Components of heart rate variability — what they really mean and what we really measure

Abstract

I n 1981, Akselrod et al’ examined the frequency components of heart rate variability (HRV) in dogs at physiologic rest, and under parasympathetic and total autonomic blockade. That study, which was later reproduced in humans,2 showed that the so-called highfrequency components of physiologic HRV (i.e., spectral components in the band of 0.15 to 0.5 Hz [approximate]) are predominantly modulated by the parasympathetic nervous system, whereas the lower frequency components (spectral band 0.05 to 0.!5 Hz) are under the iniluence of both the parasympathetic and sympathetic systems. Parasympathetic predominance of the autonomic system associated with increase of the high-frequency components of HRV has been achieved by both maneuvers, such as controlled respiration and cold stimulation of the face, and p-adrenergic receptor blockade.3,4 However, sympathetic excitation, which was reflected by increased low-frequency components of HRV, has most frequently been achieved with baroreceptor unloading.5 Thus, the highand low-frequency components of HRV were fumly associated with parasympathetic and sympathetic intluences, respectively, because of many studies that investigated the effects of different maneuvers and pharmacologic interventions on HRV After these investigations, many researchers took for granted that the spec& spectral components of HRV can be used not only as markers of the vagal and sympathetic influences on the modulations of heart rate, but also as measures of the tone of the parasympathetic and sympathetic nervous systems under various conditions. The same concept is frequently applied to some timedomain methods for HRV assessment, which are known to provide results similar to those of the speciiic spectral components. However, the idea that in all different circumstances, the HRV components closely reflect the degree of autonomic tone has no solid basis, and the purpose of this short editorial is to point out the underlying misunderstanding of this concept. The cardiac parasympathetic tone manifests in efferent vagal impulses. These impulses and their effects are very short and discrete, and their intrinsic frequency is much faster than that of high-frequency components of HRY6,7 The high-frequency componenCs of HRV correspond to the modulation of vagal tone, which is linked, for example, to respiration. These modulations of vagal efferent activity cause short-term alterations of the cycle length of sinus rhythm. Thus, the increase of highfrequency components of HRV reflects increased mod-