Spectral analysis of heart rate variability(HRV) resulted in a characteristic power spectrum with two main regions, a high frequency at 0.15-0.5Hz(HFP) corresponding to the parasympathetic system and a low frequency at 0.017-0.l5Hz(LFP) corresponding to both parasympathetic and sympathetic influences. The ratio of low : high frequency(LFP/HFP) has been postulated as an index of sympathetic activation. We propose that the LFP/HFP ratio will provide important information concerning autonomic nervous system activity during spinal anesthesia especially geriatric patients.

LFP, HFP and LFP/HFP were checked in 30 patients at the baseline and 5, 10, 15, 20, 25 and 30 minutes after subarachnoidal block. We divided patients into two groups: group 1 is 15 patients under 60 years old and group 2 is 15 patients over 65 years old, and we compared those parameters in both groups.

Spinal anesthesia did produce a significant decrease in LFP and HFP in both groups. But sympathetic-parasympathetic balance(LFP/HFP) increased at 5 minutes after spinal block. In group 1, LFP/HFP decreased at 10 minutes after spinal block, thereafter increased gradually. In group 2, LFP/HFP decreased at 10, 15 and 20 minutes and increased at 25 and 30 minutes after spinal block.

These data suggest that autonomic tone decreased with spinal anesthesia and sympathetic-parasympathetic balance decreased for a long time in geriatric patients.