The natural history of hypertrophic cardiomyopathy(HCM)is extremely heterogeneous.Many patients remain asymptomatic throughout life,some develop severe symptoms of heart failure,but others die suddenly.often in the absence of previous symptoms and at a young age.Therefore,identification of those patients at high risk of sudden death represents a major clinical problem and has become an even greater challenge since the implantable cardioverter-defibrillator(ICD)has proved to be highly effective in preventing sudden death in HCM.Patients who have survived a cardiac arrest,or one or more episodes of sustained ventricular tachycardia,are considered to be at high risk and are candidates for an ICD.However,this patient subset represents a small proportion of the HCM population.The greatest difficulty concerns the identification of high risk patients who are candidates for primary prevention of sudden death with a prophylactic ICD.Decisions are based on generally accepted clinical markers which are associated with increased risk,including:family history of sudden death,extreme left ventricular(LV)wall thickness(≥30 mm),nonsustained ventricular tachycardia on Holter monitoring,unexplained(non-neurocardiogenic)syncope particularly in young patients,and hypotensive blood pressure response to exercise.Patients with end-stage HCM or a LV apical aneurysm represent important arrhythmogenic subsets also associated with substantially increased risk.Multiple or single strong risk markers are associated with increased sudden death risk and iustify consideration for a prophylactic ICD.

Hypertrophic cardiomyopathy (HCM) is the most common cause of sudden cardiac death in young people. The implantable cardioverter-defibrillator (ICD), has recently proved to be a safe and effective therapeutic intervention in patients with HCM, both for the primary and secondary prevention of sudden death. Based on recent substantial experience, the ICD intervenes appropriately to termiante ventricular tachycardia/fibrillation (VT/VF), at a rate of 5.5%/year. ICD discharge rate is 4%/year in those patients implanted prophylactically due to one or more major risk markers, but often with considerable delays of up to 10 years before the device is required to intervene appropriately to terminate potentially letal ventricular tachyarrhythmias. Primary prevention of VT/VF occurs with similar frequency in high-risk patients having either 1,2 or ≥ 3 noninvasive risk markers, and about one-third of patients with appropriate device interventions had been implanted for only one risk factor. The ICD has proved reliable in HCM despite the extreme and complex phenotypes often present with massive degrees of left vcntricular hypertrophy, microvascular ischemia, diastolic dysfunction, or dynamic left ventricular outflow tract obstruction. Failure to convert life-threatening ventrieular tachyarrhythmias to normal rhythm is extraordinarily rare. In conclusion, in high-risk HCM patients, ICDs perform in a highly effective fashion,frequently preventing sudden death by aborting primary life-threatening ventricular tachyarrhythmias. A single marker of high risk can be sufficient evidence to justify the recommendation for a prophylactic ICD in selected patients with HCM.