Spectral and cross-spectral autoregressive analysis of cardiovascular variables in subjects with different degrees of orthostatic tolerance.
The mechanisms leading to vasovagal syncope are still unclear. A simple discriminating test for the identification of syncope-prone subjects is not presently available. Fifty-two subjects had a stepwise orthostatic test with 60 degrees tilt and -20 and -40 mm Hg lower-body negative pressure before the appearance of impending syncope symptoms. Spectral and cross-spectral analyses of heart period and systolic pressure time series were performed to estimate the power of the high-frequency (approximately equals 0.25 Hz) and low-frequency (approximately equals 0.1 Hz) oscillations, the coherence between heart period and systolic pressure, and the mean low-frequency and high-frequency central frequency, phase shift, and transfer function at maximal coherence. According to time to presyncope, the 52 subjects were divided into two groups: 25 with normal orthostatic tolerance, and 27 with poor orthostatic tolerance. In the supine positions, the mean central low-frequency was significantly lower in poor-tolerance group than in normal-tolerance group, discriminating poor from normal orthostatic tolerance with 80% specificity and 83% sensitivity, and was significantly correlated to time to presyncope. In the 2 to 3 minutes preceding syncope, subjects with poor orthostatic tolerance had less tachycardia, lower low-frequency power of systolic pressure, higher respiratory frequency, and a less negative phase shift in high-frequency range. In presyncope, sympathetic activation is reduced in subjects with poor orthostatic tolerance. In addition, the higher breathing frequency and the smaller negativity of phase shift in high-frequency range, which may indicate an inadequate engagement of the baroreflex, suggest a causal role of respiration in the development of syncope. Supine central values of low frequency may be proposed as a valuable clinical index of orthostatic intolerance.
Gulli
G
G
Department of Neurological Sciences and Vision, University of Verona, Italy.
Wight
V L
VL
Hainsworth
R
R
Cevese
A
A
D000328:Adult; D000368:Aged; D002319:Cardiovascular System / Q000503:physiopathology*; D004198:Disease Susceptibility; D005260:Female; D006801:Humans; D007024:Hypotension, Orthostatic / Q000150:complications / Q000503:physiopathology*; D008150:Lower Body Negative Pressure; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D013312:Stress, Physiological / Q000209:etiology / Q000503:physiopathology; D016683:Supine Position; D013575:Syncope / Q000209:etiology / Q000503:physiopathology; D018667:Tilt-Table Test; D013997:Time Factors