Long‐range correlation of muscle sympathetic nerve activity and arterial blood pressure in older humans

The application of mathematical techniques to assess the bursting pattern of muscle sympathetic nerve activity (MSNA) in young individuals has revealed a non‐random, time‐scale‐invariant (fractal) property. With increased age there is an increase in basal MSNA, however it is unknown whether this is accompanied by changes in the nonlinear, dynamic pattern of MSNA. We employed detrended fluctuation analysis (DFA) and multiscale entropy measurements to compare the long‐range temporal correlation (LRTC) of MSNA (microneurography) and blood pressure (BP; Finometer) in 7 young (25±1 yr) and 7 older (60±2 yr) subjects. An obvious LRTC in MSNA was observed in young and older groups. In the young group MSNA entropy increased over a wide range of time scales and then stabilized to a relatively constant value at scales >45, but in the older group MSNA entropy increased almost monotonically (i.e., similar to Brown (1/f 2 ) noise). DFA scaling exponent for MSNA was slightly higher in the younger group (0.98±0.03 vs. 0.95±0.04, P>0.05). Entropy values for BP were lower in older subjects for all time scales (P<0.05). These data are suggestive of a decrease of LRTC in MSNA and a reduction in nonlinear BP signal complexity with increased age. Given the potential importance of physiological variability to the functional responsiveness of the individual to perturbation, such analyses may have future diagnostic and prognostic utility.