Spectral analysis of heart rate variability was used to assess changes in activity of autonomic
nervous system (ANS) in 84 patients with history of recurrent syncope and 21 normal controls. The analysis
was done in different phases of tilt table testing procedure to attempt to relate such changes to specific types
of response to a trigger such as decrease of arterial blood pressure when standing up. Frequency domain
measurements of the high (HF), low (LF) frequency bands and the ratio LF/HF were derived from ECG
recordings and computed by Fast Fourier transform (FFT) for a sequential 5 min intervals of the whole 55 min
tilt testing procedure. A statistically significant increase of the LF component and decrease of the HF component
of the HRV spectrum were observed during the transition from the supine position to head-up tilt (HUT) in
subjects with a history of syncope. However, there were no differences in the ANS activity in comparable testing
intervals between subjects who experienced syncope, to those who did not. There were also no differences
in the ANS activity in subjects with tilt-induced syncope compared to healthy control subjects. We therefore
suggest that HRV is a suitable method for investigating the activity of ANS during the head-up tilt table testing,
however with existing methodology is not possible to predict the outcome of the head-up tilt table test in its
early stages.
