Chapter 7.2 Heart rate variability R. Baron a, * and D.J. Ewing b a Department of Neurology, Christian-Albrechts-University Kiel, Niemannsweg 147, D-24105 Kiel (Germany) b The Scottish Of®ce Department of Health, St. Andrews House, Edinburgh, EH1 3DG (UK) Method Recording of heart rate over long periods using ambulatory ECG monitoring reveals that the heart rate varies continuously, mainly in¯uenced by the cardiac sympathetic and parasympathetic innerva- tion. Therefore, analysis of heart rate variation provides techniques for the investigation of cardiac autonomic innervation. The R-R intervals (inverse of heart rate) yield detailed beat-to-beat informa- tion and its variation is the most useful non-inva- sive index of cardiac autonomic neuropathy (Ewing et al. 1981; Low et al. 1990; Ewing 1993). Indication Patients with disorders which might involve damage of the autonomic nervous system should be tested. These include peripheral polyneuropa- thies caused by diabetes mellitus, alcoholism, chronic renal failure, AIDS, leprosy, the Guillain± Barre  syndrome and paraneoplastic neuropathies. Furthermore, central lesions of autonomic centres due to multiple sclerosis or syringomyelia and degenerative disorders of the autonomic nervous system, i.e. primary autonomic failure, also reduce heart rate variability. More recent indications include primary cardiac disorders and the effects of antiarrhythmic drugs on heart rate. Physiological background Heart rate variation during normal activity (time- domain technique) During normal daily activities a beat-by-beat variation in heart rate (or change in the R-R interval length) occurs that is partly determined by the balance between the slowing effect of the autonomic parasympathetic and the accelerating effect of the sympathetic innervation, as well as by humoral mechanisms, and the intrinsic rhythmicity of the cardiac pacemaker tissue. Physiological variations of the heart rate include changes associated with respiration (so-called sinus arrhythmia, 0.15±0.45 Hz), as well as slower alterations associated with blood pressure ¯uctuations and barore¯ex mechan- isms (0.1 Hz) and with hormonal changes and ther- moregulation (mainly the renin-angiotensin system, 0.05 Hz) and also very slow variations in response to day and night. When subjects are supine, parasym- pathetic activity is most prominent with only minimal sympathetic activity. The time-domain technique is only one method of looking at heart rate variability during normal 283 Recommendations for the Practice of Clinical Neurophysiology: Guidelines of the International Federation of Clinical Physiology (EEG Suppl. 52) Editors: G. Deuschl and A. Eisen q 1999 International Federation of Clinical Neurophysiology. All rights reserved. Published by Elsevier Science B.V. * Correspondence to: Priv.-Doz. Dr. med. Ralf Baron, Klinik fu Èr Neurologie, Christian-Albrechts-Universita Èt Kiel, Niemannsweg 147, D-24105 Kiel (Germany).