Chronotropic incompetence consists of an insufficient increase in heart rate during effort, and its presence is recognized as a common feature in patients with heart failure due to left ventricular systolic dysfunction, apparently suggesting a worse prognosis. Little is known about the possible benefits of its reversal in such patients.

The investigators working hypothesis is that the modulation of chronotropic response, as obtained by means of atrial rate-adaptive pacing may improve functional capacity in persons with chronic heart failure and chronotropic incompetence.

To explore this hypothesis,the investigators will enroll 20 patients with NYHA II/III heart failure, low left ventricular ejection fraction (<40%) and chronotropic incompetence (Maximal heart rate <80% of predicted value in a symptom-limited incremental test), who already underwent implantation of dual-chamber implantable defibrillator for prevention of sudden cardiac death. The study will have a randomized, double-blind, cross-over design.

The procedures, to be carried out at one month from each reprogramming (VVI backup pacing vs. AAI-R "active" pacing), will comprise: blood sampling for NT-proBNP, incremental symptom-limited cardiopulmonary exercise testing (CPX), constant-workload cardiopulmonary test (50% of max WR), quality-of-life questionnaire, 24-hour ECG monitoring.

The primary end-point will be peak oxygen consumption on CPX. Secondary end-points will include acute response to reprogramming, and data derived from constant-WR tests, Holter monitoring and QoL.

Inclusion Criteria:

• NYHA II/III chronic heart failure on optimal medical therapy
• Sinus Rhythm
• Left ventricular ejection fraction less than 40 %
• Chronotropic incompetence on maximal exercise testing (maximum heart rate < 80% of predicted value)
• Age 18-75
• carrier of dual chamber ICD device
• Informed Consent

Exclusion criteria:

• Unable to perform cardiopulmonary exercise testing (for any reason)
• Absolute contraindication to maximal exercise testing
• Moderate to severe anemia (Hb<10 g/dL)
• Diagnosis of Sick Sinus Syndrome or high-degree atrioventricular block
• recent hospitalization for acute decompensated heart failure (<1 month)
• recent acute coronary syndrome (<3 months)
• Active neoplastic disease
• Active myocarditis / endocarditis
• Acute decompensated heart failure during study