TABLE 3.

Temporal changes in HF in head-up tilt tests.

	POTS n = 25	P-value	Non-POTS n = 12	P-value
Change in HF from Pre-Tilt supine to HUT 1 min	463 (391) to 402 (331)	<0.0001	364(216) to 330 (193)	0.0002
Change in HF from Pre-Tilt supine to HUT 2–4 min	463 (391) to 279 (221)	<0.0001	364(216) to 248 (135)	0.0032
Change in HF from Pre-Tilt supine to Average HUT	463 (391) to 163 (119)	<0.0001	364(216) to 180 (127)	0.0032
Change in HF from Pre-Tilt supine to HUT final	463 (391) to 85 (69)	<0.0001	364(216) to 134 (160)	0.0059
Change in HF from HUT 1 min to HUT final	402 (331) to 85 (69)	<0.0001	330(193) to 134 (160)	0.0059
Change in HF from HUT final to Post-Tilt supine	85 (69) to 236 (174)	<0.0001	134(160) to 299 (145)	0.0059

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; HF, high-frequency components of heart rate variability spectra in ms²; 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.