Our study summarizes for the first time in the current literature the value of CMR based strain by HARP, SENC and fast-SENC in the clinical setting, serving as a promising surrogate parameter for the prediction of MACE in patients both with ischemic and non-ischemic cardiac diseases as well as in asymptomatic individuals.18–20,22−26 By performing meta-analysis of individual patient data, SENC/fast-SENC provided the most robust prediction of the composite endpoint of death, myocardial infarction, coronary revascularization and hospitalization due to heart failure, beyond clinical parameters and conventional CMR parameters, such as WMA, LV-ejection fraction and LGE (HR of 4.46, 95%CI = 2.96–6.73, p < 0.001).18–20 In addition, the assessment of myocardial strain by SENC/fast-SENC predicted all-cause mortality (HR of 3.0, 95%CI = 1.2–7.6, p = 0.02).
The MESA study offers a large database with healthy patients undergoing multiple examinations including CMR, whereas long-term follow-up of up to 8 years was available in the studies included in our systematic review.22–24,26 Various strain variables assessed by HARP emerged as significant predictors of MACE and incident heart failure in asymptomatic individuals from different ethnicities, surpassing the value of clinical parameters and standard CMR variables.22–24,26 The underlying pathophysiologic mechanism of this observation is not completely understood. Possibly, regional circumferential myocardial dysfunction represents a response to increased wall stress, reflecting local alterations of myocardial properties, such as fibrosis or ischemia due to microvascular disease or CAD. This increased afterload may contribute to the development of progressive myocardial remodeling and dysfunction, triggering poorer putcomes.30 In addition, Ecc was significantly related to the LV mass index, which again underlines that the relationship between reduced strain and subclinical heart failure, which may convent to symptomatic disease due to adverse remodeling of the ventricle.30,31
Four studies on the other hand, focused on the ability of SENC/fast-SENC for the prediction of outcomes in patients who underwent clinically indicated CMR examinations. 18–20,25 In two of these studies focusing on symptomatic patients with CAD, SENC and fast-SENC respectively, outperformed the ability of WMA for the diagnostic classification and risk stratification of patients with ischemic heart disease.18,20 The results were similar although different stressors (dobutamine versus adenosine) were used for pharmacologic stimulation, which underlines the wide applicability of SENC for ischemia detection. In the two further studies, investigating patients who underwent clinically indicated CMR due to suspected ischemic and non-ischemic, structural cardiac diseases, the role HARP and fast-SENC for risk stratification was reestablished.19,25 Hereby, patients with normal myocardium > 80% by fast-SENC exhibited better outcomes compared to patients with reduced baseline strain, who experienced higher mortality, higher rates for hospitalization due to heart failure symptoms and significantly more frequent transition rates from subclinical LV-dysfunction to symptomatic heart failure.19 In addition, in our meta-analysis, based on individual patient data, SENC and fast-SENC provided the most robust prediction of MACE beyond clinical and conventional CMR parameters, exhibiting incremental value for the risk stratification of a patients with a broad spectrum of cardiac diseases.18–20 In addition, myocardial strain achieved prediction of all-cause mortality, which was not the case with conventional CMR markers.
Comparison to myocardial strain assessment by feature tracking imaging (FTI) and technical considerations. Several previous studies investigated the role of feature tracking imaging (FTI) for the risk stratification of patients with ischemic and non-ischemic heart disease.32–36 In this regards, FTI derived GLS exhibited incremental value to CMR variables such as LV-ejection fraction and late gadolinium enhancement (LGE) for the prediction of MACE, including sudden cardiac death, resuscitated cardiac arrest and hospitalization due to heart failure in patients with hypertrophic cardiomyopathy.32 In addition, LV strain parameters were independent predictors of MACE beyond clinical and conventional CMR markers, such as LVEF and LGE, in 162 patients with acute myocarditis, analyzed within a multi-center trial, while left atrial and right ventricular strain were less useful in this context.33 In the same direction, previous studies underlined the incremental prognostic value of FTI in patients with non-ischemic dilative cardiomyopathy, beyond NYHA classification, LV-ejection fraction and LGE.34 This could be confirmed in recent multi-center CMR studies, where FTI derived strain parameters surpassed the value of conventional functional CMR parameters, thus strengthening the body of evidence for the clinical implementation of strain for the risk stratification of patients with non-ischemic heart diseases.35,36 Fewer studies, however, have investigated the value of FTI for the diagnostic classification or risk stratification of patients with ischemic heart disease.37,38
From a technical point of view FTI is based on pattern matching techniques across multiple images in a cardiac cycle.39 By FTI, pixels are identified in one frame and followed in the next frames, enabling tracking of myocardial deformation with conventional cine images.40 This is the foremost advantage of FTI since it does not require additional image acquisition and can estimate myocardial strain using clinical SSFP cine images. Different software packages with FTI however, use different algorithms for the calculation of strain, which results in different numerical values. These values are also different from CMR based HARP or SENC and fast-SENC and considerations have been raised, regarding strain over- or underestimation with FTI, which may be less sensitive in terms of disease detection.41,42 In addition, strain reproducibility may be lower by FTI, compared to SENC, which may allow more comprehensive assessment of regional myocardial strain compared to FTI.43,44 Such differences may be decisive for the diagnostic classification or risk stratification of patients with ischemic heart disease.38,45 In this regard, FTI based strain exhibited lower precision than fast-SENC for the identification of segments with regional myocardial dysfunction due to ischemic heart disease.46
Considering the practical advantages of fast-SENC compared to HARP and SENC, it should be noted, that fast-SENC can be acquired during free-breathing of the patients, within a single heartbeat and high heart rates under inotropic stress CMR (> 150bpm), which is of clinical importance, especially in patients with symptomatic heart failure, arrhythmias and chronic obstructive lung disease.47 In addition, post-processing analysis with fast-SENC requires much lower time spent, compared to earlier sequences like HARP, thus increasing the potential of fast-SENC for translation into the clinical realm.44,47 Finally, the use of artificial intelligence (AI) in CMR imaging protocols evaluating potential clinical predictors in patients with cardiovascular diseases continuously increases.48 Incorporating AI in future studies may also increase the precision of strain algorithms for the risk stratification of patients, simultaneously reducing the required time spent for quantification analysis.
Limitations
Our study has some limitations. Thus, a classical meta-analysis was not possible due to substantial heterogeneity in the definition of outcomes between trials. We therefore performed individual patient data analysis from 3 studies. In this regard, the strain sequences and acquisitions differed between the 3 studies, preventing the selection of a binary illustration and a universal cut-off value. In addition, studies were conducted with different scanners and different image quality can be anticipated, which may have affected the resultant image quality and possibly slightly also affect the acquired strain values. However, our study cohort included patients with cardiac diseases based on largely heterogeneous etiologies, so that it may add important evidence for the value of myocardial strain for the risk stratification of symptomatic patients across a wide range of cardiac disorders. In addition, WMA and LGE were assessed visually in studies building the base for our meta-analysis, whereas data on T1 and T2 mapping and extracellular volume fractions (ECV) were not available. However, myocardial strain was also treated as a categorical variable in our statistical analysis, although quantification or semi quantification analysis has been available in the individual studies. In addition, T1 and T2 mapping techniques, although meanwhile established for the diagnostic work of patients with non-ischemic cardiomyopathies,8,9,49 were not widely used in studies performed more than one decade ago.