Inclusion Criteria

• Age ≥ 18 years old.
• Able to provide informed consent to enroll in the trial, or consent through a legal guardian or power of attorney.
• Previous clinical diagnosis of HF with current NYHA Class II–III symptoms.
• At least one of the following:
  • Hospitalization for decompensated HF;
  • Acute treatment for HF with an intravenous diuretic or hemofiltration;
  • Chronic treatment with a diuretic for control of HF symptoms + left atrial enlargement or elevated E/e’ ratio (≥ 14 average, ≥ 15 septal) on echocardiography;
  • Resting PCWP ≥ 15 mm Hg or LV end-diastolic pressure > 18 mmHg at catheterization for dyspnea, and/or exercise PCWP/LV end-diastolic pressure > 25 mmHg;
  • Elevated NT-proBNP level (≥ 300 pg/ml).
• Left ventricular EF ≥  40% within 12 months with clinical stability.
• Stable cardiac medical therapy for ≥ 30 days.
• Sinus rhythm.
• Chronotropic incompetence on recent (within 6 months) clinical exercise test, defined as heart rate reserve (HRR) < 0.80 or < 0.62 if on beta blockers:
  • HRR = [observed peak HR – observed rest HR]/[predicted peak HR – observed rest HR];
  • Predicted peak HR will be calculated using the formula (220-age).
• Meet both screening criteria on clinically-performed CPX within 12 months.

Exclusion Criteria:

• Discernable ventilatory anaerobic threshold on previous CPX study.
• Inability to exercise, or non-cardiac condition that precludes exercise testing.
• Any contraindication to a pacemaker system.
• Non-cardiac condition limiting life expectancy to less than one year.
• Significant left sided structural valve disease (> mild stenosis, > moderate regurgitation).
• Hypertrophic obstructive cardiomyopathy.
• Infiltrative or inflammatory myocardial disease (amyloid, sarcoid).
• Pericardial disease.
• Non-group 2 pulmonary arterial hypertension.
• Chronic stable exertional angina.
• Acute coronary syndrome or revascularization within 60 days.
• Other clinically important causes of dyspnea.
• Atrial fibrillation.
• PR interval > 210 msec.
• Resting heart rate (HR) > 100 bpm.
• A history of reduced ejection fraction (EF < 40%).
• Advanced chronic kidney disease (GFR < 20 ml/min/1.73m^2 by modified MDRD equation).
• Women of child bearing potential without negative pregnancy test and effective contraception.
• Severe anemia (Hemoglobin < 10 g/dL).
• Severe hepatic disease.
• Complex congenital heart disease.
• Listed for cardiac transplantation.
• Other class I indications for pacing.

Eligibility last updated 11/9/21. Questions regarding updates should be directed to the study team contact.