Background and Objectives. The purpose of this study is to determine if spectral analysis of beat-to-beat heart rate variability will provide important data on autonomic nervous system function during either spinal or epidural anesthesia that is not apparent from the monitoring of mean heart rate and blood pressure.

Methods. Twelve ASA I patients presenting for elective cesarean delivery (7 epidural and 5 spinal anesthetics) were prospectively selected for study. Control and postblock 10-minute segments of instantaneous heart rates were analyzed for each patient. The total spectral power (P_S, 0.02-0.5 Hz), low frequency spectral power (P_LF, 0.02-0.12 Hz), and high frequency spectral power (P_HF, 0.12-0.5 Hz) were calculated for each data segment. The complexity of the heart rate time series was estimated by calculating the approximate entropy.

Results. Neither spinal or epidural anesthesia had any effect on heart rate or blood pressure. Both spinal and epidural anesthesia did produce a significant decrease in P_S, P_LF, and P_HF. Although there were significant decreases in both the P_LF and P_HF, there was no change in the P_LF/P_HF ratio. There was a significant reduction in approximate entropy with spinal and epidural anesthesia, indicating a decrease in complexity of the heart rate dynamics.

Conclusions. These data suggest that autonomic tone decreased with spinal and epidural anesthesia but the sympathetic-parasympathetic balance (expressed as P_LF/P_HF) did not change. Spectral measures, as well as approximate entropy, provide an independent evaluation of the integrity of the autonomic nervous system and cardiovascular control mechanisms that cannot be discerned from mean heart rate and blood pressure.