Fig. 2. Heart rate variability, retrospective groups. (  A ) Low to high frequency ratio (LF/HF). (  B ) Low frequency (LF). (  C ) High frequency (HF). DBS = day before surgery; DOS-BL = day of surgery baseline; MILD = mild hypotension; MOD = moderate hypotension; PRE = after prehydration; SA+5 = 5 min after spinal anesthesia; SA+15 = 15 min after spinal anesthesia; SA+45 = 45 min after spinal anesthesia; SEV = severe hypotension. Data are presented as median, 25th and 75th percentile, and range. *P < 0.05  versus MILD. # P < 0.05  versus SEVERE. +P < 0.05, significant difference within groups between different events.