Development of ventricular fibrillations with different characteristics inthe local electrocardiogram - Large and small amplitude of activation, andits implication for implantable cardioverter defibrillator treatment

An implantable cardioverter defibrillator (ICD) was implanted in 2 patients with ventricular tachyarrhythmia related to old myocardial infarction, and defibrillation tests were attempted at the time of ICD implantation and at 2 or 4 weeks after the operation. Ventricular fibrillation (VF) was induce d by T-wave shocks, but the amplitude of the ventricular electrogram was di fferent in each VF. In most of the VFs with large ventricular electrograms, the local activity was appropriately detected. However, many undersensed b eats were observed in other VFs that had fine ventricular electrograms and a longer time was needed before delivering the shock. The amplitude of the ventricular electrogram might be small in some cases of VF and this might r esult in undersensing and/or unsuccessful defibrillation. Close attention m ust be paid to the amplitude of ventricular activation in each VF to avoid possible difficulty in ICD therapy.