The present trial is undertaken as a pilot study on selected patients, to evaluate the ability of acute hemodynamic testing to optimize pacing therapy and to predict the response of Obstructive Hypertrophic Cardiomyopathy (OHCM) to pacing, in terms of reduction of Left Ventricular Outflow Tract (LVOT) pressure gradient and functional capacity improvement.

The Primary Objective is to demonstrate a long-term reduction in LVOT obstruction at rest and/or after Valsalva/Exercise > 30% in patients responding to acute stimulation test

Inclusion Criteria:

• unequivocal diagnosis of HCM, on the basis of 2-dimensional echocardiographic demonstration of a hypertrophied (wall thickness >15 mm or > 13 mm in familial HOCM) and non dilated LV in the absence of another cardiac or systemic disease capable of producing the same magnitude of hypertrophy;
• patients in whom coronary disease has been excluded;
• presence of severe refractory symptoms (NYHA class II-IV), as evidenced by moderate-to-severe functional disability resulting from exertional dyspnea or chest pain sufficient to support a desire for alternative treatment modalities, following administration (in standard dosages) of beta-blocker and either disopyramide or verapamil, independently or in association;
• peak instantaneous LV outflow tract gradient > 50 mm Hg (resting or provoked), estimated by continuous wave Doppler;

Exclusion Criteria:

• end-stage phase of HCM (unless CRT therapy is considered);
• ejection fraction <50% (unless CRT therapy is considered);
• severe mitral valve disease (+++) not related to systolic anterior movement and/or papillary muscle abnormalities;
• age below 18 years.