The introduction of tissue Doppler imaging (TDI) strain was a great achievement in echocardiography and for the first time allowed true quantification of regional myocardial function. The central problem with this technique was the angle dependence and the unfavourable signal-to-noise ratio, which has been partially overcome with 2D speckle tracking. Figures 1 and 2 demonstrate application of speckle tracking at rest and during dobutamine stress echocardiography (DSE) in a normal subject.
Assessing Patients with Suspected Coronary Artery Disease
Given the fact that survival from revascularisation has only been demonstrated for patients with inducible ischaemia,1 the role of stress echocardiography, myocardial perfusion scintigraphy and perfusion magnetic resonance imaging (MRI) is likely to remain important in the future.
DSE has excellent rates of sensitivity and specificity2 and has been widely used for clinical and research purposes in the past. However, DSE has a long learning curve, is relatively subjective and the poor temporal resolution of the human eye is a limitation for the accurate visual assessment of the complex myocardial motion.3 Initial studies using quantitative longitudinal strain rate analysis with TDI during DSE showed high specificity and sensitivity in detecting coronary artery disease.4–6
2D speckle tracking is site specific and angle independent and allows quantitative assessment of longitudinal, transversal, radial and circumferential strain, which cannot be completely appreciated visually.7 This opens new opportunities and even further increases the accuracy of this method in ischaemia assessment.8Figure 3 demonstrates the example of a patient with a circumflex stenosis and ischaemia during DSE with post-systolic contraction of the lateral wall in the recovery period in 2D speckle tracking. In the recovery period, displacement imaging depicts post-systolic contraction of the lateral wall in red.
Speckle tracking does not replace DSE but complements it, as shown in a recently published study in which the combination of longitudinal strain and Wall Motion Score Index (WMSI) resulted in a significant incremental increase in diagnostic accuracy.9 Similar findings were found in a recently published study of Reant et al., in which the combination of circumferential and longitudinal strains with DSE resulted in very high rates of sensitivity.10
Non-invasive evaluation of regional deformation using strain rate imaging during DSE predicted the relevance of intermediate coronary stenosis compared with fractional flow reserve (FFR).6
DSE is time consuming and rather impractical in the emergency setting, and a tool to identify patients with severe coronary artery disease (CAD) would be useful. A recently published study demonstrated that peak systolic longitudinal strain (PSLS) of the left ventricle at rest using 2D speckle tracking was significantly lower in patients with left main or three-vessel CAD without regional wall motion abnormalities (RWMA) and might be useful for identifying patients with severe CAD.11
Assessing Viability
The identification of myocardial viability is very important as it determines management (revascularisation procedure versus conservative management) and in the meantime significantly affects the prognosis of the patient.12 Different imaging modalities have been used for assessment of myocardial viability in the past and the study group of the European Society of Cardiology (ESC) recently published a report on imaging techniques for the assessment of myocardial hibernation.13 Positron emission tomography (PET) imaging utilising 18F-fluorodeoxyglucose and perfusion tracers,14 myocardial perfusion scintigraphy,15 contrast-enhanced MRI (ceMRI)16 and low-dose DSE17,18 provide valuable diagnostic and prognostic information in patients with ischaemic left ventricular dysfunction and have similar accuracies.
Early studies using speckle tracking showed promising results with sensitivity and specificity rates of 70% to predict viability compared with ceMRI.19 More recent studies report even higher sensitivity and specificity rates for global and regional longitudinal strain assessed with 2D speckle tracking.20 Other investigators have suggested that levosimendan could be superior to dobutamine to predict viability21 and there might still be a role for tissue Doppler techniques.22
Speckle Tracking in Acute Coronary Syndrome
Risk stratification and determination of the time-point of revascularisation remains a challenge in patients with non-ST-elevation acute coronary syndrome (non-ST-ACS). Patients with a very high risk can potentially benefit from an aggressive invasive approach23 but identification of these subjects is difficult. Acute ischaemia induced by a balloon occlusion of the coronary artery induces significant reduction of circumferential and radial strain and displacement.24 Brunvand et al. recently published studies using speckle-tracking imaging in patients with non-ST-ACS. Strain echocardiography was shown to predict acute coronary occlusion and was an excellent predictor of final infarct size, raising the hope that patients who would benefit from immediate reperfusion therapy can accurately be identified in the future.25–27
A subgroup of patients will have an increased rate of cardiovascular adverse events after a myocardial infarction and left ventricular ejection fraction (LVEF) and WMSI have been used for risk stratification in the past. However, both LVEF and WMSI have known limitations such as poor endocardial border definition and poor reproducibility.28 A recent study of 659 patients using speckle-tracking analysis demonstrated that strain and strain rate were significantly related to all-cause mortality (primary end-point) and the composite endpoint of revascularisation, re-infarction and hospitalisation for heart failure. Most importantly, these novel parameters were superior to LVEF and WMSI in the risk stratification for long-term outcome.29 These exciting results, if confirmed in further studies, bear great potential for risk stratification and a more tailored use of invasive assessment in patients with non-ST-ACS.
Use of Speckle Tracking in Asymptomatic Patients
Given the higher sensitivity and specificity, speckle-tracking imaging would be particularly useful in patients who are asymptomatic or in those patients who are found to have normal conventional echocardiographic studies.
Patients with diabetes are very often asymptomatic despite having significant disease and early presentation of CAD and left ventricular dysfunction is not uncommon in this population. Ng et al. used speckle tracking to describe left ventricular myocardial changes in asymptomatic patients with diabetes and demonstrated impaired longitudinal systolic and diastolic function.30 Similar findings were previously shown using exercise tissue Doppler echocardiography.31
A recently published study in children with history of anthracycline therapy demonstrated impaired left ventricular deformation and dyssynchrony despite having normal left ventricular shortening fractions.32 An advantage of strain imaging for detecting subclinical myocardial disease has been shown in several studies of patients with rheumatic diseases.33–35 Due to its non-invasive nature, speckle tracking has the potential to become an important evaluation tool for asymptomatic patients in the future.
