Autonomic control of heart rate during physical exercise and fractal dimension of heart rate variability

The objectives of the present study were to investigate autonomic nervous system influence on heart rate during physical exercise and to examine the relationship between the fractal component in heart rate variability (HRV) and the system's response. Ten subjects performed incremental exercise on a cycle ergometer, consisting of a 5-min warm-up period followed by a ramp protocol, with work rate increasing at a rate of 2.0 W/min until exhaustion. During exercise, alveolar gas exchange, plasma norepinephrine (NE) and epinephrine (E) responses, and beat-to-beat HRV were monitored. HRV data were analyzed by 'coarse-graining spectral analysis' (Y. Yamamoto and R. L. Hughson. J. Appl. Physiol. 71: 1143-1150, 1991) to break down their total power (P(t)) into harmonic and nonharmonic (fractal) components. The harmonic component was further divided into low-frequency (0.0-0.15 Hz) and high- frequency (0.15-0.8 Hz) components, from which low-frequency and high- frequency power (P(l) and P(h), respectively) were calculated. Parasympathetic (PNS) and sympathetic (SNS) nervous system activity indicators were evaluated by P(h)/P(t) and P(l)/P(h), respectively. From the fractal component, the fractal dimension (D(F)) and the spectral exponent (β) were calculated. The PNS indicator decreased significantly (P < 0.05) when exercise intensity exceeded 50% of peak oxygen uptake (V̇O(2 peak)). Conversely, the SNS indicator initially increased at 50-60% V̇O(2 peak) (P < 0.05) and further increased significantly (P < 0.05) at >60% V̇O(2 peak) when there were also more pronounced increases in NE and E. The decreased PNS and the increased SNS indicators during exercise were accompanied by an increase in β (a decrease in D(F)) to the level of β > 2 (D(F) < 2). These results suggested that changes in the fractal component in HRV might be coupled to those in sympathovagal balance during physical exercise.