The optimal pacing rate during cardiac resynchronization therapy (CRT) is not known. Investigators investigate the impact of changing basal pacing frequencies on autonomic nerve function, cardiopulmonary exercise capacity and self-perceived quality of life (QoL).

This is a double-blinded randomized crossover pilot study testing two basal pacing rates: DDD-60 versus DDD-80. Each patient is paced for 2 periods of 90 days with DDD-60 and DDD-80 in a randomly sequence.

Blood samples NT-proBNP plasma levels are analysed at baseline and at the end of each phase using electrochemiluminescence-immunoassay. Patients rest for 15 minutes before the samples are taken.

These result are monitored by supervisory group to ensure that increase of more than 15% than baseline values result in termination of study.

Investigators conduct two distinct questionnaires (Minnesota Living with Heart Failure (LIfHE) questionnaire and Medical Outcome Survey Short Form (SF-36)) A symptom limiting bicycle (Ergoline, Baden-Württemberg, Germany) exercise test is performed to measure maximum work capacity and oxygen consumption at each phase.

Recordings of resting muscle sympathetic nerve activity (MSNA) are sought in all patients at both rates of pacing. All patients are studied on the same set-up in the semi-recumbent position after 30 minutes of rest. Tungsten microelectrodes (Iowa Bioengineering) are inserted into the common peroneal nerve just below the knee.

Inclusion Criteria:

• Diagnosis with idiopathic dilated cardiomyopathy
• CRT device implanted (with initial indication in accordance with European guidelines) at least 6 months prior to inclusion.
• New York Heart Association functional (NYHA) class II-IV
• Sinus rhythm,
• Hemodynamically stable
• Biventricular pacing > 90% of the time
• On optimal medical treatment for heart failure with no changes in medications during the past 3 months.

Exclusion Criteria:

• NYHA I
• unable to perform exercise test (e.g. chronic obstructive pulmonary disease, severe arthritis)
• cancer
• plasma creatinine > 200 micromole per litre
• an admission for decompensated HF or acute coronary syndrome in the preceding 3 months.