RR-interval time series often present impulses corresponding to short-duration increases or decreases in the heart rate, most probably due to bursts in autonomic activity. The time-domain heartrate variability index pNN50 is obviously linked to these spikes. As linear filtering is not appropriate for the extraction of these spikes, we developed a nonlinear algorithm for this task. In this study we demonstrate the potential of this approach to assess caffeine-induced changes in the autonomic tone, and discuss the potential of this approach.