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Sympathovagal Effects of Spinal Anesthesia Assessed by Heart Rate Variability Analysis

Heart rate variations (HRV) result from moment-to-moment changes in sympathetic and parasympathetic activity in response to many conditions. These two neural inputs to the heart can be identified by analyzing power spectra of HRV for frequency components at the vasomotor (low-frequency [LF]) and the respiratory (high-frequency [HF]) rhythms. HRV analysis has been used successfully in humans to noninvasively evaluate the autonomic responses to specific maneuvers and drugs, as well as responses to more chronic preexisting pathologic conditions. The effects of an isolated "acute" withdrawal of sympathetic activity in humans, however, have not as yet been evaluated using an autoregressive (AR) technique. We examined HRV using this technique in a group of patients receiving subarachnoid block for abdominal surgery. The sensory levels achieved were within the range of those reported to interrupt sympathetic outflow to the heart. Electrocardiograms were recorded and subjected to AR analysis. AR analysis of HRV after spinal anesthesia revealed significant decreases in both dominant frequency components (LF and HF) that occur between 0.03 Hz and 0.5 Hz. These reductions coincided with blockade of cardiac sympathetic outflow after cephalad spread of the spinal block. The power spectra were almost abolished in patients with sensory blocks reaching T1-2. The decreases in amplitude of the LF and HF components, therefore, act as markers of diminished sympathetic and parasympathetic activity to the heart, while the ratio of LF:HF indicated that sympathovagal balance was predominantly maintained during most of the block. Only during the onset of spinal block in the lumbosacral area was cardiac sympathetic activity (LF) initially increased while parasympathetic activity (HF) reflexly decreased. AR power spectral analysis of HRV provided a quantitative measure of sympathovagal activity during spinal anesthesia. (Anesth Analg 1995;80:315-21)