**1. Introduction**

This chapter discusses heart rate variability (HRV) to understand autonomic mechanisms and the use of linear analysis tools for frequency domain measures of HRV and spectral analysis by fast Fourier transform (FFT), and describes some results found in women.

Heart activity is largely modulated by the autonomic nervous system (ANS), which promotes rapid adjustments in the cardiovascular system during different stimuli (i.e., physical exercise, mental stress and postural change) (Hainsworth, 1998). HRV is a noninvasive measure used to analyze the influence of the autonomic nervous system on the heart, providing information about both sympathetic and parasympathetic contributions to consecutive heart rate (HR) oscillations. It has been proposed that a decrease in HRV is a powerful predictor of morbidity and mortality resulting from arrhythmic complications. HRV decreases with age (Catai et al., 2002; Melo et al., 2005) as a consequence of parasympathetic reduction and predominance of sympathetic modulation (Lipsitz et al., 1990; Longo & Correia, 1995; Akselrod, 1995).

The tool most commonly used in the frequency domain is spectral analysis, which consists of decomposing the HR variation in a given period into its fundamental oscillatory components, defining them by their frequency and amplitude. One of the mathematical algorithms most commonly used to determine the number, frequency and amplitude of these components is the FFT. The sum of all the components constitutes the so-called total power spectral density. Spectral analysis involves three distinct spectral components: 1) very low-frequency (VLF) fluctuations related to the renin-angiotensin system and thermoregulation; 2) low-frequency (LF) fluctuations related to the sympathetic and parasympathetic nervous systems and to baroreflex activity; and 3) high-frequency (HF) fluctuations associated with vagal activity (Longo & Correia, 1995; Task Force, 1996). The sympathovagal balance can also be expressed by the LF/HF ratio. Based on this analysis, it is possible to observe the predominance of one component over the other and the relationship between them, reflecting the autonomic modulation of the heart in the control of HR.

Spectral Analysis of Heart Rate Variability in Women 171

and then multiplying this ratio by 100. Since the LF band is modulated by both sympathetic and parasympathetic activity and the HF band is correlated with vagal cardiac control, the LF/HF ratio was calculated to determine the sympathovagal balance (Task Force, 1996). Sympathovagal balance is the ratio between LF and respiratory-frequency powers. Based on this analysis, it is possible to determine the predominance of one component over the other and the relationship between them, reflecting the autonomic modulation of the heart in the

Figure 1, which is based on an autoregressive model, illustrates the HRV power spectra at rest in the supine and sitting positions of a representative subject in different conditions.

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0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 **Frequency (Hz) Frequency (Hz)**

**Power spectral density (ms )**

**Power spectral density (ms )**

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0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 **Frequency (Hz) Frequency (Hz)**

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0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

**Frequency (Hz) Frequency (Hz)**

Fig. 1. Power spectral density of heart rate variability of a representative subject from the groups of young women (A and B), and postmenopausal women undergoing (C and D) and not undergoing (E and F) estrogen therapy, obtained at rest in the supine and sitting positions, respectively. Spectral components are shown as LF (0.04 to 0.15 Hz), HF (0.15 to 0.4 Hz) and

control of heart rate.

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**Power spectral density (ms )**

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**Power spectral density (ms )**

VLF (below 0.04 Hz). (adapted by Neves et al., 2007)

**Power spectral density (ms )2**

Given the importance of the autonomic nervous system to cardiovascular health, several analytical measures, grouped into linear and non-linear methods, can be used to assess HRV. The ECG is recorded with the subject in a steady state (when rhythms are stationary) for a sufficiently long period to determine events occurring within the frequencies of interest. R-R interval spectral power is calculated from this series of intervals using an autoregressive algorithm, which yields center frequencies and absolute power of component fluctuations (Task Force, 1996).

Sympathovagal balance (in dimensionless units) is simply the ratio of absolute LF to absolute HF power, or the LF/HF ratio. The literature on sympathovagal balance is replete with disclaimers that spectral power reflects fluctuations, not absolute levels of autonomic nerve traffic (Akselrod 1995). If mathematical manipulation of R-R interval spectral power is to inspire confidence as a robust, reliable metric, it must be grounded solidly on physiological principles. It must stand on its own and calculations of sympathovagal balance may obscure rather than illuminate human physiology and pathophysiology (Eckberg 1997).

This chapter discusses the measurement and analysis of HRV, as well as results of data for women and the relationship between aging and hormonal changes (oral contraceptives and hormone replacement therapy), which contribute to modifications of the autonomic control of the heart. Each item will be discussed in a separate subitem of this chapter.
