A system and method for determining the onset and termination of cardiac events, such as the R-wave, the T-wave and the far-field signals sensed in the atria associated with the R-wave (FFR) and T-wave (FFT). The onset is defined as the time of the first sampled point of the cardiac signal whose magnitude exceeds a pre-defined threshold for the particular event. Once the onset of an event is positively determined, the cardiac signal is sampled at given intervals. The change in magnitude of these sampled points is determined. The termination of the event is identified through an algorithm that compares the difference in magnitude of these sampled points.