Right ventricular strain and strain rate derived from bidimensional speckle tracking echocardiography (2D STE)
Speckle tracking echocardiography in a non-invasive, innovative technique that analyses the segmental myocardial deformation along different planes through the displacement of speckles [15]. Originally designed for the assessment of the LV, it is now also being applied for the analysis of RV deformation. Strain represents the percentage change in length of a myocardial segment, while strain rate represents the rate of deformation over time [40]. Both strain and strain rate are indices of myocardial contractility [41]. The measurement of RV longitudinal strain and strain rate is performed in the apical RV-focused 4-chamber view, using software dedicated for the LV assessment. The RV free wall and the interventricular septum (IVS) are each divided into three segments (basal, medial and apical), providing a six-segment model (Figure 1). Global longitudinal strain of the RV is calculated as the average of the six segmental values, while the longitudinal strain of the RV free wall (RVFW) is calculated as the average of the three segmental values of the free wall [42]. The latter is considered to be more specific for the RV [15], since the motion of the IVS contributes to both RV and LV function.
The advantages of bidimensional strain derived from STE are the angle independence, the relative load-independence, the strong correlation with RVEF measured by CMR [43] and the ability of detecting subtle myocardial abnormalities which cannot be determined using conventional parameters [44, 45]. One study showed that RVFW strain had a good correlation with the extent of myocardial fibrosis detected on CMR [46]. However, there is no uniformity in software and no reference range agreement between vendors. Other drawbacks are that strain assessment is dependent on good image quality and it neglects the contribution of the RV outflow tract (RVOT) to the global RV performance. For the longitudinal strain of the RV free wall, a value > -20% is considered abnormal [16].
Martin et al. analysed which RV strain parameter was a better predictor of hospitalizations for HF in patients with left heart disease and found out that the global longitudinal strain of the RV independently predicts readmissions, providing additional prognostic information to that obtained by TAPSE [47]. Similar findings were reported by Motoki et al., who found global RV strain to be an independent predictor of long-term adverse outcomes in patients with LVEF<35%, while RVFW strain was not. In their study, a global RV strain > -14.8% independently predicted the primary endpoint of death, cardiac transplantation or hospitalization for HF at 5 year-follow-up [48]. This is contrary to the results of another study, which found that RVFW strain was a better outcome predictor than global RV strain in HF with reduced EF, as it independently predicted total mortality and readmissions for HF even after adjustment for LV dysfunction [49]. Another prospective study showed that a RVFW strain > -21% in patients with HF is an independent predictor for a composite endpoint of death, acute HF, emergency transplantation or left ventricular assist device (LVAD) implantation at 1 year [50].
Carluccio et al. proved the superiority of RV strain over TAPSE, by following 200 patients with HF and reduced EF but preserved TAPSE (>16 mm) for a composite endpoint of death and HF rehospitalization. The authors found that the longitudinal strain of the RVFW was an independent predictor of adverse outcome, with a cut-off value for endpoint prediction of -15.3% [51]. In a recent study by Seo et al., 143 patients with DCM were prospectively followed for long-term unfavourable events (defined as all-cause death, cardiac death, aborted sudden cardiac death and HF hospitalization), for a median period of 40 months. The longitudinal strain of the RVFW was the only independent predictor of the primary outcome, with an optimal cut-off value for event prediction of -16.5% [52].
Several studies discovered independent prognostic roles for both global RV strain and RVFW strain in HF with reduced EF. Cameli et al. found that in patients with advanced systolic HF referred for cardiac transplantation, both global and free wall RV strain are independent predictors of an adverse outcome (defined as cardiac death, heart transplantation, LVAD placement, intra-aortic balloon pump implantation or acute HF), with stronger predictive power than other conventional parameters, including parameters of LV function [53]. Iacoviello et al. found that both global RV strain and RVFW strain are independent predictors of all-cause mortality in patients with HF and LVEF<45% [54]. A recent study by Houard et al. evaluated the prognostic value of bidimensional RV strain for survival prediction and compared it with conventional echocardiographic parameters and CMR in 266 patients with HF and reduced EF. The authors found out that both global RV strain and RVFW strain were independent predictors for overall mortality and cardiovascular mortality; moreover, the predictive power of RV strain was higher than that of FAC, TAPSE, CMR-derived RVEF and CMR-derived RV strain [55].