This study demonstrated that cardiac SNS activity was more elevated in patients with an LVEF of < 58% than in those with an LVEF of ≥ 58%. Furthermore, an LVEF of < 58% was significantly associated with all-cause mortality. In contrast, no significant differences were found in cardiac SNS activity in patients with an LVEF of ≥ 50% and those with < 50%, and no significant relationships were observed with all-cause mortality.
No effective treatments have been shown to improve the survival of patients with HFpEF when the cutoff value of LVEF was 50%. This is because HFpEF is a heterogeneous syndrome, and its therapeutic target is elusive. Therefore, resolving the heterogeneity of HFpEF with an improved classification may lead to improved outcomes [13]. Previously, we proposed an LVEF of 58%, rather than 50%, as a cutoff value in patients with HF [11]. This is because an LVEF of ≥ 58% is a surrogate indicator that the left ventricle has the inertia stress of late systolic aortic flow. The inertia stress of late systolic aortic flow, which is defined from the left ventricular (LV) pressure (P)–first derivative of LV pressure (dP/dt) relation, as we reported previously [10], is produced by left ventricles with good systolic function [14]. Therefore, lack of inertia stress is related to loss of elastic recoil in the left ventricle, which in turn results in the deterioration of LV relaxation [15]. This means that even with LVEF ≥ 50%, the left ventricle would not have good LV systolic function if it does not have inertia stress. In other words, left ventricles with inertia stress mean that the left ventricles have both good systolic and diastolic functions. On the other hand, cardiac SNS is activated in order to maintain systemic hemodynamics and peripheral circulation, which is one of the features of HF. Myocardial abnormality caused by LV systolic and/or diastolic dysfunction is the main cause of HF, which can be visualized using 123I-MIBG imaging as activated cardiac SNS. However, the causes of HF are quite diverse. In particular, the causes of HF in patients with high LVEF are often attributed to high blood pressure, atrial fibrillation, and aortic stiffness, including ventricular-arterial coupling [1, 16]. Our study demonstrated that cardiac SNS activity evaluated by 123I-MIBG scintigraphy was significantly lower in patients with an LVEF of ≥ 58% than in those with an LVEF of < 58%. Therefore, an LVEF of 58% is a good cutoff value to differentiate patients with HF owing to cardiac from those with HF due to non-cardiac causes. In contrast, cardiac SNS activation did not differ between patients with an LVEF of ≥ 50% and those with < 50%. This means that there was a mixed population of HF with and without cardiac dysfunction in patients with an LVEF of ≥ 50%. Seo et al., recently reported on the prognostic value of MIBG in acute decompensated HF, in which a low delayed H/M ratio was more frequent in patients with HFrEF and HFmrEF than HFpEF, using an LVEF of 50% as the cutoff. [10] This inconsistency may have resulted from differences in the patient background between the studies, such as age and NYHA class. Our study patients were older and consisted of patients with more severe HF based on the NYHA Classes compared to the abovementioned study.
No differences were found in the use of β-blockers between patients with an LVEF of ≥ 58% and those with < 58%. Nevertheless, cardiac SNS was activated in patients with an LVEF of < 58%, suggesting that β-blockers may be a possible treatment option in such patients. In contrast, the use of β-blockers was higher in patients with an LVEF of < 50% than in those with an LVEF of ≥ 50%. However, no difference in cardiac SNS activation was found between the two groups, suggesting that β-blockers may also be a possible treatment option in patients with an LVEF of ≥ 50%. Therefore, both these results suggest that the underuse of β-blockers may have occurred in patients with an LVEF of 50–58%. The reason for the high use of β-blockers in patients with an LVEF of ≥ 58% may be due to the high prevalence of atrial fibrillation in this group. In our study, there were differences in the use of angiotensin-converting enzyme inhibitors when the LVEF cutoff value was either 58% or 50%. The differences could be attributed to the potential prevention of cardiac remodeling by angiotensin-converting enzyme inhibitors in the presence of systolic dysfunction. The high use of calcium channel blockers in patients with an LVEF of ≥ 50% resulted from the higher prevalence of hypertension in this group.
