OBJECTIVES: This study hypothesized that microvolt T-wave alternans(MTWA) improves selection of patients for implantable cardioverter-defibrillator(ICD) prophylaxis, especially by identifying patients who are not likely to benefit. BACKGROUND: Many patients with left ventricular dysfunction are now eligible for prophylactic ICDs, but most eligible patients do not benefit; MTWA testing has been proposed to improve patient selection. METHODS: Our study was conducted at 11 clinical centers in the U.S. Patients were eligible if they had a left ventricular ejection fraction(LVEF) ≤ 0.40 and lacked a history of sustained ventricular arrhythmias; patients were excluded for atrial fibrillation, unstable coronary artery disease, or New York Heart Association functional class IV heart failure. Participants underwent an MTWA test and then were followed for about two years. The primary outcome was all-cause mortality or non-fatal sustained ventricular arrhythmias. RESULTS: Ischemic heart disease was present in 49% , mean LVEF was 0.25, and 66% had an abnormal MTWA test. During 20± 6 months of follow-up, 51 end points(40 deaths and 11 non-fatal sustained ventricular arrhythmias) occurred. Comparing patients with normal and abnormal MTWA tests, the hazard ratio for the primary end point was 6.5 at two years(95% confidence interval 2.4 to 18.1, p< 0.001). Survival of patients with normal MTWA tests was 97.5% at two years. The strong association between MTWA and the primary end point was similar in all subgroups tested. CONCLUSIONS: Among patients with heart disease and LVEF ≤ 0.40, MTWA can identify not only a high-risk group, but also a low-risk group unlikely to benefit from ICD prophylaxis. OBJECTIVES: This study hypothesized that microvolt T-wave alternans (MTWA) improves selection of patients for implantable cardioverter-defibrillator (ICD) prophylaxis, especially by identifying patients who are not likely to benefit. BACKGROUND: Many patients with left ventricular dysfunction are now eligible METHODS: Our study was conducted at 11 clinical centers in the US Patients were eligible if they had a left ventricular ejection fraction (LVEF) ≤ 0.40 and lacked a history of sustained ventricular arrhythmias; patients were excluded for atrial fibrillation, unstable coronary artery disease, or New York Heart Association functional class IV heart failure. Participants underwent an MTWA test and then were for about two years. The primary outcome was all-cause mortality or non-fatal sustained ventricular arrhythmias. RESULTS: Ischemic heart disease During 20 ± 6 months of follow-up, 51 end points (40 deaths and 11 non-fatal sustained ventricular arrhythmias) occurred. Comparing patients with 49%, mean LVEF was 0.25, and 66% had an abnormal MTWA test. normal and abnormal MTWA tests, the hazard ratio for the primary end point was 6.5 at two years (95% confidence interval 2.4 to 18.1, p <0.001). Survival of patients with normal MTWA tests was 97.5% at two years. The strong association among patients with heart disease and LVEF ≤ 0.40, MTWA can identify not only a high-risk group, but also a low-risk group unlikely to benefit from ICD prophylaxis .