CONTEXT: Recently, the implantable cardioverter-defibrillator (ICD) has been 
promoted for prevention of sudden death in hypertrophic cardiomyopathy (HCM). 
However, the effectiveness and appropriate selection of patients for this 
therapy is incompletely resolved.
OBJECTIVE: To study the relationship between clinical risk profile and incidence 
and efficacy of ICD intervention in HCM.
DESIGN, SETTING, AND PATIENTS: Multicenter registry study of ICDs implanted 
between 1986 and 2003 in 506 unrelated patients with HCM. Patients were judged 
to be at high risk for sudden death; had received ICDs; underwent evaluation at 
42 referral and nonreferral institutions in the United States, Europe, and 
Australia; and had a mean follow-up of 3.7 (SD, 2.8) years. Measured risk 
factors for sudden death included family history of sudden death, massive left 
ventricular hypertrophy, nonsustained ventricular tachycardia on Holter 
monitoring, and unexplained prior syncope.
MAIN OUTCOME MEASURE: Appropriate ICD intervention terminating ventricular 
tachycardia or fibrillation.
RESULTS: The 506 patients were predominately young (mean age, 42 [SD, 17] years) 
at implantation, and most (439 [87%]) had no or only mildly limiting symptoms. 
ICD interventions appropriately terminated ventricular tachycardia/fibrillation 
in 103 patients (20%). Intervention rates were 10.6% per year for secondary 
prevention after cardiac arrest (5-year cumulative probability, 39% [SD, 5%]), 
and 3.6% per year for primary prevention (5-year probability, 17% [SD, 2%]). 
Time to first appropriate discharge was up to 10 years, with a 27% (SD, 7%) 
probability 5 years or more after implantation. For primary prevention, 18 of 
the 51 patients with appropriate ICD interventions (35%) had undergone 
implantation for only a single risk factor; likelihood of appropriate discharge 
was similar in patients with 1, 2, or 3 or more risk markers (3.83, 2.65, and 
4.82 per 100 person-years, respectively; P = .77). The single sudden death due 
to an arrhythmia (in the absence of advanced heart failure) resulted from ICD 
malfunction. ICD complications included inappropriate shocks in 136 patients 
(27%).
CONCLUSIONS: In a high-risk HCM cohort, ICD interventions for life-threatening 
ventricular tachyarrhythmias were frequent and highly effective in restoring 
normal rhythm. An important proportion of ICD discharges occurred in primary 
prevention patients who had undergone implantation for a single risk factor. 
Therefore, a single marker of high risk for sudden death may be sufficient to 
justify consideration for prophylactic defibrillator implantation in selected 
patients with HCM.