Clinical implications of present physiological understanding of HRV components.

Time and frequency domain analysis of heart rate variability (HRV) is a non invasive technique capable of providing information on autonomic modulation of the sinus node and of stratifying risk after myocardial infarction and in heart failure. One of the basic assumptions used to explain the negative predictive value of reduced HRV was the concept that overall HRV was largely dependent on vagal mechanisms and that a reduction in HRV could reflect an increased sympathetic and a reduced vagal modulation of sinus node; i.e., an autonomic imbalance favouring cardiac electrical instability. This initial interpretation was challenged by several findings indicating a greater complexity of the relationship between neural input and sinus node responsiveness as well as the possible interference with non neural mechanisms.Nevertheless, the prognostic value of time and geometric parameters of HRV has been consistently confirmed. More complex is the interpretation of spectral parameters particularly when they are computed on 24-hour recordings. Under controlled conditions, instead, the computation of low and high frequency components and of their ratio seems to provide information on sympatho-vagal balance in normal subjects as well as in most patients with preserved left ventricular function, thus providing an unique tool to investigate neural control mechanisms. More recently, analysis on nonlinear dynamics of HRV has been utilized to describe the fractal-like characteristics of the variability signal and has been shown to identify patients at risk for sudden cardiac death. In conclusion, in spite of an incomplete understanding of the physiological significance of HRV parameters, this non invasive methodology is of substantial utility to evaluate autonomic control mechanisms and to identify patients with an increased cardiac mortality.