Spectral analysis of heart rate variability has been used to study autonomic balance in humans, and it is generally accepted that the high frequency (HF) component (0.15-0.4 Hz) can be used as a marker for vagal modulation of heart rate. Although it is tempting to use the low frequency (LF) component (0.04-0.15 Hz) as a marker for sympathetic activity, its specificity is less clear. Pagani and colleagues9,10 have hypothesized that when the LF component is expressed in normalized units (LFnu) it becomes a better marker of sympathetic modulation of heart rate. For most studies using spectral analysis, the LF/HF ratio is used and considered by many to be a good index of sympathovagal balance.6,7,9,10