Influence of erythrocyte aggregability and plasma fibrinogen concentration on CBF with aging

The influence of the rheological properties of the blood on cerebral perfusion is still unresolved. Data on normal subjects are lacking and difficulties arise regarding the effect of blood viscosity owing to its close relationship with hematocrit. For these reasons we have studied the relationship between two rheological hematocrit-independent parameters and CBF in normal subjects of various ages. 36 normal volunteers, aged 20-74, free from risk factors, have been studied. CBF was measured by the Xenon inhalation method. Erythrocyte aggregability was expressed as Mean Erythrocyte Aggregation Index (MEA). Plasma fibrinogen concentration was evaluated by the coagulative method in 26 subjects. No correlation was found between CBF and MEA or fibrinogen in the subjects under the age of 45. A significant negative correlation was found between CBF and MEA (p = 0.015) and between CBF and fibrinogen (p = 0.011) in the subjects over 45. These data show that cerebral perfusion is influenced by the rheological properties of the microcirulation only with aging. We suggest that a "rheological autoregulation" exists and that it works properly in youth, only to be lost with physiological aging. This finding can be of significance in the pathogenesis of cerebrovascular disease processes in humans.