[Assessment of the changes in autonomic nervous function during head up tilt test in syncopal patients using spectral...

To study the pathological mechanism of vasovagal syncope, spectral analysis of heart rate variability was used to evaluate the changes of autonomic function during head up tilt test in patients with unexplained syncope.

27 patients with recurrent episodes of unexplained syncope underwent 70 degrees head up tilt test. Spectral analysis was used to assess the changes in autonomic function before tilt testing, immediately after tilting, just before the occurrence of syncope or at the end of the test, during the syncope period or at the end of the test and after testing in supine rest. At the same time, haemodynamic changes were recorded. Spectral power in very low frequency (VLF, 0.003 - 0.04 Hz), low frequency (LF, 0.04 - 0.15 Hz) and high frequency (HF, 0.15 - 2.00 Hz) were computed with Fast Fourier Transform analysis, and LF and HF were normalized: LF norm = 100 x LF/(TP-VLF) and HF norm = 100 x HF/(TP-VLF).

12 patients (mean age 40 +/- 10 years) showed a negative response and 15 patients (mean age 37 +/- 9 years) showed a positive response. Both systolic and diastolic blood pressure decreased in all the patients [(118.00 +/- 10.42-->81.00 +/- 12.36) mm Hg, P < 0.01 and (76.00 +/- 8.40-->52.00 +/- 10.95) mm Hg, P < 0.01] and heart rate decreased in 8 patients (53%). No significant difference in all the indices of spectral analysis was observed in supine position between the subjects with positive and negative test results. LF norm in both the groups did not alter during the entire tilt procedure. The decreased HF norm and increased LF/HF persisted throughout head up tilt test in the negative patients. In the positive patients, similar patterns of changes were observed before the occurrence of positive symptoms, and during the occurrence of the symptoms. HF norm abruptly rose (10.47 +/- 4.04-->32.95 +/- 10.48) and obviously exceeded that before tilt testing (23.44 +/- 4.20-->32.95 +/- 10.48, P < 0.05) and LF/HF dropped (3.28 +/- 0.39-->1.07 +/- 0.31, P < 0.01). At supine rest just after test, all the indices in both groups came back.

In the supine position, autonomic function is similar between positive and negative subjects. Positive patients have a different pattern of response to the tilting test. The pathological mechanism leading to vasovagal syncope appears to be related with the abrupt and excessive increase of vagal activity.