Strain is a unitless measurement of dimensional or deformational change; speckle-tracking echocardiography is the most widely used technique to assess strain, with demonstrated clinical utility in a variety of settings. This paper reviews the diagnostic and prognostic impact of echocardiographic assessment of myocardial strain. The following are key points to remember:

1. Definition/nomenclature:
   • Image-processing algorithms on digital two-dimensional echocardiography identify and track small, stable myocardial footprints (or speckles) generated by ultrasound-myocardial tissue interactions within a region of interest. Distances between speckles are tracked frame-to-frame over the cardiac cycle, and distances between speckles provide information about myocardial deformation.
   • By convention, positive values are assigned to lengthening, thickening, or clockwise rotation, whereas negative values are assigned to shortening, thinning, or counterclockwise rotation. Greater degrees of deformation are expressed as a numerically lower (more negative) strain value.


2. Variability and normal ranges:
   • There is substantial intervendor variability in strain values that are related to post-processing differences.
   • Current 2015 American Society of Echocardiography guidelines recognize the heterogeneity for normal values between published reports, and do not define normal ranges. However, as a guide, a value less than -20% is considered likely to be normal.


3. Technical factors that influence strain. Strain may be influenced by image quality, choice of a segmental model, selection of image clips, selection of landmarks and segmental contouring, selection of the region of interest, selection on timing during the cardiac cycle, recognition of poor tracking, and technical differences between vendors.


4. Clinical factors that influence strain. Strain may be influenced by racial/ethnic/international differences, age and sex, hemodynamic factors, cardiovascular risk factors, medications, dialysis, pregnancy, and endurance athletics.


5. Clinical utility of strain. Most clinical strain data come from retrospective, nonrandomized studies. Reduction of absolute strain is a marker of most acute and chronic myocardial diseases.
   • Cardiac amyloid. Global longitudinal strain (GLS) is abnormal among patients with cardiac amyloid, sometimes despite preserved left ventricular ejection fraction (LVEF). Cardiac amyloid has a characteristic strain pattern with apical sparing.
   • Hypertrophic cardiomyopathy (HCM). Among patients with HCM, a reduction in GLS is associated with worse cardiovascular outcomes, including heart failure.
   • Hypertension. Among patients with hypertension, GLS is reduced compared to normal controls, and further reduced among patients with heart failure with preserved EF.
   • Athletes. Physiologic hypertrophy/athlete’s heart is associated with significantly higher GLS compared to patients with HCM.
   • Cardio-oncology. GLS is used among patients receiving chemotherapy as part of surveillance for evidence of cardiotoxicity.
   • Aortic stenosis. Among patients with aortic stenosis, GLS may be a marker of subclinical LV systolic dysfunction, and a means to predict clinical endpoints and postoperative LV functional recovery.
   • Ischemic heart disease. Abnormalities in GLS, including early systolic stretch, low systolic shortening, and post-systolic shortening (tardokinesis) have been reported in patients during ischemia.


6. Other cardiac chambers:
   • Left atrium (LA). Acquired from an apical window, LA strain relates to LA deformation, and has been reported to be inversely related to LA pressure.
   • Right ventricle (RV). RV free-wall strain is measured from an RV-focused apical four-chamber view, and has been associated with prognosis in a variety of conditions including heart failure, pulmonary embolism, and arrhythmogenic RV cardiomyopathy.


7. Future directions. The following are areas for potential future improvement in or greater adoption of speckle-tracking echo:
   • Improved tracking and border recognition may lead to shorter analysis times.
   • Greater automation may lead to more widespread adoption of the technique.
   • Collaboration between vendors could serve to reduce intervendor variability.
   • Three-dimensional strain may overcome limitations caused by out-of-plane speckle motion.

Keywords: Athletes, Aortic Valve Stenosis, Cardiomyopathy, Hypertrophic, Cardiomyopathies, Cardiotoxicity, Diagnostic Imaging, Echocardiography, Heart Failure, Heart Valve Diseases, Hypertension, Hypertrophy, Myocardial Ischemia, Renal Dialysis, Risk Factors, Stroke Volume, Systole

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