This investigation is a prospective, randomized, single-blinded and multicenter design.

The purpose of this study is to evaluate the benefit of atrial-synchronous biventricular (BiV) pacing in severely symptomatic hypertrophic obstructive cardiomyopathy (HOCM) patients with severe Left Ventricular Outflow Tract (LVOT) obstruction implanted with a Cardiac Resynchronization Therapy - Pacing (CRT-P) device.

Randomization

• Implant will be performed (CRT-P).
• Patients will be randomized 1:1 to either Treatment Group (Optimized Biventricular (DDD) pacing) or Control Group (Back-up Atrial (AAI) pacing) during the first 12 months:
• Treatment Group. The patient´s device is programmed to optimized DDD BiV pacing
• Control Group. The patient´s device is programmed to back-up pacing AAI.
• After 12 months, the patients initially randomized to the Treatment Group (Optimized DDD Pacing) will continue in the same group. The patients initially randomized to Control Group (AAI Back-up Pacing) will be changed to the Treatment Group (Optimized DDD Pacing). And all the patients will be followed 12 months more.

Data collection

• In clinic Follow ups (FU): Enrollment, Baseline, Implant, Pre-discharge, 3 months (M), 12 M, 15 M and 24 M.
• Phone call FU: 6 M, 9 M, 18 M and 21 M.

Inclusion Criteria:

1. Unequivocal diagnosis of hypertrophic cardiomyopathy (HCM), on the basis of 2-dimensional echocardiographic demonstration of a hypertrophied (wall thickness ≥15 mm) and nondilated Left Ventricle (Left Ventricle End Diastolic Diameter (LVEDD) < 55mm) confirmed by a Core Lab Echo (Appendix I).
2. Significant resting (not provoked) LV outflow tract obstruction, on the basis of peak LVOT gradient ≥50 mmHg, estimated by continuous wave Doppler and confirmed by a Core Lab Echo (Appendix I).
3. Presence of refractory symptoms (exertional dyspnea or chest pain) despite of optimal treatment with betablockers and/or verapamil for at least 3 months (NYHA class >II).
4. Patients that refuse or have contraindication for septal myectomy or septal ablation (i.e., comorbidity, inappropriate coronary anatomy for septal ablation), that prefer cardiac pacing, or that are at high risk for developing heart block following septal myectomy or septal ablation.

6. Patient's age is 18 years or greater. 7. Patients must indicate their understanding of the study and willingness to participate by signing the appropriate informed consent form.

8. Patients must be willing and able to comply with all study requirements.

Exclusion Criteria:

1. Known causes of cardiac hypertrophy as infiltrative cardiomyopathy, aortic stenosis and severe uncontrolled hypertension.
2. Permanent or persistent atrial fibrillation.
3. Previous septal myectomy or septal ablation.
4. Any indication for permanent pacing, except for HOCM.
5. Any indication for an Implantable Cardioverter Defibrillator (ICD).
6. Systemic disease that would preclude completion of the protocol.
7. Any disability or limitations to correctly understand or complete the study (physical, intellectual, logistical).
8. Patients with a life expectancy <24 months (based on investigator assessment).
9. Patients who are or may potentially be pregnant.