Background: Implantable cardioverter-defibrillators (ICD) are increasingly used for prevention of sudden cardiac death (SCD). Although mortality risk reduction is about the same in primary and secondary prevention trials (~30%), we hypothesised that the incidence and the nature of ventricular arrhythmias is different in high risk ICD recipients without prior arrhythmias compared to patients who presented with life threatening arrhythmias.
Methods: A hundred consecutive ICD recipients were allocated to 2 groups: 1) secondary prevention: an ICD was implanted for secondary prevention of episodes of ventricular tachycardia (VT) or ventricular fibrillation (VF). 2) primary prevention: patients at high risk of SCD without prior arrhythmias. They were prospectively followed and the incidence of appropriate ICD therapies was determined by reviewing stored electrograms.
Results: During a mean follow-up of 20 (10) months, the overall mortality was 5% and 5% of the patients underwent heart transplantation. Of the 67 secondary prevention patients, 40% (n = 27) had VT/VF triggering ICD therapy, whereas only 15% (n = 5) of the 33 primary prevention patients had VT/VF triggering ICD therapy (p <0.05). The adjusted hazard ratio for arrhythmias triggering ICD interventions in the primary prevention group was 0.345 (95% confidence interval 0.132 to 0.902, p = 0.03).
Conclusions: The risk of developing arrhythmias triggering appropriate ICD intervention was 65% lower among the primary prevention patients than in secondary prevention patients. Importantly, ICD therapies are not correlated with lives saved, and efficacy of ICD therapy in primary and secondary prevention cannot be drawn from these data. However, the low incidence of ICD use in primary prevention patients emphasises that efforts should be made to develop better instruments for stratification.