| Objective(1) To investigate the meaning of LF and HF in short-term power spectral analysis, frequency-domain measures of HRV are analyzed from normal subjects before and after autonomic nervous system (ANS) blockade with intravenous propranolol and atropine, respectively.(2) To study the pathological mechanism of vasovagal syncope, spectral analysis of heart rate variability was used to value changes of autonomic function during head up tilt test in patients with unexplained syncope.Methods(1) Seven normal volunteers (5 men; mean age 18+/-3 years) were studied. Five-minute ECG recordings were made before and after pharmacological autonomic nervous system (ANS) blockade with intravenous propranolol and atropine. Frequency domain measurements of the very low frequency (VLF, 0.003-0.04Hz), low frequency (LF, 0.04-0.15 Hz) and high frequency (HF, 0.15-0.40 Hz) bands calculated from the R-R interval tachograms, and LF and HF were normalized: LFnorm=100 X LF/(TP-VLF) and HFnorm=100 X HF/( TP-VLF), and the LF/HF ratio was also calculated.(2) twenty-seven patients with recurrent episodes of unexplained syncope underwent 70? head up tilt test and spectral analysis was used to assess changes in autonomic function before tilt testing, immediately after tilting, just before the occurrence of syncope or the end of the test, during syncope period or the end of test and after testing in the supine rest. At the same time, haemodynamic changes were recorded.Results(1) After atropinization, HFnorm significantly decreased (28 +6.7-17 + 2.3), and no changes in LFnorm were observed, and LF/HF increased (1.21 + 0.17- 1.87 +0.20); in contrast, after abolishing the sympathetic influencewith propranolol, no changes in HFnorm were observed, LFnorm significantly decreased (36 + 7.8-~20 + 6.8), and and LF/HF decreased (1.24+0.20-0.75 +0.16). In a week interval, spectral indices were similar in seven volunteers before autonomic nervous system (ANS) blockade with drugs.(2) 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 patients (118.00+10.42-81.00 + 12.36 mmHg, P<0.01, and 76.00 + 8.40- 52.00 + 10.95mmHg, P<0.01, respectively) and heart rate decreased in 8 patients (53%). No significant difference in all indices of spectral analysis was observed in the supine position between subjects with positive and negative test results. LFnorm in both groups did not alter during the whole tilt procedure. The decreased HFnorm and increased LF/HF persisted throughout head up tilt test in the negative patients. In positive patients, similar patterns of changes were observed before positive symptoms occurred; but during symptoms occurrence, HFnorm abruptly rose (10.47+4.04-32.95 +10.48) and obviously exceeded that of before tilt testing (23.44+4.20-32.95 + 10.48, P<0.01) and LF/HF dropped (3.28+0.39- 1.07+0.31, P<0.01). At supine rest just after test, all indices in both groups come back.Conclusion(1) HFnorm reflects the modulation of parasympathetic nerve and LFnorm reflects the changes of sympathetic nerve.(2) Power spectral analysis has favourable repeatability.(3) 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 relative with the abrupt and excessive increase of vagal activity.
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