Background: Patients at increased risk for sudden cardiac death (SCD) may receive an implantable cardioverter defibrillator (ICD). The primary criterion for a primary prevention ICD implantation is a left ventricular ejection fraction (LVEF) below 35%, but refinement of ICD criteria is important since only a small proportion of ICD patients receives appropriate device therapy (ATP or a shock) during follow-up. Post-extrasystolic potentiation (PESP) may be a new risk marker for SCD. PESP is defined as a temporary increase in contractility that follows an extrasystolic beat (ESB) and is associated with myocardial calcium handling. In heart failure, changes in calcium homeostasis may lead to afterdepolarisations and thus predispose for SCD. PESP can be measured indirectly and non-invasively as post-extrasystolic blood pressure potentiation (PESP-BP). Abnormal PESP-BP was previously found to be an independent predictor of increased mortality in post-myocardial infarction patients with a reduced LVEF. However, it is unknown if this increased mortality in heart failure patients with abnormal PESP-BP is caused by an increased risk of SCD.

Hypothesis: The investigators hypothesize that PESP-BP might be a new predictor of the occurrence of SCD, and can be used to enhance patient selection for primary prevention ICD therapy.

Design: During scheduled device replacement ESB with various extrasystolic and post-extrasystolic coupling intervals will be evoked by electrical stimulation via the right atrial and ventricular device leads of the patient. Throughout the stimulation study blood pressure will be measured non-invasively a continuous electrocardiogram will be recorded. Either before or after the procedure, patients will undergo a 30-minutes assessment of spontaneous ESB, again with blood pressure and ECG recordings.

Study population: 30 patients who are scheduled for device replacement or reposition, are eligible for this study; (1) 10 ICD patients who previously received appropriate device therapy (ADT); (2) 10 ICD patients who are free from ADT and (3) 10 dual-chamber pacemaker patients (control group).

Outcomes: (1) Evoked PESP-BP (i.e. blood pressure differences between baseline, ESB and post-ESB); (2) Spontaneous PESP-BP (i.e. blood pressure differences between baseline, ESB and post-ESB); (3) Timing parameters (in ms): the basic cycle length interval; Extra-systolic interval (ESI); Post-extrasystolic interval (PESI).

Inclusion Criteria:

• Implanted with a dual-chamber device
• For ICD patients: a LVEF ≤35%, measured recently
• For Pacemaker patients: a LVEF > 50% measured both recently
• A device follow-up of at least one year must be available
• Optimal (stable) medical therapy
• Sinus rhythm

Exclusion Criteria:

• Age <18 or incapacitated adult
• Unknown left ventricular function prior to device implantation
• Patients unwilling to participate
• Documented atrial fibrillation
• Second or third degree atrioventricular (AV) conduction disorders;
• Patients with a cardiac resynchronization therapy (CRT-D) or one-chamber device
• Hypertrophic cardiomyopathy
• Conditions with insufficient blood flow to the fingers, e.g. M. Raynaud or conditions with extreme vasoconstriction

30 patients who are scheduled for device replacement or reposition, are eligible for this study; (1) 10 ICD patients who previously received appropriate device therapy (ATP/shock); (2) 10 ICD patients who are free from ADT and (3) 10 dual-chamber pacemaker patients (control group)