Fractal properties of human muscle sympathetic nerve activity

Abstract

Muscle sympathetic nerve activity (MSNA) in resting humans is characterized by cardiacrelated bursts of variable amplitude that occur sporadically or in clusters. The present study was designed to characterize the fluctuations in the number of MSNA bursts, interburst interval, and burst amplitude recorded from the peroneal nerve of 15 awake, healthy human subjects. For this purpose, we used the Allan and Fano factor analysis and dispersional analysis to test whether the fluctuations were time-scale invariant (i.e., fractal) or random in occurrence. Specifically, we measured the slopes of the power laws in the Allan factor, Fano factor, and dispersional analysis curves. In addition, the Hurst exponent was calculated from the slope of the power law in the Allan factor curve. Whether the original time series contained fractal fluctuations was decided on the basis of a comparison of the values of these parameters with those for surrogate data blocks. The results can be summarized as follows. Fluctuations in the number of MSNA bursts and interburst interval were fractal in each of the subjects, and fluctuations in burst amplitude were fractal in four of the subjects. We also found that fluctuations in the number of heartbeats and heart period (R-R interval) were fractal in each of the subjects. These results demonstrate for the first time that apparently random fluctuations in human MSNA are, in fact, dictated by a time-scale-invariant process that imparts "long-term memory" to the sequence of cardiac-related bursts. Whether sympathetic outflow to the heart also is fractal and contributes to the fractal component of heart rate variability remains an open question.Allan factor; dispersional analysis; Fano factor; heart rate variability; long-range correlations; time-scale invariance MUSCLE SYMPATHETIC NERVE ACTIVITY (MSNA) in resting humans is characterized by cardiac-related bursts of variable amplitude that occur sporadically or in clusters (1,9,10,18,20). In some subjects, cardiac-related burst amplitude waxes and wanes on the time scale of the respiratory cycle (9, 10, 18, 23). Whether respiratory-related or not, a hallmark of human MSNA is the variability of cardiac-related burst amplitude and interburst interval. In general, the muscle nerve fails to generate a burst in a variable number of cardiac cycles, thus leading to an MSNA burst-to-cardiac cycle ratio that is typically Ͻ1.0 in healthy human subjects.The primary aim of the present study was to define the basis for the fluctuations in the number of MSNA bursts, interburst interval, and burst amplitude. Two possibilities were considered. First, the variabilities might be best described by a random process in which events are uncorrelated (6, 27). The second possibility is that long-range correlations exist among events. If such correlations extend over more than one time scale, the fluctuations would be best modeled as a time-scaleinvariant (i.e., fractal) process in which the present value of the measured property is related not only to recent events but al...

Fractal fluctuations in human respiration

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