[Stored electrograms to differenciate between adequate and inadequate ICD therapy].

The introduction of implantable cardioverter defibrillators in 1980 by Mirowski et al, offered a new therapeutic device for the treatment of ventrikular tachyarrhythmias. In the beginning it was only possible to evaluate arrhythmic events by analysis of the therapy-counter in combination with clinical symptoms. Even rapid ventricular tachyarrhythmias may not produce significant symptoms prior to ICD shock in more than 50% of patients. On the other hand the ICD device will initiate inappropriate therapy due to sensing error and non sustained tachyarrhythmias in 20-40% of patients. Third generation ICD devices provide sophisticated diagnostic information by stored electrograms. Intracardiac electrograms are recorded by the bipolar tip electrode of the sensing lead, located in the right ventricel (near field) or by the electrodes used for energy delivery (far field). These ECG recordings provide valuable information to evaluate the type of arrhythmia, trigger mechanisms and therapeutic efficacy. The arrhythmia leading to device therapy is judged by cycle length and stability and morphology of the intracardiac signal. A correct arrhythmia classification using electrogram analysis is possible in 92-98% of arrhythmias. Evaluation is limited in 5-10% of patients with bundle branch block or aberand conduction. Inappropriate ICD therapy is caused by atrial fibrillation in more than 50% and by supraventricular tachycardia or sinus tachycardia in about 20%. Sensing of artefacts can be attributed to lead failure in about 17%. The distinction between appropriate and inappropriate therapeutic intervention by the ICD allows the prevention of unnecessary shock delivery and early diagnosis of lead failure. The development of dual chamber systems with enhanced storage capacity and improved algorithms for detection of arrhythmias may further decrease the incidence of inappropriate ICD therapies.