Heart rate variability.

THE RHYTHM OF THE HEART has not only fascinated cardiologists but also inspired poets and musicians. Indeed, the periodic beat of the heart was used to define the speed of music. In music notation, the traditional Italian term “moderato” originally referred to one beat of the measure per walking pace (76–80 paces/min) or heartbeat ( 72 beats/min). The use of the heartbeat to define the speed of music may imply that the periodicity of the beat of the heart is very constant. However, this is not necessarily the case. In fact, loss of heart rate variability can indicate severe cardiovascular diseases and reliably predict poor outcome of such conditions (18, 22, 27, 47a). This In Focus article reviews sources of heart rate variability, its role as a prognostic marker for cardiovascular diseases, and its application in estimation of cardiac autonomic nervous system activity. All of these topics have been addressed intensely in articles published in the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology during the last two years. In healthy subjects, the sinoatrial node located at the posterior wall of the right atrium initiates each beat of the heart. Due to the unstable membrane potential of the myocytes located in this region, action potentials are generated periodically at a fairly constant frequency. This relatively constant frequency generated by the autorhythmicity of the sinoatrial node is modulated by many factors that add variability to the heart rate signal at different frequencies. According to the Task Force of The European Society of Cardiology and The North American Society of Pacing and Electrophysiology (47a) these frequencies are classified into 1) ultra-low frequencies (ULF; 5-h cycle length) that include the circadian rhythm (6, 9, 34, 54); 2) very low frequencies (VLF; 25-s cycle length) that are supposed to be affected by temperature regulation (1, 7, 34, 52, 54) and humoral systems (9, 36); 3) low frequencies (LF; 6-s cycle length in humans) that are sensitive to changes in cardiac sympathetic (and presumably parasympathetic) nerve activity (27, 30); and 4) high frequencies (HF; 2.5to 6.0-s cycle length in humans) that are synchronized to the respiratory rhythm (5) and are primarily modulated by cardiac parasympathetic innervation (38). The most prominent oscillation in the ULF band of the heart rate spectrum is the circadian rhythm. The autonomic nervous system contributes significantly to circadian heart rate variability. Using long-term recordings in conscious rabbits, Barrett et al. (6) demonstrated a strong circadian rhythm in heart rate, mean arterial blood pressure, renal blood flow, and renal sympathetic nerve activity. The importance of this study is that it clearly demonstrates that sympathetic nerve discharges exhibit a strong circadian rhythmicity. The paraventricular nucleus of the hypothalamus (PVN) appears to play a central role in mediating the circadian rhythm of autonomic nervous system activity. First, GABAergic and glutamatergic neurons project from the suprachiasmatic nuclei of the hypothalamus (SCN) to spinal-projecting neurons of the PVN (12). The SCN is the major central oscillator that triggers the day/night cycle. It receives photic input from the retina (47) and drives many neuroendocrine, metabolic, autonomic, and behavioral circadian rhythms (14, 16, 21, 31, 35, 44, 46, 50). In addition, microinjections of the inhibitory neurotransmitter GABA into the PVN of anesthetized rats elicit dosedependent decreases in renal sympathetic nerve activity, whereas bicuculline (a GABA antagonist) increases renal sympathetic nerve activity (56). Second, from the PVN, neurons project to the nucleus of the solitary tract (that integrates inputs from the baroreceptors), the nucleus ambiguus (origin of preganglionic parasympathetic neurons to the heart), the rostroventrolateral medulla (location of sympathetic premotor neurons), and the intermediolateral cell column of the thoracolumbar spinal cord (location of preganglionic sympathetic neurons). Thus PVN neurons can modulate autonomic nervous system activity by sending inputs to major sites of autonomic nervous system regulation. As an example, a pivotal role of the PVN for sympathoexcitation during parturition was recently demonstrated in sheep. The increase in sympathetic nerve activity that accompanies birth in maternal animals was prevented by stereotactic lesioning of the PVN (43). Taken together, a major component of the circadian heart rate variability is elicited by diurnal fluctuations in autonomic nervous system activity, generated by corresponding fluctuations of neuronal activity within the PVN, which depend on circadian inputs originating from the SCN. It has been suggested that thermoregulation affects VLF heart rate variability (10, 26). Cooling the heart causes bradycardia, a mechanism used in heart surgeries. Conversely, fever is known to increase heart rate. Raising body core temperature from 36.0 to 36.6°C caused an increase in heart rate by almost 40 beats/min in male subjects (1), whereas acutely reducing ambient temperature from thermoneutral conditions (35°C) to 29, 23, and 17°C, reduced heart rate from 400 to 250 beats/min in 8-day-old rats (7). In addition, lowering temperature in the isolated working Address for reprint requests and other correspondence: H. M. Stauss, Dept. of Exercise Science, The Univ. of Iowa, Iowa City, IA 52242 (E-mail: [email protected]). Am J Physiol Regul Integr Comp Physiol 285: R927–R931, 2003; 10.1152/ajpregu.00452.2003.