• Clinically Significant Adverse Events (all cause mortality, hospitalization...) measured from randomization until 17 m follow-up.
  

• Number of each of the components of the CSAE
  
• Atrial tachyarrhythmias related (burden, num episodes...)
  
• Ventricle tachyarrhythmias related (burden, number of episodes...)
  
• NYHA, LVEF, CAD, exercise capacity, medication
  
• Quality of live questionnaires
  
• all measured at baseline, 8 months ant 17 months.
  

Single chamber (SC) implantable cardioverter defibrillators (ICDs) have several limitations that might be relevant during follow up, like atrial pacing requirements, inadequate therapies, sustained atrial tachyarrhythmias and difficulties to achieve an accurate diagnosis of the arrhythmia. Dual chamber (DC) ICDs offer an attractive rationale solution, although controversy remains if the costs and complexity of the implants offer a real clinical advantage.

The DATAS is a, prospective, multicentre, randomized, open labelled study, with three arms (two of them cross-over and the third parallels the other two)

The primary objective of this trial is to determine whether use of DDED ICD results in a significant decrease in the number of primary end points.

The primary end point is a composite comprising four so-denominated "Clinically Significant Adverse Events":

1. all-cause mortality
2. invasive intervention, hospitalization (>24 h) or prolongation of hospitalization due to cardiovascular cause
3. inappropriate shocks (two or more episodes with inappropriate shocks)
4. sustained symptomatic atrial tachyarrhythmias that (a) require urgent termination or (b) last more than 48 h leading to therapeutic intervention.

Secondary objectives are:

1. Number of each of the components of the CSAE.
2. Arrhythmia related: atrial tachyarrhythmia, frequency and burden, ventricular tachyarrhythmia frequency and burden number of appropriate shocks, number of inappropriate shocks, need for reprogramming, need for medication/RFA for arrhythmia control, pacemaker syndrome and development of dual chamber pacing indication.
3. Cardiovascular related: NYHA functional class, exercise capacity, left ventricular ejection fraction (LVEF), reduction of medication (diuretics.).
4. Quality of life: evaluated by the SF-36, Minnesota living test, with heart failure and Symptom Checklist instruments.

Inclusion Criteria:

• Meet the Class I implantation criteria for single chamber implantable cardioverter defibrillator according to the guidelines (ACC/AHA).

Exclusion Criteria:

• Permanent atrial fibrillation
• Patients without structural heart disease
• Patient meets implantation criteria for dual-chamber pacing (symptomatic sinus node disease, all 2nd AV block [except asymptomatic Mobitz I] and all 3rd degree AV block ).
• Patient with previous system implanted (ICD or pacemaker).
• Patients with biventricular stimulation or re-synchronization.
• Patient has a mechanical right heart valve.
• Patient has medical conditions that would preclude the testing required by the protocol, or limit study participation.
• Patient is unwilling or unable to cooperate or give written informed consent, or the patient is a minor and legal guardians refuse to give informed consent.
• Patient is or will be inaccessible for follow-up at the study center.
• Patients who are enrolled or planning to enroll in other clinical trials during the clinical study.