SHort Of Pace - Heart Failure (SHOP-HF)

Inclusion Criteria: * Stable symptomatic heart failure (NYHA functional class ≥II) during the last month. * Diagnosis criteria of HFpEF according to ESC guidelines: 1. symptoms and signs of HF. 2. left ventricular ejection fraction ≥50% by Simpson method. 3. proBNP \>125 pg/mL in the last month. 4. at least one additional criterion: 1. relevant structural heart disease (left ventricular hypertrophy and/or left atrial enlargement); and/or 2. Diastolic dysfunction. * Left ventricular hypertrophy was defined as wall thickness \> 10 mm. * Small ventricular volume was defined as indexed left ventricular end diastolic volume \< 45 ml/m2. * Adults ≥18 years old. * Previous admission for acute heart failure. * Chronotropic incompetence assessed by CPET, defined as: \[(HRmax - HRrest)\]/\[(220 - age) - (HRrest)\] \< 0.62. * Resting heart rate \< 65 bpm in sinus rhythm or 70 bmp in atrial fibrillation. Exclusion Criteria: * Inability to perform a valid baseline exercise test. * Significant primary pulmonary disease; including pulmonary arterial hypertension, chronic thromboembolic pulmonary disease or chronic obstructive pulmonary disease. * Patient with prior history of left ventricular ejection fraction \<50%. * History of an acute coronary syndrome in the previous 12 months. * Effort angina or signs of ischemia during CPET. * RER threshold at \<1.05 at the CPET. * Significant primary moderate to severe valvular disease. * Any other comorbidity with a life expectancy lower than 1 year. * Heart rate at rest \> 75 lpm. * Other pacemaker indication. * Pregnant women. * Baseline rhythm different from sinus rhythm or atrial fibrillation. * Active treatment with beta-blockers, digitalis or non dihidropiridine calcium channel blockers.