Linear and nonlinear properties of heart rate variability: Influence of obesity

Physiological systems are best characterized as time-varying processes exhibiting rhythmic and complex behavior. The interaction among system variables, external noise, and state changes modulates the overall variability of physiological signals such as heart rate, arterial pressure, and respiration, which may therefore present both linear and nonlinear patterns. To describe the complex and periodic dynamics of living systems, various analytical tools have been employed, especially in the cardiovascular field. Among them, power spectral analysis (PSA) and recurrence quantification analysis (RQA) have been used to describe, respectively, linear and nonlinear dynamics of heart rate variability (HRV). PSA and RQA were applied to the R-wave peak interval (RR interval) time series as derived by continuous electrocardiographic (ECG) monitoring (250-Hz frequency sampling). We analyzed 21 ECG tracings recorded during 60 min of quiet, supine rest. Subjects were divided into two groups, 13 obese and 8 lean, on the basis of their body mass index (BMI ??28 kgom-2). The obese subjects displayed lower heart rate variability, mainly in the vagally modulated frequency component, with a relative sympathetic dominance. Furthermore, they showed greater periodicity and lower complexity properties than lean subjects. The PSA parameters related to periodicity were well correlated with the RQA parameters related to complexity. In the obese, loss of chaotic properties seems to be strictly linked with a reduced vagal response, as shown by the correlation between chaotic parameters, such as ENT and MEAND, and the LF/HF ratio. RQA parameters evaluated in the two groups were significantly different whereas LF/HF ratio, although higher in the obese group, did not reach statististical significance presumably because of the small sample size. Our findings indicate that (1) RQA is a valuable tool to study heart rate varibility; (2) autonomic outflow modulates both linear responses to endogenous stimuli and nonlinear properties of heartbeat; (3) obesity, a clinical condition with an excess of cardiac morbidity and mortality possibly due to autonomic dysfunction, is characterized by consensual changes in both linear and nonlinear characteristics of heart rate variability.