Review
Heart rate variability explored in the frequency domain: A tool to investigate the link between heart and behavior

https://doi.org/10.1016/j.neubiorev.2008.07.006Get rights and content

Abstract

The neural regulation of circulatory function is mainly effected through the interplay of the sympathetic and vagal outflows. This interaction can be explored by assessing cardiovascular rhythmicity with appropriate spectral methodologies. Spectral analysis of cardiovascular signal variability, and in particular of RR period (heart rate variability, HRV), is a widely used procedure to investigate autonomic cardiovascular control and/or target function impairment. The oscillatory pattern which characterizes the spectral profile of heart rate and arterial pressure short-term variability consists of two major components, at low (LF, 0.04–0.15 Hz) and high (HF, synchronous with respiratory rate) frequency, respectively, related to vasomotor and respiratory activity. With this procedure the state of sympathovagal balance modulating sinus node pacemaker activity can be quantified in a variety of physiological and pathophysiological conditions. Changes in sympathovagal balance can be often detected in basal conditions, however a reduced responsiveness to an excitatory stimulus is the most common feature that characterizes numerous pathophysiological states. Moreover the attenuation of an oscillatory pattern or its impaired responsiveness to a given stimulus can also reflect an altered target function and thus can furnish interesting prognostic markers. The dynamic assessment of these autonomic changes may provide crucial diagnostic, therapeutic and prognostic information, not only in relation to cardiovascular, but also non-cardiovascular disease. As linear methodologies fail to provide significant information in conditions of extremely reduced variability (e.g. strenuous exercise, heart failure) and in presence of rapid and transients changes or coactivation of the two branches of autonomic nervous system, the development of new non-linear approaches seems to provide a new perspective in investigating neural control of cardiovascular system.

Section snippets

Conceptual background

The neural regulation of cardiac function is mainly determined, in its efferent side, by the interaction of sympathetic and vagal mechanisms (Fig. 1). In most physiological conditions, the activation of either sympathetic or vagal outflow is accompanied by the inhibition of the other suggesting the concept of sympathovagal balance, as a horizontal beam pivoted at its center (Malliani, 2000). This reciprocal organization, alluding to a synergistic design, seems instrumental to the fact that

Methodology

Variable phenomena such as heart period or arterial blood pressure can be described not only as a function of time (i.e. in the time domain), but also the sum of elementary oscillatory components, defined by their frequency and amplitude (i.e. in the frequency domain).

The analysis of HRV is usually performed off-line with computerized techniques. It is impossible, in this context, to address the various approaches for which we refer to previous articles (Pagani et al., 1986, Malliani et al.,

Practical aspects

In principle, spectral analysis, used to detect possible rhythmicities hidden in the signal, necessitates stationary conditions that, in strict terms, are unknown to biology.

A crucial procedure to be simultaneously performed is to obtain some measurement of respiratory rate in order to assess its synchronization with HF component (Malliani et al., 1991, Montano et al., 1994) (Fig. 2). Conversely, when the frequency of respiration decreases enough to approach the LF rhythm in such a way that HF

Physiological studies

The core hypothesis of the proposed approach was that the sympathovagal balance can, on the whole, be explored in the frequency domain. Quite numerous data support the assumptions that (1) the respiratory rhythm of heart period variability, defined as HF spectral component, is a marker o vagal modulation (Akselrod et al., 1981, Pagani et al., 1986, Malliani et al., 1991, Montano et al., 1994); (2) the rhythm defined as LF, present in RR and SAP variabilities and corresponding to vasomotor waves

Physical exercise: the complex simplicity of cardiovascular neural control

Spectral analysis of short-term HRV has been shown to be capable to detect and track the complex adaptational changes in sympatho-vagal balance attending regular physical training, a core component of primary and secondary cardiovascular prevention and of an optimal care management in cardiovascular diseases. This has been demonstrated from patients to high-performance world class athletes, implying relevant clinical information and practical applications.

In hypertensive patients who feature

General considerations

In spite of an always more diffuse use of spectral methodology and of a Task Force attempt (1996), true standard values (Fagard et al., 1999) corresponding to normal or abnormal conditions are not yet available. This is not surprising and, in a sense, is only partly detrimental. Indeed, what is to be measured is the dynamic equilibrium of the sympathovagal balance and the range of its excursions that can be extremely wide. Quite obviously this complex ensemble of properties is affected by a

Future perspectives

Linear analysis of HRV can therefore furnish non-invasive indexes of cardiac autonomic modulation in the presence of rhythmic variability. On the other hand, in settings characterized by rapid and non-repetitive changes, like the periods preceding cardiac events, or in conditions characterized by co-activation of the two branches of the ANS, non-invasive standard measurements of these control systems give less reliable information than during more stable periods (Porta et al., 2000).

We have

Conclusions

Frequency domain analysis of heart rate variability is a physiological and clinical tool having the merit of being totally non-invasive and of providing a global, although indirect, evaluation of autonomic modulation of heart period. In several instances the assessment of sympathovagal balance obtained with this procedure seems to reflect an even more general equilibrium ranging from quiet to excitation.

This approach, together with other methods based on non-linear dynamics, pertains to the

Acknowledgments

N.M. was partly supported by Italian Space Agency DCMC Grant; F.I. was supported by Italian Space Agency DCMC grant I/006/06/0.

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    To the memory of Alberto Malliani, an extraordinary mentor, physician, scientist and friend.

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