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 re sponse to orthostatic stress in unexplained syncope. Subjects-69 subjects, mean (SD) age 42 (18) years, undergoing 60 degrees he ad 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 ca lculated, as well as the LF/HF power ratio, reflecting: the sympathovagal b alance. All variables were measured just before tilt during the last five m inutes of the supine position, during the first five minutes of head up til t, and just before the end of passive tilt. Results-Symptoms occurred in 42 subjects (vasovagal syncope in 37; psychoge nic syncope in five). Resting haemodynamics and HRV indices were similar in subjects with and without syncope. Immediately after assuming the upright posture, adaptation to orthostatism differed between the two groups in that the LF/HF power ratio decreased by 11% fi om supine (from 2.7 (1.5) to 2.4 (1.2)) in the positive test group, while it increased by 11.5% (from 2.8 ( 1.5) to 3.1 (1.7)) in the negative test group (p = 0.02). This was because subjects with a positive test did not have the same increment in LF power w ith tilting as those with a negative test (11% v 28%, p = 0.04), while HF p ower did not alter. A decreased LF/HF power ratio persisted throughout head up tilt and was the only variable found to discriminate between subjects w ith positive and negative test results (p = 0.005, multivariate analysis). During the first: five minutes of tilt, a decreased LF/HF power ratio occur red in 33 of 37 subjects in the positive group and three of 27 in the negat ive group. Thus a decreased LF/HF ratio had 89% sensitivity, 89% specificit y, a 92% positive predictive value, and an 86% negative predictive value. Conclusions-Through the LF/HF power ratio, spectral analysis of HRV was hig hly correlated with head up tilt results. Subjects developing syncope late during continued head up tilt have a decrease in LF/HF ratio immediately af ter assuming the upright posture, implying that although symptoms have not developed the vasovagal reaction may already have begun. This emphasises th e major role of the autonomic nervous system in the genesis of vasovagal (n eurally mediated) syncope.