The roots of education are bitter, but the fruit is sweet.
Since the initial report of speckle-tracking echocardiography (STE), a plethora of articles have demonstrated its superiority to standard echocardiographic parameters, including left ventricular (LV) ejection fraction, in many cardiovascular conditions. Because of its better precision, higher sensitivity, and incremental prognostic value, experts have discussed whether STE is ready for prime time for clinical use. Global longitudinal strain (GLS) appears to be the best candidate. A growing number of institutions are reporting GLS in the clinical setting, especially for the identification of subclinical diseases and further risk stratification of patients.
One of the obstacles to the clinical use of strain has been intervendor disagreement on strain measurements, which is caused mainly by postprocessing. To achieve a consensus on methodology for the quantitative evaluation of cardiac mechanics, the European Association of Cardiovascular Imaging and the American Society of Echocardiography invited vendors to participate in a concerted effort to reduce intervendor variability of strain measurement. The initial target was GLS, as the largest clinical experience has focused on this strain measurement. The fruits of these efforts are becoming apparent for resting images. Reproducibility of GLS measurements among vendors is mostly superior to that of conventional echocardiographic measurements. Another recent comparison between GE and Philips software demonstrated a reduction of variation in these measurements across time, with the measured variation being no greater than that of other measurements, including LV ejection fraction.
As one of the main aims of deformation analyses using STE is to quantify myocardial contraction and wall motion, it is sensible to apply this method for the identification of myocardial ischemia. This approach can be used either at rest or in conjunction with stress. A meta-analysis including 1,385 symptomatic patients showed that GLS at rest can detect moderate to severe obstructive coronary artery disease (CAD) with decent accuracy: pooled sensitivity of 74.4%, specificity of 72.1%, a positive likelihood ratio of 2.9, a negative likelihood ratio of 0.35, area under the curve of 0.81, and a diagnostic odds ratio of 8.5. Considering that these studies were performed under resting conditions, this level of discrimination is satisfactory; however, there is room for improvement. Thus, the application of STE in conjunction with stress echocardiography is an attractive option.
Among the modalities of stress echocardiography, pharmacologic stress STE would be the next step, mainly because of feasibility. STE has been applied for both dipyridamole and dobutamine stress echocardiography (DSE). GLS, particularly assessed by comparing peak-dose with low-dose values, improves the accuracy of dipyridamole stress echocardiography in the detection of single-vessel CAD compared with visual assessment. DSE seems to be a particularly appealing approach to test quantitative approaches because one can repeat image acquisition until optimal-quality images have been captured, not only at baseline during the resting state but also during peak stress and early recovery. Assessing systolic or postsystolic strain by STE during early recovery after DSE helps detect hemodynamically significant CAD compared with visual assessment. STE is also feasible during DSE after ST-segment elevation myocardial infarction to detect significant angiographic CAD at follow-up.
Another reason favoring pharmacologic stress is mimicking findings from exercise stress. Some patients without significant CAD may develop an abnormal LV contractile response to exercise. A well-known condition associated with an abnormal response is a hypertensive response to exercise, or underlying cardiomyopathy in the absence of ischemia, because exercise may result in worsening of regional and global systolic function in myopathic ventricles. A recent study revealed that an abnormal response to exercise in the absence of established causes is associated with resting LV long-axis dysfunction and is usually seen in women.
As the use of contrast agents is recommended when two or more contiguous segments are not detected in any three of the apical views, we are tempted to use these agents. There is, however, a caveat for the use of contrast agent when speckle-tracking analysis is planned: these contrast agents work as another source of speckle, which significantly interferes with tracking quality. These contrast microbubbles should be eliminated before applying STE. Destroying microbubbles is done most effectively using a three-dimensional probe with a high–mechanical index flash.
In this issue of JASE , investigators from Kitakyushu, Japan, report on a study testing their hypothesis that strain imaging also requires expertise, using 37 patients who underwent both DSE and coronary angiography. They compared the diagnostic accuracy of DSE to detect significant CAD from two perspectives: (1) expert versus nonexpert and (2) visual assessment versus strain assessment. A bull’s-eye map of longitudinal strain and a postsystolic shortening map were assessed at baseline and peak stress by five examiners, three experts and two fellows, using vendor-dependent (GE) software in three apical views. The fellows had completed level 3 training and had >300 cases of experience in STE. Myocardial ischemia by STE was defined as a >1% reduction in longitudinal strain with an increase in postsystolic shortening in at least two contiguous segments in the coronary artery territory at peak stress compared with baseline. Twenty-five patients had significant CAD (>70% stenosis on coronary angiography). The main findings were threefold: (1) The diagnostic accuracy of visual assessment by the experts was not significantly different from that of the fellows. (2) STE provided no additional benefit over visual analysis, on both a patient basis and a vessel basis, for experts and fellows. (3) However, the diagnostic accuracy of speckle-tracking DSE was better among the experts than the fellows because of lower specificity among the fellows than the experts. The investigators concluded that expertise affects strain evaluation for detecting significant CAD during DSE.
Their finding of comparable accuracy between the experts and fellows with level 3 training supports the adequacy of the current training scheme. However, the significant effect of expertise on the accuracy of STE suggests that this advanced imaging method still requires an advanced level of expertise. This is concordant with a report from an international multicenter trial. Although agreement on GLS was significantly greater than that on LV ejection fraction, regardless of image quality or experience, there was still a significant trend toward lower interobserver variability in GLS measurements with increasing experience. In addition, training improved agreement on segmental strain regardless of experience. As CAD is a disease that is usually manifested regionally, training may have a role from this perspective. In addition, future technical innovations on the horizon should enhance reproducibility and reduce the need for manual editing during strain analysis.
The advances in STE software interfaces allow even a high school student (with a good science background) to assess longitudinal systolic strain. From my own 3-year experience in providing hands-on sessions to Australian cardiology fellows and registrars, >90% of them can demonstrate the apical-sparing pattern in myocardial amyloidosis after a few hours of training. Nevertheless, STE still requires expertise. In other words, although the plethora of recent publications may appear to suggest that STE is the “new penicillin,” good for almost every ailment, speckle-tracking for DSE is not a panacea. Twenty-five years ago, Picano et al . clearly demonstrated the importance of expertise in stress echocardiography. At that time, stress echocardiography was considered an advanced technique, similar to strain imaging in the current era. Picano et al .’s article, at least partially, influenced the current approach to training in echocardiography. These new observations on the effect of expertise in STE should do the same for future versions of the approach to echocardiography training, to improve the diagnostic yield of testing and to provide better patient care.
Dr. Negishi is supported by an award from the Select Foundation, which had no role in the preparation of this work.