There was a significant difference in all-cause mortality when the LVEF cutoff value was 58%, whereas there was no significant difference in all-cause mortality when the LVEF cutoff value was 50%. We previously reported that significant differences were found in all-cause mortality and subsequent HF when the LVEF cutoff value was 58% [11]. However, these differences in the composite all-cause mortality and HF readmission were not found in this study. This discrepancy may have resulted from differences in the characteristics of the target patients, such as age and underlying disease. This may also be partly because of the higher incidence of subsequent HF hospitalizations compared to the previous study. In this present study, the proportion of deaths and HF hospitalizations in the LVEF of 50–58% group was higher than in the LVEF of > 58% group, as in the LVEF of < 50% group. Therefore, a larger number of patients may have led to a different conclusion on the composite endpoint. In previous studies using 123I-MIBG, it has already been established that activated cardiac SNS is an indicator of prognosis in both chronic HF and acute decompensated HF [7–10], and the findings of our study are consistent with those of previous studies. The ADMIRE-HF study, which prospectively assessed the event rates in patients with symptomatic HF using 123I-MIBG, showed significantly lower event rates in the delayed H/M ratio ≥ 1.60 group than in the delayed H/M ratio < 1.60 group [9]. The Japanese pooled study also reported that a delayed H/M ratio of 1.68 was a prognostic indicator [8]. Furthermore, a delayed H/M ratio of > 2.0 has been reported to have a low risk of cardiac mortality (< 5%/5 years) [7]. In our study, the delayed H/M ratio for detecting an LVEF of 58% was 1.9, and no patients with an LVEF of > 58% died from cardiac causes.
We believe that an LVEF cutoff of 58% is a good candidate to reclassify HFpEF patients based on cardiac SNS activation. The use of β-blockers for the treatment of HF patients with an LVEF of 50–58% should also be reconsidered. While there have been meta-analyses showing the potential efficacy of β-blockers in HFpEF cases [17, 18], positive outcomes of β-blockers have not been reported in HFpEF treatment [19, 20]. One possible reason for the ineffectiveness of β-blockers in HFpEF patients is the existence of chronotropic incompetence. Chronotropic incompetence is the inability of the heart to increase its rate with increased activity and is an independent predictor of overall mortality [21, 22]; β-blockers, in the presence of chronotropic incompetence, prevent a compensatory increase in heart rate. Atrial fibrillation is common in patients with HFpEF [23], and we previously reported the relationship between an increase in heart rate and exercise tolerance in patients with atrial fibrillation with preserved LVEF; an adequate increase in heart rate is important to maintain exercise tolerance in such patients [24]. It has also been reported that lenient heart rate control is as effective as strict heart rate control in patients with permanent atrial fibrillation [25]. Therefore, it is understandable that a compensatory increase in heart rate would be needed, especially in patients with HF. In our study, the heart rate tended to be lower in patients with an LVEF of ≥ 58% than in those with an LVEF of < 58%, despite the higher prevalence of atrial fibrillation. These findings indicate that β-blockers may have been overused in the treatment of patients with an LVEF of ≥ 58%. Therefore, a reduction in the use of β-blockers should be considered, especially in patients with atrial fibrillation and an LVEF of > 58%. In contrast, β-blockers may be useful in patients with atrial fibrillation and an LVEF of 50–58% because the activated cardiac SNS could be a therapeutic target. We have recently reported that β-blockers may be beneficial in HFpEF patients with atrial fibrillation [26]. Thus, future studies are needed for validating the reclassification of HFpEF with an LVEF of 58%, and for examining the usefulness of β-blockers in patients with an LVEF of 50–58%, especially those with atrial fibrillation.
This study has few limitations. First, this was a single-center, retrospective, observational study that included a limited number of patients. Second, the same cutoff value of LVEF at 58% was used, even though the targeted patients were different from our previous study. However, the left ventricle, which preserves the inertia stress of late systolic aortic flow with good left ventricle systolic and diastolic function, is not likely to be different depending on the underlying heart disease. We believe that an LVEF of 58% is a reliable value.
Cardiac SNS activity was more elevated in patients with an LVEF of < 58% than in those with an LVEF of ≥ 58%. Furthermore, an LVEF of < 58% was significantly associated with all-cause mortality. An LVEF of 58% is therefore a better cutoff value for reclassifying HFpEF patients based on cardiac SNS activation.