SPECTRAL CHARACTERISTICS OF HEART-RATE-VARIABILITY BEFORE AND DURING POSTURAL TILT - RELATIONS TO AGING AND RISK OF SYNCOPE

Fourier analysis of heart rate (HR) may be used to characterize overall HR variability as well as low- and high-frequency components attributable to sympathetic and vagal influences, respectively. We analyzed HR spectral characteristics of 12 healthy young (18-35 years) and 10 healthy old (71-94 years) subjects before and during 60° head-up tilt. Total spectral power in the 0.01-0.40-Hz frequency range and low-frequency (0.06-0.10 Hz) and high-frequency (0.15-0.40 Hz) components of the HR power sepctrum were significantly lower in old than in young subjects in supine and upright positions. To characterize and compare overall HR variability in young and old subjects, we computed the regression lines relating the log amplitude of the log frequency of the supine HR spectra (l/f(x) plots). The regression lines for old subjects were lower and steeper (mean slope, -0.78 [5%, 95% confidence limits (CL), -0.73, -0.83]) than in young (mean slope, -0.67 [CL, -0.62, -0.72]), indicating not only reduced overall spectral amplitude but also relatively greater attenuation of high-frequency HR components in the old subjects. This finding illustrates a novel way to quantify the loss of autonomic influences on HR regulation as a function of age. During postural tilt, HR variability was unchanged in the old subjects. For the entire group of young subjects, total HR variability increased during tilt. Six young subjects developed vasovagal syncope during tilt, enabling us to examine differences in the HR spectra of these subjects while they were asymptomatic before syncope. Subjects with syncope had a significant increase in total and low-frequency HR variability during tilt, whereas those without syncope had no significant change in the HR spectrum. Thus, the old subjects have reduced supine HR variability and absent or attenuated low-frequency activation during tilt. Young subjects who develop syncope during tilt demonstrate prominent low-frequency activation, which may be associated with heightened sympathetic activity. Our findings may provide a physiological explanation for resistance to vasovagal syncope in old age.