Intracardiac QRS Electrogram Width—An Arrhythmia Detection Feature for Implantable Cardioverter Defibrillators: Exercise Induced Variation as a Base for Device Programming

Delivery of inappropriate therapy of implantable cardioverter defibrillators (ICD) due to inaccurate arrhythmia detection represents a major clinical problem. Different arrhythmia detection criteria such as the "stability" of the cycle length or the suddenness of "onset" of tachycardia have been implemented in ICD software to prevent inappropriate therapy. The new Medtronic model 7223Cx ICD offers an additional detection parameter (QRS width), which reflects changes in the duration of ventricular depolarization as a tool to distinguish supraventricular from ventricular tachycardias. Although this criterion can be programmed based on ECG parameters derived from resting ECGs, this may not be sufficient since QRS width is subject to considerable changes due to transient myocardial ischemia, changes in autonomic tone, or frequency dependent effects of antiarrhythmic drugs. The present study aimed to determine frequency dependent changes in QRS width in individual patients at rest and during symptom-limited exercise testing in 16 patients with documented ventricular tachycardia (N = 13) or ventricular fibrillation (N = 3). The optimal EGM slew threshold and the individual variation of QRS width were determined. Measurements obtained at the end of the implantation procedure were compared to those performed at hospital discharge. The majority of patients showed a wider variation in QRS duration as measured from 30 consecutive cycles during exercise as compared to rest. For example, the QRS range (i.e., the difference between the maximal and the minimal QRS width measured) averaged 7 +/- 3 ms at rest and increased to 11 +/- 3 ms during exercise (P = 0.004) with an increase of > or = 4 ms observed in 11 (69%) of 16 patients. In 13 (81%) of 16 patients a reprogramming of at least one QRS width parameter from its value at the time of implantation was necessary. Thus, the QRS width measured from the intracardiac EGM shows significant intraindividual variations in different physiological conditions. For optimal programming of the QRS width parameter, measurements obtained during exercise are important.