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Figure 4 Heart period and systolic blood pressure variability changes during head up tilt (HUT). (A) Changes in low frequency (LF) heart period variability (HPV) to tilting. The LF power initially increased significantly in both groups to a similar extent. After being upright, LF HPV gradually decreased in the HUT+ group. The decrease in the LF HPV did not reach statistical significance until the time of the faint. *p < 0.0001, early-tilt vs. supine (HUT+ group) and p < 0.0001, early-tilt vs. supine (HUT– group); #p = 0.0186 for faint vs. early-tilt (HUT+ group) and p = 0.0003 for faint/late tilt (HUT+ vs. HUT– group). (B) Changes in LF blood pressure variability (BPV) to tilting. LF power initially increased in both groups during early tilt. After being upright, LF BPV gradually decreased in the HUT+ group. LF BPV decreased below the baseline supine value of BPV at the mid-tilt interval and during the faint. The decrease in the LF BPV started shortly after the early-tilt period. In the HUT– group, LF BPV continued to increase and remain at an increased level for the duration of the tilt. *p < 0.0001 HUT+ vs. HUT– group (mid-tilt); #p = 0.0001 faint vs. early-tilt (HUT+ group).