TABLE 5.

Temporal changes in LF/HF in head-up tilt tests.

	POTS n = 25	P-value	Non-POTS n = 12	P-value
Change in LF/HF from Pre-Tilt supine to HUT 1 min	13.1 (7.0) to 15.4 (7.8)	<0.0001	13.4 (5.2) to 15.5 (5.6)	0.0005
Change in LF/HF from Pre-Tilt supine to HUT 2–4 min	13.1 (7.0) to 21.2 (10.6)	<0.0001	13.4 (5.2) to 19 (7.1)	0.0002
Change in LF/HF from Pre-Tilt supine to Average HUT	13.1 (7.0) to 34.4 (15.1)	<0.0001	13.4 (5.2) to 28.5 (12.3)	0.0032
Change in LF/HF from Pre-Tilt supine to HUT final	13.1 (7.0) to 44.4 (18.2)	<0.0001	13.4 (5.2) to 39.6 (19.8)	0.0059
Change in LF/HF from HUT 1 min to HUT final	15.4 (7.8) to 44.4 (18.2)	<0.0001	15.5 (5.6) to 39.6 (19.8)	0.0059
Change in LF/HF from HUT final to Post-Tilt supine	44.4 (18.2) to 21.8 (12.6)	<0.0001	39.6 (19.8) to 16.5 (7.7)	0.0020

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; LF,: low-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.