ANALYSIS OF HEART-RATE DYNAMICS AS A MEASURE OF AUTONOMIC TONE IN OBSTETRICAL PATIENTS UNDERGOING EPIDURAL OR SPINAL-ANESTHESIA

Background and Objectives. The purpose of this study is to determine i f spectral analysis of beat-to-beat heart rate variability will provid e important data on autonomic nervous system function during either sp inal or epidural anesthesia that is not apparent from the monitoring o f mean heart rate and blood pressure. Methods. Twelve ASA I patients p resenting for elective cesarean delivery (7 epidural and 5 spinal anes thetics) were prospectively selected for study. Control and postblock 10-minute segments of instantaneous heart rates were analyzed for each patient. The total spectral power (Ps, 0.02-0.5 Hz), low frequency sp ectral 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 comple xity of the heart rate time series was estimated by calculating the ap proximate entropy. Results. Neither spinal or epidural anesthesia had any effect on heart rate or blood pressure. Both spinal and epidural a nesthesia 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 significa nt reduction in approximate entropy with spinal and epidural anesthesi a, indicating a decrease in complexity of the heart rate dynamics. Con clusions. 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 a s approximate entropy, provide an independent evaluation of the integr ity of the autonomic nervous system and cardiovascular control mechani sms that cannot be discerned from mean heart rate and blood pressure.