Patients with heart failure and an implantable cardioverter-defibrillator (ICD) for primary prevention are at increased mortality risk after receiving shock therapy. We sought to determine the prognostic significance of ICD therapies, both shock and antitachycardia pacing, delivered for different ventricular arrhythmia (VA) rates.
We evaluated mortality risk among 1790 ICD -implanted patients from MADIT -CRT (Multicenter Automatic Defibrillator Implantation Trial With Cardiac Resynchronization Therapy). For the first analysis, patients were divided into mutually exclusive groups by the rate of treated VA only: slow VA (<200 beats per minute) and fast VA (≥200 beats per minute or ventricular fibrillation). In a secondary analysis, both the type of ICD therapy and VA rate were used. The reference group was always patients who had no ICD therapy. ICD therapy for fast VA was associated with increased mortality risk (hazard ratio [HR]=2.27; 95% CI, 1.48 to 3.48; P<0.001). However, mortality risk after ICD therapy for slow VA was similar to the risk related to no ICD therapy (HR=1.45; 95% CI=0.86-2.44; P=0.162). Consistently, shocks (HR=2.96; 95% CI, 1.91 to 4.60; P<0.001) and antitachycardia pacing (HR=2.22; 95% CI; 0.96 to 5.14; P=0.063) for fast VA were both associated with increased mortality risk. Shocks and antitachycardia pacing for slow VA were not significantly associated with increased mortality risk (HR=1.43 [95% CI, 0.52-3.92; P=0.489]; and HR=1.43; 95% CI, 0.80 to 2.56; P=0.232, respectively).
In patients with mild heart failure receiving ICD for primary prevention, mortality is associated with the rate of underlying VA rather than the type of therapy. These findings suggest that fast VA is a marker for increased mortality rather than shock therapy directly contributing to increased risk.