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. 1999 Sep;82(3):312–318. doi: 10.1136/hrt.82.3.312

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

C Kouakam 1, D Lacroix 1, N Zghal 1, R Logier 1, D Klug 1, P Le Franc 1, M Jarwe 1, S Kacet 1
PMCID: PMC1729176  PMID: 10455081

Abstract

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 head up tilt, and just before the end of passive tilt.
RESULTS—Symptoms occurred in 42 subjects (vasovagal syncope in 37; psychogenic 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% from 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 with tilting as those with a negative test (11% v 28%, p = 0.04), while HF power did not alter. A decreased LF/HF power ratio persisted throughout head up tilt and was the only variable found to discriminate between subjects with positive and negative test results (p = 0.005, multivariate analysis). During the first five minutes of tilt, a decreased LF/HF power ratio occurred in 33 of 37 subjects in the positive group and three of 27 in the negative group. Thus a decreased LF/HF ratio had 89% sensitivity, 89% specificity, a 92% positive predictive value, and an 86% negative predictive value.
CONCLUSIONS—Through the LF/HF power ratio, spectral analysis of HRV was highly correlated with head up tilt results. Subjects developing syncope late during continued head up tilt have a decrease in LF/HF ratio immediately after assuming the upright posture, implying that although symptoms have not developed the vasovagal reaction may already have begun. This emphasises the major role of the autonomic nervous system in the genesis of vasovagal (neurally mediated) syncope.


Keywords: heart rate variability; vasovagal syncope; head up tilt test

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Figure 1  .

Figure 1  

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ECG recordings for heart rate variability analysis. HUT, head up tilt.

Figure 2  .

Figure 2  

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Changes in low frequency (LF), high frequency (HF), and LF/HF power ratio with upright tilt in the study group. HUT, head up tilt; *p < 0.05, upright v supine; †p < 0.05, positive HUT v negative HUT.    

Figure 3  .

Figure 3  

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Heart rate variability power spectrum calculated by fast Fourier transform algorithm. Example of supine (A) and upright (B) spectrum of a patient with a negative head up tilt test, compared with those of a patient with a positive test (C, D).

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