Inadequate sympathovagal balance in response to orthostatism in patients with unexplained syncope and a positive head up tilt test

Aim—To analyse the immediate response of heart rate variability (HRV) in response to orthostatic stress in unexplained syncope. Subjects—69 subjects, mean (SD) age 42 (18) years, undergoing 60° head up tilt to evaluate unexplained syncope. Methods—Based on 256 second ECG samples obtained during supine and upright phases, spectral analyses of low (LF) and high frequency (HF) bands were calculated, as well as the LF/HF power ratio, reflecting the sympathovagal balance. All variables were measured just before tilt during the last five minutes of the supine position, during the first five minutes of