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Analysis of 24-h Holter recordings was a pre-specified substudy of SHIFT (Systolic Heart Failure Treatment with the If Inhibitor Ivabradine Trial) for exploring the heart rhythm safety of ivabradine and to determine effects of ivabradine on 24-h, daytime, and night-time heart rate (HR) compared with resting office HR.The 24-h Holter monitoring was performed at baseline and 8 months after randomization to ivabradine (n = 298) or matching placebo (n = 304) titrated maximally to 7.5 mg b.i.d. in patients with baseline HR ≥70 b.p.m. Patients received guideline-based optimized heart failure therapy including ACE inhibitors and/or ARBs in 93% and beta-blockers at maximally tolerated doses in 93%. After 8 months, HR over 24 h decreased by 9.5 ± 10.0 b.p.m. with ivabradine, from 75.4 ± 10.3 b.p.m. (P < 0.0001), and by 1.2 ± 8.9 b.p.m. with placebo, from 74.8 ± 9.7 b.p.m. (P < 0.0001 for difference vs. ivabradine). HR reduction with ivabradine was similar in resting office and in 24-h, awake, and asleep recordings, with beneficial effects on HR variability and no meaningful increases in supraventricular or ventricular arrhythmias. At 8 months, 21.3% on ivabradine vs. 8.5% on placebo had ≥1 episode of HR <40 b.p.m. (P < 0.0001). No episode of HR <30 b.p.m. was recorded; 3 (1.2%) patients had RR intervals >2.5 s on ivabradine vs. 4 (1.6%) patients on placebo. No RR intervals >3 s were identified in patients taking ivabradine.Ivabradine safely and significantly lowers HR and improves HR variability in patients with systolic heart failure, without inducing significant bradycardia, ventricular arrhythmias, or supraventricular arrhythmias.