The high frequency component of heart rate variability reflects cardiac parasympathetic modulation rather than parasympathetic 'tone'.

This study was designed to evaluate the effect of modulating cardiac parasympathetic input on the high frequency component of heart rate variability. We stimulated the right vagus nerve with three different stimulation patterns in anaesthetized, vagotomized and spinal anaesthetized dogs. We kept the mean stimulation frequency constant; controlled the amplitude of modulation with programmed stimulation patterns, and analysed the resulting heart rate variability by power spectral analysis. Constant frequency vagal stimulation increased the cardiac interval, but did not change heart rate variability markedly. There was a slight increase, from 11 +/- 2 to 27 +/- 11 ms2, in the high frequency component. However, when the instantaneous stimulation frequency oscillated between 4 and 17 Hz during 5 s period, we could produce a marked heart rate variation, with 91 +/- 9% of the variation corresponding to the frequency of the modulation (0.20 Hz). The high frequency component was 12932 +/- 7701 ms2. With an increased magnitude of modulation, i.e. the difference between minimum and maximum instantaneous frequency, the high frequency component increased to 32711 +/- 17943 ms2. Thus, the high frequency component of heart rate variability reflects the magnitude of fluctuation in the cardiac parasympathetic input rather than parasympathetic 'tone'.