Under pathological conditions, the autonomic nervous system, which continually regulates cardiac function, may have adverse effects on pump function and electrical activation. The sinus rhythm is an indirect marker of the different components of the autonomic nervous system and changes in the heart rate reflect the permanent variation of vagal and sympathetic tone. Analysis of variability in the temporal or frequential domaines, provides information about the modulation of the different components of the autonomic nervous system. In the post-infarction period, studies have shown that the heart rate variability is an independent predictive factor of cardiovascular mortality. The data available in patients with cardiac failure is limited. There is a decrease in heart rate variability and this decrease seems to be associated with a higher risk.