Effects of Atrial Rate-adaptive Pacing on Exercise Capacity in Patients With Chronic Heart Failure Complicated by Chronotropic Incompetence

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.