The Lyapunov Exponent of Heart Rate Dynamics as a Sensitive Marker of Central Autonomic Organization: An Exemplary Study of Early Multiple Sclerosis

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Abstract
In a study of central autonomic dynamics in early multiple sclerosis, we measured the temporal oscillations of the momentary heart rate (heart rate dynamics). 11 young patients suffering from relapsing remitting definite multiple sclerosis in relapse-free and early stages of illness and 11 healthy controls were examined under vagotonic and sympathicotonic conditions. The temporal structure of the heart rate dynamics was operationalized phase-space analytically through the estimation of the largest Lyapunov exponent. Positive Lyapunov exponents were found in all participants under all conditions indicating deterministically chaotic heart rate oscillations. The variance analysis of these exponents detected no significant effect of sympathetic or vagal activity (experimental condition) but a significant group difference (p < .02). The multiple sclerosis patients were characterized by significantly lower Lyapunov exponents than the healthy controls. This finding suggests a more stable and thus less adaptive central-autonomic organization in early multiple sclerosis.