 0.4% to 14.3 6 0.3%) (P < .001). The optimal cutoff value for the rest-to-stress change in longitudinal strain for detection of disease was 1.9%; this cutoff value yielded sensitivity of 87%, specificity of 46%, and accuracy of 60%. Conventional wall motion analysis yielded sensitivity of 11%, specificity of 94%, and accuracy of 64%. Multivariate analysis showed that the change in longitudinal strain had incremental value for the prediction of significant disease compared with the combination of clinical variables and visual wall motion analysis. Joyceetal. alsoinvestigated theutility of the1.9% threshold value of the rest-to-stress change in longitudinal strain for the detection of disease in the three individual coronary artery territories. Using receiver operating characteristic analysis, areas under the curve ranged from 0.41 to 0.82 among the seven segments of the left anterior descending coronary artery territory. Areas under the curve ranged from 0.42 to 0.74 in the left circumflex coronary artery territory (five segments) and the right coronary artery territory (five segments). Using the segments in each territory with the optimal cutoff value identified by receiver operating characteristic analysis, accuracy for the detection of disease was 67%, 50%, and 42% in the left anterior descending, left circumflex, and right coronary artery territories, respectively. Interobserver variability was 16.7% for global longitudinal strain at peak stress, whichtranslates to a 2.4% absolute difference in strain. Interobserver variability for regional strain was in a similar range. The investigators offer a mixed conclusion, stating that strain analysis is a promising new technique but acknowledging that its low specificity, segment-to-segment heterogeneity, and limitations of the technology currently prevent the routine clinical adoption of this method.