Morphological stability of bipolar and unipolar endocardial electrograms

Implantable Cardioverter Defibrillators (ICD) are widely used for sudden cardiac death prevention. In most ICD algorithms, decision making includes a morphological analysis of the unipolar and/or bipolar electrograms (EGM). The principle of such algorithms is to create a "normal" template by averaging normal sinus rhythm heartbeats, for comparison to each arrhythmic heartbeat. The present study addresses the stability of unipolar and bipolar EGMs with respect to the posture of the patient, and the temporal evolution of the EGM shapes during sinus rhythm. We show that unipolar EGMs are slightly affected by position changes, while bipolar ones are unaffected. Moreover, the morphological variability of both EGMs is significant during the first post-implant month and very small after a few months. Collectively, these findings provide important information for the design of a statistically valid template updating procedure for morphological algorithms in ICDs.