This study evaluates the impact of a new pacing mode avoiding unnecessary ventricular stimulation in combination with advanced dual chamber detection with slow VT management on the clinical outcome for hospitalization and mortality and inadequate therapy in medically stable, ICD-indicated patients with impaired left ventricular function (LVEF ≤ 40%) who do not have pacing indications and no indication for Cardiac Resynchronization Therapy (CRT). It compares a new pacing mode avoiding ventricular stimulation when not needed combined with dual chamber detection with a pure ventricular back up pacing and single chamber detection criteria with pure ventricular back up pacing. Therapies are compared in a prospective, randomized, single-blinded, parallel trial with a 24-month randomized treatment period. Randomization follows a 1:1 ratio. ICD therapy is enabled for all patients throughout the study. All patients receive optimal drug therapy for arrhythmia and heart failure treatment.

All patients will receive an implantable cardioverter defibrillator OVATIO™ DR model 6550 or a later Sorin Group device offering the same functions. After Enrolment visit but before implant, patients will be randomized in two arms according to the parallel study design. Whenever possible before implant there will be the first Holter recording for the Tvar risk stratification procedure. In case Tvar recording could not be performed before implant it has to be performed before patient leaves the hospital post implant in unpaced rhythm.

The dual-chamber arm will be programmed to 3 detection zones with PARAD+ activated.

The TDI for the slow VT zone will be set to 500 ms (120 bpm - or in case the resting rate is higher than 90 bpm it is recommended to adjust this parameter to: resting rate + 30 bpm) and at least one ATP program activated as specified in table 1.

A VT zone with a TDI of 353 ms (170 bpm) in case of no history of VT or a TDI cycle length equalling slowest documented VT interval (spontaneous or induced) plus 50 ms is required. In this 2nd VT zone therapies need to be activated in this group.

AAIsafeR2 mode will be activated with a basic rate of 60 bpm. The single-chamber arm will be programmed to optimal detection with Acceleration (Onset), Stability and Long Cycle Search (VTLC) activated. A VT zone is requested in this group, with the same programming procedures as described above. Therapies will be set according to the clinical judgment of the participating investigators but a Slow VT-zone with TDI 500 ms in monitoring setting at least is required.

Inclusion Criteria:

• Patient has been prescribed the implantation for an ICD system accordingly to the relevant currently-approved ACC/AHA guidelines 1 or ESC guidelines 35 or any relevant currently-approved local guidelines for the implantation of an ICD-system
• Impaired left ventricular function demonstrated by a left-ventricular ejection fraction (LVEF) ≤ 40 %, measured by angio-scintigraphy, echocardiography, or contrast ventriculogram.
• An optimal (as determined by the enrolling physician) medical regimen.
• Patient has received all relevant information on the study, and has signed and dated a consent form.

Exclusion Criteria:

• Any generally accepted indication for standard cardiac pacing, or any contraindication for standard cardiac pacing.
• Any indication for CRT accordingly to the relevant currently-approved ACC/AHA1 or ESC35 guidelines for the implantation of a CRT system.
• Any contraindication for ICD therapy and the implant of a dual chamber ICD.
• ICD replacement
• Chronic atrial arrhythmias or cardioversion for atrial fibrillation within the past month.
• A PR interval > 250 ms or AR interval > 300 ms measured at implant.
• Hypertrophic obstructive cardiomyopathy.
• Acute myocarditis.
• Unstable coronary symptoms or myocardial infarction within the last month.
• Recent (within the last month) or planned cardiac revascularization or coronary angioplasty.
• Recently performed (in the last month) or planned cardiac surgery
• Already included in another clinical study.
• Life expectancy less than 24 months.
• Inability to understand the purpose of the study or refusal to cooperate.
• Inability or refusal to provide informed consent and, if not part of the informed consent, a Health Insurance Portability and Accountability Act (HIPAA) authorization.
• Unavailability for scheduled follow-up at the implanting or cooperating center.
• Age of less than 18 years.
• Pregnancy