Syncope in High-Risk Cardiomyopathy Patients With Implantable Defibrillators: Frequency, Risk Factors, Mechanisms, and Association With Mortality

BACKGROUND: There is a relative paucity of studies investigating the mechanisms of syncope among heart failure patients with implantable cardioverter-defibrillators, and it is controversial whether nonarrhythmogenic syncope is associated with increased mortality. METHODS AND RESULTS: The Multicenter Automatic Defibrillator Implantation Trial-Reduce Inappropriate Therapy (MADIT-RIT) randomized 1500 patients to 3 different implantable cardioverter-defibrillator programming arms: (1) Conventional programming with therapy for ventricular tachycardia ≥170 bpm; (2) high-rate cutoff with therapy for ventricular tachycardia ≥200 bpm and a monitoring zone at 170 to 199 bpm, and (3) prolonged 60-second delay with a monitoring zone before therapy. Syncope was a prespecified safety end point that was adjudicated independently. Multivariable Cox models were used to identify risk factors associated with syncope and to analyze subsequent risk of mortality. During follow-up, 64 of 1500 patients (4.3%) had syncope. The incidence of syncope was similar across the 3 treatment arms. Prognostic factors for all-cause syncope included the presence of ischemic cardiomyopathy (hazard ratio [HR], 2.48; 95% confidence interval [CI], 1.42-4.34; P=0.002), previous ventricular arrhythmias (HR, 2.99; 95% CI, 1.18-7.59; P=0.021), left ventricular ejection fraction ≤25% (HR, 1.65; 95% CI, 0.98-2.77; P=0.059), and younger age (by 10 years; HR, 1.25; 95% CI, 1.00-1.52; P=0.046). Syncope was associated with increased risk of death regardless of its cause (arrhythmogenic syncope: HR, 4.51; 95% CI, 1.39-14.64, P=0.012; nonarrhythmogenic syncope: HR, 2.97; 95% CI, 1.07-8.28, P=0.038). CONCLUSIONS: Innovative programming of implantable cardioverter-defibrillators with therapy for ventricular tachycardia ≥200 bpm or a long delay is not associated with increased risk of arrhythmogenic or all-cause syncope, and syncope caused by slow ventricular tachycardias (<200 bpm) is a rare event. The clinical risk factors associated with syncope are related to increased cardiovascular risk profile, and syncope is associated with increased mortality irrespective of the cause. CLINICAL TRIAL REGISTRATION URL: http://www.clinicaltrials.gov. Unique identifier: NCT00947310.