This paper presents several mathematical methods for analysis of electrocardiogram digital data. The measurement of beat to beat fluctuations known as Heart Rate Variability becomes a non-invasive diagnostic technique to study the cardiac autonomic regulation. The analysis was done by software developed by the author. The article presents the results of linear methods, nonlinear methods and wavelet analysis of Heart Rate Variability data in healthy and diseased subjects. The obtained results and the performed comparative analysis demonstrate the possibility for effective application of the considered methods in new cardiovascular information systems.