Significance and assessment of autonomic indices in cardiovascular reactions.

The purpose of this review was to integrate recent evidence supporting the reliability of noninvasive measures of parasympathetic and sympathetic activity. Literature concerning spectral analysis of heart period (HP) variability is reviewed with special emphasis on works revealing neural mediation of high-frequency and mid-frequency components of HP power spectrum and suggesting their use as a tool to assess autonomic balance. Problems of derivations of autonomic indices based on impedance cardiography and HP variance analysis are discussed. Advantages of parametric time series (autoregressive-AR) models are described with the objective of providing an informed basis for choosing among methodological alternatives. Two original approaches developed in our laboratory are outlined, namely the algorithms for systolic time interval assessment based on impedance cardiogram as well as the AR method developed for heart period power spectral density estimation.