Functional TCD: regulation of cerebral hemodynamics--cerebral autoregulation, vasomotor reactivity, and neurovascular coupling.

Three main mechanisms influence cerebral hemodynamics, with the aim of adapting the cerebral blood flow to the metabolic demand of the brain. Cerebral autoregulation ensures stable perfusion of the brain, independent of the systemic blood pressure. Vasomotor reactivity reflects the hemodynamic responses to modifications of the arterial pCO2/pH of the brain tissue. Neurovascular coupling adapts the perfusion to increased metabolic demand as a consequence of enhanced brain activity to permit reasonable functioning of cells. Different methods using transcranial Doppler sonography have been developed to characterize these mechanisms in healthy subjects and under pathologic conditions. The most established applications in clinical settings are described, and the results of specific research studies are briefly reported.