Spectral analysis of heart rate variability in the analysis of autonomic nervous system activity during tilt-table testing in patients with unexplained syncope

Spectral analysis of heart rate variability was used to assess changes in autonomic nervous system (ANS) function before and during postural tilt in 44 patients with vasovagal syncope and 20 control subjects. In the patients the mean values of LF and HF frequency bands decreased in response to tilting and the LF/HF ratio increased, though the latter change was not statistically significant. All parameters showed a significant increase just before the onset of syncope. In the controls there was an increase in the LF and LF/HF ratio and a decrease in the HF immediately after tilting, but no further significant changes during the test. Syncopal patients have a different pattern of response to the orthostatic stimulus, in that they do not show the increase in sympathetic tone observed in normal individuals immediately after tilting while there is a shift in the balance of ANS activity towards the sympathetic system shortly before the onset of syncope.