Time and frequency domain assessment of heart rate variability: a theoretical and clinical appreciation.

Non-invasive techniques for assessing heart rate variability can be used either diagnostically, as in identification of autonomic neuropathy associated with diabetes mellitus or tissue rejection following cardiac transplantation, or as a prognostic indicator in coronary artery disease. The methodology is based upon calculation of successive R-R intervals from an electrocardiogram, which can then be plotted as a frequency histogram (time domain analysis), undergo power spectral analysis to yield information in the frequency domain or be applied to chaos theory. In this review, several parameters are discussed which can be derived to quantify heart rate variability in the time and frequency domains; the latter providing information on autonomic balance. In the frequency domain up to three peaks may be observed, with the peak below 0.15 Hz being mediated by sympathetic and parasympathetic activity and peaks above 0.15 Hz being of vagal origin. The effects of different physiological and pathophysiological conditions on various indices of heart rate variability, and the use of heart rate variability analysis as a pharmacological method to assess the impact of drug therapy on sympathovagal balance are discussed.