Heart rate variability and head-up tilt testing in patients with syncope of undetermined etiology.

Indices of heart rate variability are non-invasive indicators of neural control of the heart. To investigate the significance of changes in heart rate variability in neurally mediated syncope, we performed head-up tilt testing in 45 patients with syncope of undetermined etiology. Seventeen patients showed a negative response and 28 showed a positive response; 18 had a vasodepressor response (systolic blood pressure dropped > or = 50% without a decrease in heart rate) and 10 had a vasovagal response (systolic blood pressure dropped > or = 50% with a decrease in heart rate of > or = 30%). The mean RR-interval, the standard deviation of normal sinus RR-intervals (standard deviation of RR-interval) and power spectra were measured in consecutive 2 min periods throughout the study. Power spectra consisted of low frequency (0.04-0.15 Hz), high frequency (0.15-0.40 Hz) and total spectra (0.01-1.0 Hz). Both high frequency spectra and the low/high frequency spectra ratio significantly changed with head-up tilt testing regardless of the response. However, high frequency, low frequency and total spectra increased in relation to symptoms. These changes were most profound in the high frequency spectra of subjects with a vasovagal response. Since high frequency spectra reflect parasympathetic tone, a profound change in the high frequency spectra implies that parasympathetic activities play a significant role in patients with a vasovagal response. The assessment of heart rate variability during head-up tilt testing can provide new insight into the pathogenesis of syncope of undetermined etiology.