The analyses of beat to beat fluctuations of heart rate known as heart rate variability (HRV) become a non-invasive clinical tool to study the modulation that the autonomic nervous system exerts on the cardiovascular system. HRV presents a complex behaviour that may contain hidden information, which may not extractable with conventional methods of HRV analysis. Linear HRV measures are not able to detect these subtle but important properties. Thus several methods to describe non-linear heart rate...