The following conclusions were reached based on the study results evaluated above: 1) The 3DE-derived RV-fwLS was an appropriate parameter for predicting patients with impaired RV systolic function and more powerful than 2DE-derived RV-fwLS. 2) There were moderate to good correlations of 3D-derived RV longitudinal strain to CMR-derived RVEF, and the relation of 3DE-derived RV-fwLS to CMR-derived RVEF was greater than other measurements of RV systolic function. 3) RV dysfunctional patients with PH showed significant lower values in 3DE-derived RV-fwLS than patients without PH.
The RV performance is increasingly being recognized as a major predictor of long-term outcomes in various pathological conditions [19–21]. An analysis of RV performance based on geometric models in conventional 2DE method that could not assess the anterior and posterior walls or outflow tract of RV adequately, as these territories are poorly visualized in 2D planes. Given the irregular shape of RV, 3DE-derived strain parameters could avoid this shortcoming to better reflect the true contractive condition of RV. To the best of our knowledge there is, as of yet, no published research assessing RV-fwLS in a 3DE environment for patients with RV dysfunction referred to chronic left-sided heart failure. We reported that 3DE-derived RV-fwLS has a better correlation to CMR-derived RVEF, and could detect more deformational features useful in identifying patients with RV dysfunction than 2DE-derivedRV-fwLS. However, Smith et al. demonstrated 3DE-derived global longitudinal strain showed only moderate correlation to 3DE-derived RVEF, it might because of different system and vendor [13].
We observed that the correlation of 2DE or 3DE-derived RV longitudinal strain to CMR-derived RVEF was moderate to good. The RV myocardium mainly consists of myocardial fibers with the epicardial circumferential to oblique alignment and the subendocardial longitudinal alignment [3]. Longitudinal contraction contributes more to RV systolic function than transverse shortening, accounting for nearly 80% of overall RV function. Therefore, the dominant RV myocardial contraction is longitudinal direction [6, 7]. Our study also indicated that the magnitude of the longitudinal strain in patients with RV dysfunction was significantly reduced relative to control subjects. In the present study, the variables of RV end-diastolic pressure and tricuspid regurgitation were not taken into analysis of the predictors of 3DE-derived RV-fwLS. In fact, RV strain parameters are less load-dependent [4]. Meanwhile, the reference values of 3DE-derived longitudinal strain of RV should be established in subsequent studies.
Both ventricles share the oblique fibers in the interventricular septum, which may contribute to their independent contractions [22]. Owing to this important physiologic coaction between LV and RV, the contractile performance of RV is closely linked to the global performance of LV. In our study population, all RV dysfunctional patients were associated with chronic left heart failure. This might weaken the diagnostic accuracies of the corresponding deformation parameters in interventricular septum to discover patients with RV dysfunction. Furthermore, the LV becomes more spherical and interventricular septal myocardial fibers become less oblique because of chronic left heart failure, reducing their mechanical advantage to reflect the longitudinal function. In contrast, the RV-fwLS could analyze the RV mechanism free of influence from LV, are expected to provide more information for the actually contractive condition of RV. In our opinion, it was not appropriate to include the interventricular septum for calculating the global longitudinal strain of RV, as its inclusion would inevitably taint the data for RV analysis.
Along with the findings by Giusca et al. [23] and Wald et al. [24], we also found that conventional RV measurements (such as TAPSE or FAC) had relatively lower correlations to CMR-derived RVEF than RV deformation parameters. TAPSE is easily limited by regional abnormalities in the RV free wall tethering or tricuspid regurgitation, and RV-FAC has a major shortcoming of inter-measurement variability caused by its high dependence on the specific imaging plane. It is obvious that underestimations of TAPSE and RV-FAC could result from its load dependency [4]. Overestimation of TAPSE may be caused by an increasing variability of the apical rotation [5] or the influence of medical therapy on RV regional motion to interfere its measurements. Nevertheless, RV free wall longitudinal strain is theoretically angle-independent and less susceptible to be influenced by cardiac translational motion, and as such may provide more accurate information concerning RV deformation pattern.
In our study cohort, patients with RV dysfunction were divided into two subgroups (those with and without PH) according to the aforementioned cutoff value. RV dysfunctional patients with PH presented with significantly lower magnitudes of longitudinal strain in RV free wall. Increased LV filling pressures could lead to increased pulmonary artery pressure and further impair RV function by increasing the afterload. Our findings supported that 2DE or 3DE-derived RV-fwLS could identify a more advanced stage of cardiopulmonary involvement in patients associated with chronic left heart disease. Prior studies have showed that 2DE-derived RV longitudinal strain could help to predict morbidity and mortality in patients with PH [15, 20]. We hope to explore the potential ability of 3DE-derived RV-fwLS to predict prognosis among these patients in our future studies.
Limitations
Several limitations of the current study should be noted. Firstly, this was a single-territory study only with a small number of subjects, so further studies with a larger amount of subjects are needed to confirm our preliminary findings. Secondly, CMR has been considered as the gold standard for determining RV volumes and RVEF, this modality is limited by its high expense and low availability. However, CMR examinations were not available to all the subjects in this study, we also discovered that there was no difference in the clinical and conventional echocardiographic baseline data between subgroups with and without taking this examination. Thirdly, a portion of subjects in our study had been diagnosed with diabetes mellitus or hypertension which could potentially affect RV function. While excluding these subjects from the comparison did not alter the analysis of RV functional parameters between RV dysfunctional patients and control subjects. Fourthly, we did not assess 3D-derived RV myocardial deformation in radial and circumferential directions in this study. In fact, 3DE-derived radial and circumferential strains of RV were not analyzed owing to methodological limitations of this technology. Longitudinal shortening of the RV has already been shown to be a more important contributor to the RV systolic function compared to circumferential shortening [25]. Fifthly, patients with preserved RV function and chronic left heart failure were excluded from the further research, so the results were non suitable to this subgroup. Finally, our study subjects featured different types of etiologies, a larger study population with a pure etiology will be required to validate our findings.