The purpose of this study is to determine the role of ivabradine in cardiac resynchronization recipients with an unsatisfactory percentage biventricular pacing.

The study protocol 60 patients with heart failure NYHA (New York Heart Association) II-IV treated with optimal medical therapy as clinically indicated who received CRT-D device more than 3 months ago. Patients with biventricular pacing <95% will and heart rate <70 at rest and >50% of heart rate in device memory >70 will receive ivabradine. The minimal follow-up of patients in the study will be at least six months.

Approximately 30% of CRT recipients do not respond to therapy. One of the causes ot that is unsatisfactory percentage of biventricular pacing. Patients may loose biventricular pacing because of inadequate sinus tachycardia.

Ivabradine may prevent inadequate sinus tachycardia and improve quality of live of CRT recipients.

According to current guidelines ivabradine is recommended in patients with symptomatic heart failure with heart rate at rest >70.

The hypothesis of this study is that ivabradine may increase percentage of biventricular pacing in CRT recipients who have unsatisfactory percentage of biventricular pacing and >50% of heart rate in device memory >70.

Inclusion Criteria:

• heart failure NYHA II-IV
• left ventricular ejection fraction =<35%
• CRT-D implanted over 3 months ago
• optimal CRT-D parameters
• biventricular pacing <95% despite the optimal parameters of the device
• optimal pharmacotherapy with the highest well-tolerated beta-adrenolytic dosage
• heart rate at rest below 70 bpm
• over 50% of heart rhythm over 70 bpm at interrogation with the device

Exclusion Criteria:

• persistent atrial fibrillation/flutter
• device associated ineffective resynchronization
• contraindications to ivabradine