Evaluate Safety and Efficacy of the OPTIMIZER® System in Subjects With Moderate-to-Severe Heart Failure: FIX-HF-5C

Inclusion Criteria: 1. Subjects who are 18 years of age or older 2. Subjects who are either male or female. Females of childbearing potential must be using a medically approved method of birth control and must agree to continue to use birth control throughout the study, or must be surgically sterilized (tubal ligation, hysterectomy) or post-menopausal for at least 1 year. 3. Condition 1. Subjects who have a baseline ejection fraction greater than or equal to 25% and less than or equal to 45% by echocardiography determined by the echocardiography core laboratory. 2. Subjects who have been treated for heart failure for at least 90 days (including treatment with a β-blocker for at least 90 days unless the subject is intolerant) and are in New York Heart Association functional Class III and IV at the time of enrollment. 3. Subjects receiving appropriate, stable medical therapy during the 30 days prior to enrollment for treatment of heart failure according to the region- specific guideline recommendations. For patients with EF≤35%, this regimen shall consist of the appropriate doses of diuretics, ACE-inhibitor or angiotensin II receptor blocker and β-blocker. Stable is defined as no more than a 100% increase or 50% decrease in dose. 4. Subjects who, in the opinion of the Principal Investigator (based on the current guidelines for clinical practice ), have a clinical indication for an implanted cardiac defibrillator (ICD, e.g., EF≤35%) and/or pacemaker, must have an existing device or agree to undergo implantation of such a device unless the patient refuses to undergo the implantation of such device for personal reasons. 5. Subjects who are willing and able to return for all follow-up visits. Exclusion Criteria: 1. Subjects whose baseline peak VO2 is \<9 or \>20 ml O2/min/kg. 2. Subjects who have a potentially correctible cause of heart failure, such as valvular heart disease or congenital heart disease. 3. Subjects who have clinically significant angina pectoris, consisting of angina during daily life (i.e., Canadian Cardiovascular Society Angina score of II or more), an episode of unstable angina within 30 days of enrollment, or angina and/or ECG changes during exercise testing performed during baseline evaluation. 4. Subjects who have been hospitalized for heart failure which required the use of inotropic support within 30 days of enrollment. 5. Subjects who have a clinically significant amount of ambient ectopy, defined as more than 8,900 PVCs per 24 hours on baseline Holter monitoring. 6. Subjects having a PR interval greater than 375 ms. 7. Subjects who have chronic (permanent or persistent) atrial fibrillation or atrial flutter or those cardioverted within 30 days of enrollment. 8. Subjects whose exercise tolerance is limited by a condition other than heart failure (e.g., angina, COPD, peripheral vascular disease, orthopedic or rheumatologic conditions) or who are unable to perform baseline stress testing. 9. Subjects who are scheduled for a CABG or a PTCA procedure, or who have undergone a CABG procedure within 90 days or a PTCA procedure within 30 days of enrollment. 10. Subjects who have a biventricular pacing system, an accepted indication for such a device, or a QRS width of 130ms or greater. 11. Subjects who have had a myocardial infarction within 90 days of enrollment. 12. Subjects who have mechanical tricuspid valve. 13. Subjects who have a prior heart transplant. 14. Subjects on dialysis. 15. Subjects who are participating in another experimental protocol. 16. Subjects who are unable to provide informed consent.