Nonlinear transcranial Doppler analysis demonstrates age-related changes of cerebral hemodynamics

We studied the age- and gender-related changes in complexity of arterial blood flow by analyzing the maximum velocity waveforms. Data were collected by insonation of the middle cerebral artery blood flow by a 2-MHz transcranial pulsed Doppler system. Following the paradigm of nonlinear dynamical systems or "chaos" theory, complexity is best evaluated by estimating the correlation dimension (D2) and the largest Lyapunov exponent (lambda 1). Forty healthy persons (male/female ratio: 1/1; mean age 48.6 y; range 19-86 y) were studied. No gender-related differences were observed. The age-effect showed a diminishing lambda 1 [df(1, 36) = 5.687; p < 0.022] and an increasing D2 at higher age [df(1, 36) = 4.997; p < 0.032]. The age-related decline of the lambda 1 implies a more prominent periodicity, explained by reduced fluctuations in R-R intervals and an altered gain of the baroreceptor reflex. The increased D2 might be related to more prominent vessel wall oscillations due to the increased vessel wall stiffness at higher age.