[The effect of orthostatic provocation on the spectral pattern of heart rate variability in healthy subjects]

Investigations were carried out in 74 persons (32 female and 42 male, mean age: 39.7 +/- 10.0 years, range: 16 to 68 years) free of diseases known to affect autonomic function. After a quarter of rest in quite, air conditioned room, while on voluntary breathing in supine, and later on in vertical position, time-series of a few minutes of successive R-R intervals were collected in each person and than stored for further off line analysis. Multivariate autoregressive modeling approach with parametric decomposition of peaks has given a basis for computation of spectral power of heart rate variability. In each body position 150 seconds long, stationary epoch of R-R intervals free of ectopy and artifacts were selected, and spectral power were calculated in three frequency bands: very low frequency (VLF): below 0.03 Hz (rejected from further analysis), low frequency (LF): from 0.03 to 0.15 Hz, and high frequency (HF): 0.15 to 0.35 Hz. Than, computed powers were expressed in either absolute units, or as normalized values, that is as percentages of total power less VLF. Breathing was also monitored in each case and spectral characteristics of its frequency also given in ranges defined for heart rate variability. In vertical, as compared to supine position, mean R-R interval decreased: 856 +/- 119 ms vs. 695 +/- 111 ms (p < 0.0005). The power of LF increased: 8.1 +/- 7.3 BPM2 vs. 19.2 +/- 14.6 BPM2, (p < 0,005), and simultaneously the power of HF dropped: 4.7 +/- 3.6 BPM2 vs. 2.7 +/- 2.5 BPM2, (p < 0.0005). In normalized units LF power enhanced: 63 +/- 385 vs. 87 +/- 42%, (p < 0.0005), and HF dropped: 36.7 +/- 32% vs. 12.3 +/- 12%, (p < 0.0005). Autonomic balance expressed as LF/HF ratio was higher in vertical position: 2.0 +/- 1.6 vs. 11.0 +/- 8.7, (p < 0.0005). In 73 out of 74 subjects (98.6%) the power of LF increased, and in 71 (95.9%)-decreased. In one person who had lower power of LF in vertical than supine position, the increase of HF power was noticed. Mean percentage increase of LF in the whole group was 209.4 +/- 243%, but after rejection of the result mention above the mean value increased to 218 +/- 22.1%. Mean percentage decrease of HF power in the whole group was 34.7 +/- 37.7%, and increased to 39.5 +/- 26.9% after rejection of three results with changes in opposite direction. Mean values of respiratory power did not change with switching of posture neither in LF:14.4 +/- 6.6% vs. 17.8 +/- 7.1%, (NS), nor in HF: 77.7 +/- 9.2% vs. 73.6 +/- 12.1%, (NS).