Implantable cardioverter defibrillators (ICDs) can store intracardiac electrograms (EGMs) in sinus rhythm (SR), at the onset of spontaneous ventricular tachyarrhythmias (VT) or during their course. This allows the investigation of unknown features of the heart electrical activity associated with different cardiac rhythms. In this study we propose a non conventional cardiac electrical activity characterization (CEAC) that extracts quantitative information about the power spectrum wideness and variability of the beat-by-beat morphology. We analyze 293 EGMs from 40 patients who underwent implantation of St Jude Medical-Ventritex ICDs that allow the storage of EGMs with two different modes of recording: bipolar (BIP) and unipolar or far-field (FF). The EGMs are studied with this CEAC by (1) exploring differences between the CEAC measured from FF and BIP EGMs during similar cardiac rhythms, and (2) investigating the mode of recording that allows a better separation between SR and VT rhythms. Results show that, with similar cardiac rhythm, the CEACs from FF or BIP recordings are different (for SR rhythm: sensitivity 81.5%, specificity 93.6%; for VT rhythm: sensitivity and specificity 100%); thus FF and BIP EGMs analyze different aspects of cardiac activity. The CEAC applied to FF EGMs distinguishes better EGMs obtained during SR from VT rhythms (VT vs SR with sensitivity 92.7% and specificity 79.7%) than when it is applied to BIP signals (VT vs SR with sensitivity 60% and specificity 73.3%).