Understanding autonomic sympathovagal balance from short-term heart rate variations. Are we analyzing noise?

Heart rate variations reflect the output of the complex control of the hear t mediated by the autonomic nervous system. Because of that, they also enco de different types of information, namely the efferent outflow of reflex me chanisms involved in the beat-to-beat control of cardiac function, the effe rent activity of neurohumoral elements involved in the control of other car diovascular parameters and random noise resulting from the hysteresis of th e different controllers. The degree to which power spectrum estimation meth ods will uncover the periodic component of heart rate variations is in dire ct relation with the status of the system under study. Although the utility of spectral methods is now established in mammalian research, very little is known on the utility of these techniques in non-mammalian cardiovascular research. This review covers this space by discussing the physiological si gnificance of heart rate variations in non-mammalian vertebrates. A detaile d account of the different steps of the technique, its limitations and the ways to overcome these problems are also presented. These are: the recordin g of the cardiac event signal, the detection and digital processing methods , the satisfaction of stationarity conditions, the problem of spectral leak age and the different methods to estimate the power spectrum. (C) 1999 Else vier Science Inc. All rights reserved.