Spectral analysis of heart rate in vasovagal syncope: the autonomic nervous system in vasovagal syncope.

Spectral analysis of heart rate fluctuations was used to investigate the role of the autonomic nervous system in the pathogenesis of vasovagal syncope. Nine adolescents with a history of at least three episodes of vasovagal syncope and nine age-matched healthy controls were studied. All subjects were tested in supine position and at a 60 degrees inclination for 60 min or less if syncope developed. Blood pressure and heart rate were measured, while the ECG and respiration traces were recorded on magnetic tape for later spectral analysis. Baseline heart rate was lower in control subjects than in patients, increased with tilt in both groups, and remained lower in the control subjects throughout the experiment. Baseline systolic and diastolic blood pressure was similar in both groups. Diastolic blood pressure initially increased with tilt in all subjects and decreased significantly thereafter in patients. Pulse pressure was lower in patients throughout the experiment. The heart rate power spectra displayed a higher baseline level of low frequency fluctuations in the control group. The high frequency fluctuations component was similar in all subjects. The results of the test, regarding haemodynamic parameters and autonomic control of the heart rate, as expressed by low and high frequency fluctuations, are consistent with a reduced sympathetic reserve in the individuals with previous episodes of syncope.