LOW-FREQUENCY SPECTRAL POWER OF HEART-RATE-VARIABILITY IS NOT A SPECIFIC MARKER OF CARDIAC SYMPATHETIC MODULATION

Background: Heart rate variability in the frequency domain has been pr oposed to reflect cardiac autonomic control. Therefore, measurement of heart rate variability may be useful to assess the effect of epidural anesthesia on cardiac autonomic tone, Accordingly, the effects of pre ganglionic cardiac sympathetic blockade by segmental epidural anesthes ia were evaluated in humans on spectral power of heart rate variabilit y, Specifically, the hypothesis that cardiac sympathetic blockade atte nuates low-frequency spectral power, assumed to reflect cardiac sympat hetic modulation, was tested. Methods: Ten subjects were studied while supine and during a 15-min 40-degrees head-up tilt both before and af ter cardiac sympathetic blockade by segmental thoracic epidural anesth esia (sensory block: C6-T6). EGG, arterial pressure, and respiratory e xcursion (Whitney gauge) were recorded, and a fast-Fiourier-transforma tion was applied to 512-s data segments of heart rate derived from the digitized ECG at the end of each intervention. Results: With cardiac sympathetic blockade alone and the subjects supine, both low-frequency (LF, 0.06-0.15 Hz) and high-frequency (HF, 0.15-0.80 Hz) spectral pow er remained unchanged. During tilt, epidural anesthesia attenuated the evoked increase in heart rate (+11 . min(-1) +/- 7 SD vs. +6 +/- 7, P = 0.024). However, while during tilt cardiac sympathetic blockade sig nificantly decreased the LF/HF ratio (3.68 +/- 2.52 vs. 2.85 +/- 2.15, P = 0.041 vs. tilt before sympathetic blockade), a presumed marker of sympathovagal interaction, absolute and fractional LF and HF power di d not change. Conclusions: Although preganglionic cardiac sympathetic blockade reduced the LF/HF ratio during tilt, it did not alter spectra l power in the LF band during rest or tilt, Accordingly, low-frequency spectral power is unlikely to specifically reflect cardiac sympatheti c modulation in humans.