BackgroundHeart Rate Score (HRSc), the percent of atrial depolarizations in the largest paced/sensed 10-bpm histogram bin recorded in cardiac devices, is associated with several adverse outcomes but it remains uncertain if HRSc independently predicts atrial high-rate episodes (AHREs) in patients with sinus node dysfunction (SND) undergoing pacemaker (PM) implantation. ObjectiveTo determine if initial HRSc post-PM implant predicts new-onset AHREs in patients with SND. MethodsPatients had Boston Scientific PMs implanted for SND from 2012-2021 at Cleveland Clinic, University of Occupational and Environmental Health, Japan, Kyushu Rosai Hospital, and JCHO Kyushu Hospital. Patients were excluded if they had atrial fibrillation before PM implant or AHREs within 3-months post-implant. Subsequent AHREs post-implant were evaluated and correlated with HRSc. ResultsOver 48.9 (IQR 25.7-50.4) months, 130 consecutive PM patients (76±10 years, 40% male) had a median initial HRSc of 74(57-86)%. AHREs defined by >1%, >6h/day burden, and ATR events>24h developed in 27/130(21%), 15/130(12%), and 9/130(7%), respectively. For each definition, patients with HRSc≥80% had higher occurrence of AHREs than those with HRSc<80% (both p=0.008, log-rank test). After adjusting for age, race, comorbidities, left ventricular ejection fraction, left atrial diameter, and cumulative %RA/RV pacing, initial HRSc ≥80% (HR:3.33, 95% CI:1.35-8.18; P=0.009) and male sex (HR:2.59, 95% CI:1.06-6.33; P=0.04) independently predicted AHREs. ConclusionHRSc≥80% is associated with new-onset, device-determined AHREs for patients undergoing PM implant for SND. HRSc may have prognostic and therapeutic implications.