Ranolazine in Diastolic Heart Failure

Inclusion Criteria: 1. Males or females aged \> 40 years 2. Clinical symptoms of heart failure (NYHA class II-III) at time of screening (e.g., dyspnea, paroxysmal nocturnal dyspnea, orthopnea, bilateral lower extremity edema) 3. Left ventricular ejection fraction (LVEF) \> 45% at screening 4. With: * E/E' \> 15 measured by Tissue Doppler echocardiography at screening * NT-pro-BNP \> 220pg/mL at screening * Average resting LVEDP \>18 mm Hg (refer to continued eligibility criteria), * Average resting time constant of relaxation (tau) \> 50 ms at time of cardiac catheterization (refer to continued eligibility criteria) 5. Signed informed consent Exclusion Criteria: 1. Acute cardiac decompensation requiring mechanical ventilation 2. Hypotension with blood pressure \< 90/50 mm Hg 3. Primary hypertrophic or restrictive cardiomyopathy or systemic illness associated with infiltrative heart disease (e.g., cardiac amyloidosis) 4. Pericardial constriction 5. Hemodynamically significant uncorrected obstructive or regurgitant valvular disease 6. Cor pulmonale or other causes of right heart failure not associated with left ventricular dysfunction 7. Clinically significant pulmonary disease in the opinion of the Investigator or requiring home oxygen or oral steroid therapy 8. History of serious cardiac dysrrhythmias including atrial fibrillation with resting heart rate of \> 100 beats per minute 9. Need for treatment with Class I or III antiarrhythmic medications 10. Implantable pacemaker, cardioverter-defibrillator, or left ventricular assist device 11. Clinically significant chronic hepatic impairment (Child-Pugh Class B \[moderate\] or Class C \[severe\]) 12. Severe renal insufficiency defined as creatinine clearance ≤30 mL/min as calculated by Cockcroft-Gault formula or Modified Diet in Renal Disease (MDRD) equation. 13. History of congenital or a family history of long QT syndrome, or known acquired QT interval prolongation 14. Inability to exercise due to other co-morbidities that may affect performance of cardiopulmonary exercise test (CPET) (e.g., osteoarthritis, peripheral vascular disease) 15. Current treatment with potent and moderate CYP3A inhibitors 16. Current treatment with potent CYP3A inducers (e.g., rifampin/rifampicin, St. John's Wort, carbamazepin/carbamazepine) 17. Prior treatment with ranolazine 18. Other conditions that in the opinion of the investigator may increase the risk to the patient (e.g. pts with weight ≤60 kg), prevent compliance with study protocol or compromise the quality of the clinical trial Continued Eligibility Criteria: Patients must continue to meet eligibility criteria and have an average (of 3 measurements) resting LVEDP \> 18 mm Hg and resting tau \> 50 ms at time of cardiac catheterization to receive study drug.