TABLE 4.

Temporal changes in LF in head-up tilt tests.

	POTS n = 25	P-value	Non-POTS n = 12	P-value
Change in LF from Pre-Tilt supine to HUT 1 min	457 (183) to 465 (178)	0.0013	450 (171) to 464 (166)	0.0730
Change in LF from Pre-Tilt supine to HUT 2–4 min	457 (183) to 431 (159)	0.0539	450 (171) to 420 (136)	0.3872
Change in LF from Pre-Tilt supine to Average HUT	457 (183) to 363 (154)	0.0073	450 (171) to 388 (152)	0.1938
Change in LF from Pre-Tilt supine to HUT final	457 (183) to 306 (183)	0.0073	450 (171) to 379 (190)	0.1133
Change in LF from HUT 1 min to HUT final	465 (178) to 306 (183)	0.0005	464 (166) to 379 (190)	0.1133
Change in LF from HUT final to Post-Tilt supine	306 (183) to 379 (209)	0.0001	379 (190) to 432 (148)	0.1133

Values are means of the HRV parameters with standard deviation. P-values were computed with the sign test; POTS, subjects with postural tachycardia syndrome; non-POTS, subjects without postural tachycardia syndrome; LF, low-frequency components of heart rate variability spectra in ms^2; Pre-Tilt supine, 2 min before upright position in recumbent position; HUT 1 min, during the first minute of upright position; HUT 2–4 min, from the second to fourth minutes of upright position; HUT final, during the last minute of upright position; Post-Tilt supine, from the first to third minutes after returning to the recumbent position; Average HUT, the average of the parameters during the whole time in the upright position.