Introduction: The American Heart Association and European Society of cardiology guidelines reclassified heart failure according to left ventricular ejection fraction, recognizing patients with mid-range EF (mrEF; 40% to 49%) as a distinct group. However, studies on the clinical characteristics of mid-range EF patients and the occurrence of cardiovascular events in acute MI patients are insufficient. Methods: We categorized 6,553 patients with acute myocardial infarction (AMI) from the Korea AMI-National Institutes of Health between November 2011 and December 2015 into three groups (reduced EF ; LVEF < 40% at admission, mild-reduced EF ; LVEF 40 to 49%, preserved EF ;LVEF ≥50%). The primary endpoint was defined as any death at two-year. Secondary endpoints were defined as any myocardial infarction, any revascularization, patient-oriented composite outcome(POCE). Results: Compared to patients with other two groups, the reduced EF group had a highest overall mortality, POCE, and any MI (24.7% vs 8.3% vs 4.6%, p < 0.0001, 33.0% vs 15.6% vs 12.4%, p<0.0001, 3.9% vs 2.7% vs 2.6%, p<0.0046). When mid-range EF group was designated as a reference, in multivariate analysis including all variables, significant differences with HFrEF group was found. (Hazard ratio ). When compared with HFpEF, only Model 1 and Model 2 showed a significant statistical difference (Model 1 ; 0.65 (0.53-0.81), Model 2 ; 0.56 (0.56-0.86). Conclusions: Followed up for two years, significant differences in survival rates were observed between the mid-range EF, reduced EF, and preserved EF group. After adjusting for common prognostic factors affecting the overall mortality rate, the reduced EF group had a significantly higher mortality rate than mid-range EF group, but no significant difference was observed between the preserved EF and the mid-range EF group.

<b>Background/Objectives:</b> Pacemaker-associated heart failure (PaHF) is a recognized complication of chronic ventricular pacing, yet its long-term incidence and prognostic impact remain incompletely defined. Previous studies on PaHF have been largely limited by small sample sizes, single-center designs, and insufficient long-term or time-dependent analyses. We aimed to evaluate the incidence, clinical predictors, and mortality risk of PaHF in a nationwide real-world cohort. <b>Methods:</b> Using the Korean National Health Insurance Service database, we identified 32,216 patients who underwent de novo pacemaker implantation between 2008 and 2019 without prior heart failure. <b>Results:</b> During a median follow-up of 3.8 years, 4170 patients (12.9%) developed new-onset PaHF and 6184 (19.2%) died. PaHF was independently associated with increased all-cause mortality (adjusted hazard ratio [HR]: 3.11, 95% confidence interval [CI]: 2.93-3.32, <i>p</i> < 0.001), even after accounting for immortal-time bias and relevant covariates. The incidence of PaHF and its associated mortality risk both peaked within the first six months post implantation and remained persistently elevated throughout follow-up; notably, PaHF-associated mortality showed a late resurgence. Sensitivity and subgroup analyses consistently demonstrated higher mortality among patients with PaHF across a wide range of clinical characteristics. <b>Conclusions:</b> In this large, nationwide cohort, the development of PaHF was associated with a substantial and sustained increase in mortality risk following pacemaker implantation. Given the persistent and dynamic nature of this risk, longitudinal monitoring of cardiac function and individualized pacing strategies may be warranted to mitigate long-term adverse outcomes. Additionally, these findings provide real-world benchmarks to guide future pacing strategies and surveillance efforts.