Figure 4. Power spectral analysis of RR interval of the data shown in Figure 3at baseline (BL; left) and 3 h postoperatively (R3; right). The patient given placebo (A and B) showed a profound postoperative reduction of power across low-frequency (0.00-0.15 Hz) and high-frequency (0.15-0.50 Hz) ranges, consistent with the reduced variability of RR interval observed in Figure 3B. The patient given clonidine (C and D) showed a preservation of power across the high- and low-frequency ranges in the postoperative period, consistent with the large amplitude of RR interval variation observed in Figure 3D.