Autonomic function in adolescents with orthostatic dysregulation measured by heart rate variability.

We evaluated the responses of autonomic nervous system to transient positional changes and daily changes in orthostatic dysregulation (OD) patients in order to clarify the mechanisms underlying the appearance of symptoms. The control group consisted of 16 healthy adolescents (8 males and 8 females), and the OD group consisted of 25 adolescents (13 males and 12 females). Each subject underwent continuous electrocardiographic monitoring during the head-up tilt test, followed by electrocardiographic monitoring for 24 h. Low frequency power (LF) and high frequency power (HF) were calculated as indices of autonomic function. During 0-5 min in the standing position in the tilt test, HF was higher in the OD group than in the control group (180+/-110 ms(2) vs. 42.6+/-54.1 ms(2); p<0.05). The LF/HF ratio during standing for 15-20 min was higher in OD patients than in the controls (4.75+/-3.45 vs. 1.67+/-1.21; p<0.05). The 24 h analysis showed that HF during sleep was significantly lower in the OD patients than in the controls (516+/-290 ms(2) vs. 1,290+/-429 ms(2); p<0.05); the LF/HF ratios were consistently higher in the OD patients than in the controls (4.13+/-3.41 and 2.92+/-2.00 vs. 2.46+/-0.89 and 1.35+/-1.54 in waking and sleeping states, respectively; p<0.05). This study showed that OD patients have less variability of the parasympathetic nervous system as well as hyperactivity of the sympathetic nervous system. This autonomic dysfunction is responsible for the symptoms of OD.