[A spectral analysis of heart rate variability during postural tilt]

We analyzed the spectral characteristics of heart rate variability during the 4-minute periods just before and after postural tilt and during the 4-minute period just before the occurrence of syncope or the end of the test. 6 subjects with syncope had a progressive increase in low-frequency segment during these three periods, and highfrequency segment was significantly lower than that of the 10 subjects without syncope during the three periods. Those with syncope had a significantly higher segment ratio of low-frequency segment to high-frequency than those without syncope during these three periods. Thus, the imbalance of sympathetic and parasympathetic neural tones plays an important role in the syncope induced during postural tilt. The ratio of low-frequency segment to high-frequency is a practical measure which reflects the accuracy and sensibility of interaction of sympathetic and parasympathetic neural tones.