Diastolic Dysfunction
Heart failure with preserved ejection fraction (diastolic heart failure) continues to have a rising incidence and at the same time evidence-based therapy that alters morbidity or mortality is lacking.36 Patients with diastolic dysfunction have a significantly increased risk of morbidity/mortality even in the absence of symptomatic heart failure.37 Traditionally, echocardiography with mitral inflow and pulmonary venous flow pattern and, more recently, tissue Doppler imaging has been used to diagnose and grade diastolic dysfunction. Several studies have assessed the additional benefit of speckle tracking on top of the already-established echocardiographic methods to diagnose diastolic dysfunction. While most studies suggest an advantage of speckle tracking,38–40 other studies have reported that longitudinal, radial and circumferential strain are not different between patients with diastolic dysfunction and control subjects.41
Speckle Tracking in Valve Disease
Aortic Stenosis
In patients with aortic stenosis (AS), lower average longitudinal strain is related to higher left ventricular mass, concentric geometry and more severe AS.42 Interestingly, in severe AS patients, impaired left ventricular strain and strain rate exist although ejection fraction is preserved and aortic valve replacement leads to a significant improvement of strain measures.43,44 Future studies should focus on whether strain imaging can predict left ventricular recovery after valve replacement and whether this can be achieved without DSE.
Mitral Regurgitation
Mitral valve replacement bears a significant amount of morbidity and mortality and prediction of post-operative outcomes would be a useful tool in the identification of those patients who truly benefit from surgery. Lancellotti et al. demonstrated in 2008 that in asymptomatic patients with degenerative mitral regurgitation (MR), exercise left ventricular longitudinal contractile recruitment during exercise predicts post-operative left ventricular dysfunction.45 Kim et al. published a paper of 59 patients in 2009 and showed that left- ventricular short-axis function is a useful marker of left ventricular contractility in patients with chronic, severe MR and left ventricular long-axis function becomes depressed earlier in the chronic remodelling process.46
Speckle-tracking echocardiography can be used to predict a decrease in LVEF over the medium term after mitral valve replacement.47
Speckle Tracking to Guide Cardiac Resynchronisation Therapy
Patients with New York Heart Association (NYHA) functional class III or IV who are on established medical therapy and have a prolonged QRS (>120ms) and a decreased ejection fraction (EF <35%) benefit from cardiac resynchronisation therapy (CRT).48 Different echocardiographic techniques have been used in the past to quantify left ventricular mechanical dyssynchrony and predict patient response.49–51 The Predictors of Responders to Cardiac Dyssynchrony (PROSPECT) study suggested that echocardiographic dyssynchrony, such as tissue Doppler, did not have enough predictive value to challenge routine selection criteria for CRT.
Speckle-tracking echocardiography using radial, circumferential, transverse and longitudinal strain has been successfully employed in several studies to assess dyssynchrony. The recently published Speckle Tracking and Resynchronization (STAR) study was a prospective, multicentre trial and demonstrated that dyssynchrony detected by radial and transverse strains is associated with EF response and long-term outcome following CRT.52 Mechanical dyssynchrony can be widely observed in heart failure patients with a narrow QRS complex but so far CRT studies in these patients have not been successful. A recent study showed that intraventricular measures of mechanical dyssynchrony using speckle tracking may be useful in predicting left ventricular reverse remodelling at six-month follow-up in heart failure patients with a narrow QRS complex.53
The Future of Echocardiography
In a recent comprehensive review on the future of echocardiography, Marvick points out that quantification and 3D echocardiography (3D echo) will be of great importance in clinical decision-making in the next few years. Automation and quantification are likely to attenuate skill dependence in future years and echocardiography will finally overcome its label of an ‘old technology’.54
3D echo has been shown to be associated with a marked decrease in inter-observer variability compared with 2D even in the assessment of rather complicated structures like the right ventricle.55 We presented normal values and reproducibility of strain parameters derived from 3D speckle tracking of the left ventricle at the Annual Meeting of the ESC in Sweden.56 Left ventricular torsion is the wringing motion of the left ventricle around its long axis and is related to fibre orientation in the sub-epicardial and sub-endocardial wall.57 To be clinically useful it should be quantified as a circumferential–longitudinal shear angle. Therefore, torsion is related to the deformation process within the myocardial wall and may be considered as a marker for cardiac disease. Speckle tracking has been shown to be a technique for quantification of left ventricular torsion and good correlation with cardiovascular magnetic resonance (CMR) studies has been demonstrated.8,58 Three-dimensional speckle tracking is a promising tool to overcome the lack of a reference coordinate system in two-dimensional scanning. The expanding role of left ventricular torsion in the analysis of myocardial dysfunction is nicely described in a review by Rüssel et al.59 However, whether this analysis can add to the diagnostic accuracy of a stress test has not clearly been documented so far.
In summary, given the major social and economic importance of CAD for our societies, quick and effective evaluation and management of stable and unstable patients is essential. Testing for myocardial ischaemia using speckle tracking has the potential to become an excellent tool to guide interventional cardiologists as to whether treatment of a coronary stenosis will have an impact on the symptoms and prognosis of patients.
Three-dimensional speckle tracking is likely to emerge as an even more important non-invasive technology in the future. Advantages are that it is widely available (compared with CMR scanners) and does not use ionising radiation (compared with computed tomography [CT] or PET).