Alternatively, reduced high-frequency power after sympathetic blockade may also be explained by diminished “accentuated antagonism,” a phenomenon described more than three decades ago (4). Heart rate response to vagal nerve stimulation is accentuated when sympathetic tone is elevated, and vice versa. Thus, cardiac sympathetic withdrawal by high spinal or epidural blockade may have resulted in diminished beat-to-beat fluctuations of R-R intervals without alteration of actual vagal nerve activity. To draw a definitive conclusion regarding the mechanism, determinations of central vagal/sympathetic outflow would be mandatory by an animal experiment.
http://www.anesthesia-analgesia.org/content/100/4/1216.2.full