Ventricular fibrillation with small amplitude of activation and its implications for implantable cardioverter defibrillator treatment

An implantable cardioverter defibrillator (ICD) was implanted in a patient with ventricular fibrillation (VF) related to old myocardial infarction. Du ring VF, amplitude of ventricular activation was small, and the ventricular sensitivity at 1.2 mV failed to detect several small ventricular activatio ns. When the sensitivity was changed to 0.3 mV, both under- and oversensed beats occurred during VF, and at the ventricular sensitivity of 0.15 mV, th e undersensed beats disappeared while oversensed beats markedly increased. Defibrillation test was repeated one and four weeks after the implantation, and these inappropriate beats were minimized at the ventricular sensitivit y of 0.3 mV. We should pay attention to the amplitude of ventricular activa tion to avoid possible trouble in ICD therapy.