## 4 Discussion  
The main findings of this study address the heterogeneity of RVGLS and LVGLS responses to increases in HR. Most of the participants in this study exhibited significant reductions in LVGLS in response to increases in HR, together with improvements in both RVGLS and RVfwLS. The relevance of these findings can be considered from methodological, pathophysiologic, and clinical perspectives. From a methodological perspective, our study focused on the analysis of LV and RV strain as assessed by STE. This methodology was validated using synthetic data sets and in vitro and in vivo studies, as well as in clinical settings [11; 12; 13; 14; 15; 16]. Several studies performed over the last years demonstrated the advantages of evaluating both LV and RV longitudinal strain. Specifically, these studies revealed that this method provides reproducible results, is highly sensitive for detecting mild ventricular dysfunction, and provides relevant prognostic information [2; 7; 17; 18]. For these reasons, we chose these measures to evaluate the response of ventricular function to changes in HR in our study. It is also important to recognize that we were able to evaluate strain in response to HR without any modifying of adrenergic stimuli. Left ventricular longitudinal strain correlates with exercise capacity and predicts exercise tolerance in patients affected by CHF as well as in healthy subjects [19; 20]. However, there are few data about the heart rate response of LV and RV longitudinal strain in CHF patients. For this reason, we evaluated HR response by excluding the possible influence of adrenergic stimuli in terms of vasoconstriction, isotropic effect, and other modifications induced on the cardiovascular system. All of the participants enrolled in our study were managed with a pre-existing implantable device, which facilitated progressive increases in HR through atrial pacing. To achieve a more accurate estimate of HR-dependent ventricular function, we enrolled patients diagnosed with CHF managed with ICD or CRT-D devices, as well as those with preserved left and right ventricular function who were clinically managed with a pacemaker. We used the AutoStrain application (TOMTEC), which is a validated semi-automatic system that can reduce interobserver variability when analyzing variations in strain measurements. However, and despite the reliability of these new software applications, other factors may contribute to both inter- and intra-operator variability [16], including imaging-modality and software-related variations. To address this concern, we tested the inter-observer reproducibility of both RV and LV longitudinal strain measures in 19 participants and confirmed the very high reproducibility and the accuracy of the software employed in this study. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{4}{c}{**HR RVfwLS Changes**} \\ \hline  & **Worsened** & **Unchanged** & **Improved** & **ANOVA** \\  & **(n = 7)** & **(n = 13)** & **(n = 25)** & **\(p\)** \\ \hline PASP (mmHg) & \(49\pm 13\) & \(30\pm 9\) * & \(31\pm 6\) * & \textless{}0.001 \\ TR (a.u.) & \(2.6\pm 1.1\) & \(1.8\pm 1.2\) & \(1.6\pm 0.9\) & \(0.841\) \\ RVGLS (\%) & \(-11.3\pm 5.1\) & \(-16.0\pm 2.3\) * & \(-16.4\pm 3.3\) * & \(0.005\) \\ RVfwLS (\%) & \(-15.3\pm 6.4\) & \(-22.7\pm 3.7\) * & \(-21.6\pm 4.6\) * & \(0.004\) \\ RVGLS/PASP & \(-0.27\pm 0.19\) & \(-0.57\pm 0.18\) * & \(-0.55\pm 0.13\) * & \(<0.001\) \\ RVfwLS/PASP & \(-0.37\pm 0.24\) & \(-0.81\pm 0.26\) * & \(-0.72\pm 0.17\) * & \(<0.001\) \\ \hline \hline \end{tabular} For abbreviations see Tables 1 and 2. * post-hoc test \(<0.05\) vs. the group that exhibited a worsening of RVfwLS with increasing HR. \(p\) refers to ANOVA analysis for continuous variables and to Pearson Chi-square for categorical variables.  
\end{table} 
Table 4: _Cont._Apart from the methodological aspects, the results of our study have highly relevant pathophysiological implications. Previous studies that aimed to evaluate HR-related changes in LV function generated largely conflicting results.