## 4 Discussion
[PERSON] et al. [21] examined the impact of HR on deformation indices and the force-frequency relationship based on data obtained by LV catheterization. Among their results, they observed a systematic HR-related decline in longitudinal strain values with no significant variations of strain rate and a modest increase in LV dP/dtmax. By contrast, [PERSON] et al. [22] evaluated the responses of a group of twenty-one patients early after cardiac surgery and reported that systolic and early diastolic strain rates were dependent on both preload and HR. They found that a pacing-induced increase in HR resulted in an elevated rate of LV systolic strain, although it did not have any impact on longitudinal strain. These findings were consistent with those of [PERSON] et al. [23], who demonstrated that atrial pacing resulted in improvements in early diastolic relaxation in patients with both normal and depressed LV systolic function. The differences observed between these studies might be explained by different preload conditions; we note that, in contrast to the methodology of [PERSON] et al. [21], [PERSON] et al. [22] maintained constant cardiac filling pressures by fluid infusion. In the latter study, the fact that LVGLS was not impaired during atrial pacing thus confirms the hypothesis that LVGLS is a preload-dependent parameter. These findings are consistent with our results. Likewise, and consistent with the findings reported by [PERSON] et al. [22], most of our study participants exhibited deteriorations in LVGLS at higher stimulation frequencies, notably those carrying a diagnosis of CHF. We also identified deteriorations in LVGLS at higher HR; this response is most apparent among the participants who presented with diminished LV function at baseline, as determined by measures of longitudinal strain, Tei index, and LVEF. The pathophysiological relevance of HR-related worsening in LVGLS is also demonstrated by the relationship of this response with increased E/e', which is considered to be a parameter that reflects LV filling pressures [6]. This relationship suggests that patients who exhibit HR-related reductions of LV systolic function as assessed by LVGLS might benefit from strategies that maintain lower HR, consistent with currently recommended therapeutic strategies [10; 24]. These findings might be considered when designing ICD atrial paced rhythm programs. Our findings could represent the first step toward a rational programming of implanted devices to improve cardiac performance and functional capacity. Moreover, our findings might also be considered from the perspective of the RV. HR-dependent RV function may also be a fundamental parameter for defining the optimal stimulation frequency. To the best of our knowledge, there are no previous studies that have explored HR-dependent changes in RV strain. Both RVGLS and RVfwLS have been identified as accurate measures of RV systolic function and prognosis [3; 4]. In this study, we found that HR-dependent RVGLS and RVfwLS responses differ from those of LVGLS, as they can both worsen but also improve at higher stimulation frequencies. These results permit us to speculate on the interventricular interdependence of RV function and the different mechanics associated with ventricular contraction. Earlier studies have revealed that RV function is coupled with low hydraulic impedance of the pulmonary circulation and that right-sided pressures are significantly lower than those on the left side of the heart [24]. Compared with the LV, RV function is more frequently influenced by increased preload and even more so by afterload. Slight variations in pulmonary pressure can impair RV function and global cardiac performance, even in the setting of normal preload [25]. The ability of the RV to increase strain at higher HR may suggest a role for this compartment as a reservoir. Higher HRs would favor RV emptying in support of pulmonary and systemic circulation as well as LV filling. This phenomenon may be facilitated by differential arterial pressures experienced by the RV and the LV.