This study is designed as a multicenter, prospective, controlled, randomized, two-arms, cross-over, two parallel cohorts, efficacy Clinical research with CE-marked medical device . The crossover trial will be performed in a blind fashion, so each participant will be evaluated during both blinded pacing-on and pacing-off phases, each of which will last 3 months. The population includes patients with the diagnosis of stable HFpEF according to criteria of the European Society of Cardiology and New York Heart Association (NYHA) functional class II-III/IV, left ventricular hypertrophy with small ventricular volume, sinus rhythm with low basal heart rate and chronotropic incompetence (CI).

All patients will undergo pacemaker implantation, and computer-generated randomization sequence previously designed will be used to allocate participants (in a 1:1 ratio) to receive: (a) programming of the pacemaker with pacing On followed by pacing Off in two periods of 3 months; or (b) programming of the pacemaker with pacing Off followed by pacing On in two periods of 3 months. The study will be conducted in three centers in Spain. Discounting the time due to staggered entry, the total duration of a patient's follow-up will be 7 months.

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.