Figure 3. Low-frequency (0.06–0.15 Hz) and high-frequency (0.15–0.8 Hz) spectral power densities of heart rate variability along with the LF/HF ratio during a 40 degrees head-up tilt before (open symbols) and during (closed symbols) preganglionic cardiac sympathetic blockade by epidural anesthesia. Individual changes and mean plus/minus SD from 10 awake subjects. Neither epidural blockade alone nor tilt during epidural blockade evoked significant alterations in absolute power spectral variability measures. However, cardiac sympathetic blockade significantly reduced the increase in the LF/HF ratio evoked by tilt. *P < 0.05.