chapter  12
20 Pages

## Assessing Causal Interactions among Cardiovascular Variability Series through a Time-Domain Granger Causality Approach

Head-Up Tilt Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .237 12.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .238 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .238

Cardiovascular variables exhibit spontaneous, nonrandom, beat-by-beat fluctuations about their mean value and these variations are referred to as cardiovascular variability (Cohen and Taylor, 2002). Since the magnitude of these fluctuations depends on the state of the autonomic nervous system (Akselrod et al., 1981; Malliani et al., 1991), spontaneous cardiovascular variability has been basically exploited to noninvasively infer it. For example, the amount of the fluctuations of the heart period (HP) in the high frequency (HF, around the respiratory rate) band is considered as an index of vagal modulation directed to the heart, while the magnitude of the fluctuations of systolic arterial pressure (SAP) in the low-frequency (LF, from 0.04 to 0.15 Hz) band is generally employed as an index of sympathetic modulation directed toward the vessels (Akselrod et al., 1981; Cooke et al., 1999; Pagani et al., 1986). Since the evaluation of the autonomic control is mainly based on the assessment of the magnitude of HP and SAP fluctuations in the LF and HF bands, spectral analysis is the most widely utilized tool in cardiovascular variability studies.