SYMPATHOVAGAL EFFECTS OF SPINAL-ANESTHESIA ASSESSED BY HEART-RATE-VARIABILITY ANALYSIS

Heart rate variations (HRV) result from moment-to-moment changes in sy mpathetic and parasympathetic activity in response to many conditions. These two neural inputs to the heart can be identified by analyzing p ower spectra of HRV for frequency components at the vasomotor (low-fre quency [LF]) and the respiratory (high-frequency [HF]) rhythms. HRV an alysis has been used successfully in humans to noninvasively evaluate the autonomic responses to specific maneuvers and drugs, as well as re sponses 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 patient s receiving subarachnoid block for abdominal surgery. The sensory leve ls achieved were within the range of those reported to interrupt sympa thetic outflow to the heart. Electrocardiograms were recorded and subj ected to AR analysis. AR analysis of HRV after spinal anesthesia revea led significant decreases in both dominant frequency components (LF an d HF) that occur between 0.03 Hz and 0.5 Hz. These reductions coincide d with blockade of cardiac sympathetic outflow after cephalad spread o f the spinal block. The power spectra were almost abolished in patient s with sensory blocks reaching T1-2. The decreases in amplitude of the LF and HF components, therefore, act as markers of diminished sympath etic and parasympathetic activity to the heart, while the ratio of LF: HF indicated that sympathovagal balance was predominantly maintained d uring most of the block. Only during the onset of spinal block in the lumbosacral area was cardiac sympathetic activity (LF) initially incre ased while parasympathetic activity (HF) reflexly decreased. AR power spectral analysis of HRV provided a quantitative measure of sympathova gal activity during spinal anesthesia.