[Variability of the RR interval: a measure of systemic sympathetic activity].

Beat-to-Beat variability may now be measured, from 24h ECG recordings (Holter monitoring), in the time domain (by statistical indexes) and in the frequency domain (spectral analysis). This variability gives essential informations about the interactions of different cardiovascular variables, and especially about the balance of the two components of the sympathetic nervous system. It is generally thought that a malfunction of the ortho/parasympathetic balance favours ventricular fibrillation. So the potential interest of this new technique as a predictor of sudden cardiac death seems enormous.