The purpose of this research study is to evaluate the safety of a new pacing method (Fusion Pacing) for heart failure patients with an ejection fraction (EF) greater than 50% over a period of 4 months, and determine the effects on different measures of heart failure. This new pacing method may allow the heart to pump more efficiently. Participation in this study will last approximately 8 months.

Patients with an EF greater than 50% and a regional mechanical delay of >= 65ms (an indication of mechanical dyssynchrony) will receive an implantable cardioverter defibrillator (ICD) with a special pacing feature. When the feature is active, the device wil pace the left ventricle to target improved pump efficiency. Patients will have the feature active for 4 months and inactive for 4 months. The order will be randomized and the patient will not know whether the pacing is active or not. Echocardiography, biomarkers, exercise testing, quality of life (QOL) testing and device information will be used to compare the patient's status while the feature is active vs. inactive.

Inclusion Criteria:

• Patients with quantitative ejection fraction (EF) of greater than 50 percent
• Patients with active heart failure (symptomatic American Heart Association/American College of Cardiology stage C) despite optimal medications
• Patients greater than 18 years of age
• Patients willing and able to give informed consent
• Patients available for appropriate follow-up times for the length of study
• Patients able to complete cardiopulmonary exercise (CPX) testing
• Patients expected lifespan is greater than 12 months beyond study enrollment
• Patients with a regional mechanical delay (RMD) (septal-lateral) greater than 65 milliseconds

Exclusion Criteria:

• Patients who have undergone cardiac surgery 60 days prior to enrollment
• Patients who have had a myocardial infarction 90 days prior to enrollment
• Patients with hemodynamically significant uncorrected cardiac valvular disease
• Patients with active myocarditis and amyloidosis (if documented)
• Patients with a permanent pacemaker
• Patients with creatinine greater than 2 milligrams per milliliter or estimated glomerular filtration rate (eGFR) less than 30 to 50 milliliters per hour
• Patients with permanent or chronic sustained atrial fibrillation (fibrillation lasting 7 days or more at a time)
• Patients with a PR interval greater than 250 milliseconds
• Patients with severe chronic obstructive pulmonary disease
• Patients with high risk coronary artery disease
• Patients who are or expect to become pregnant during this study
• Patients enrolled in concurrent studies which could confound the results of this study
• Patients with hypertrophic cardiomyopathy (obstructive or non-obstructive)
• Patients with infiltrative disease or restrictive cardiomyopathy
• Patients with indication for percutaneous coronary intervention (PCI) at enrollment