Progressive increase in cardiac sympathetic activity precedes vasovagal episodes

To assess the time course of the neural changes preceding vasovagal episodes we studied a group of 8 champion swimmers who had fainted during a 90° head-up tilt test. Two different moments of the tilt period were considered: early tilt (T(E)) when all subjects were still asymptomatic, and pre-syncopal tilt (T(PS)), corresponding to the 256 beats immediately preceding the onset of fainting (mean latency 8 ± 1 min). Sympathetic and vagal activities to the heart were assessed by the power of, respectively, the low frequency (LF(RR), about 0.1 Hz) and the high frequency (HF(RR), about 0.25 Hz) components obtained by means of spectral analysis of spontaneous RR interval variability. During T(E), RR was reduced from 909 ± 42 to 636 ± 25 msec. As to the spectral components, HF(RR) decreased from 49 ± 6 to 20 ± 4 normalized units (nu, p < 0.05) while LF(RR) increased from 46 ± 7 to 77 ± 4 nu (p < 0.05). During T(PS) there was a further reduction of RR interval (610 ± 25 msec) and of HF(RR) (11 ± 1 nu), while LF(RR) further increased to 86 ± 2 nu (p < 0.05). Therefore, the neural changes preceding a vasovagal event seem to be characterized by a progressive enhancement of sympathetic activity to the sino-atrial node up to the onset of fainting.