![]() ![]() Despite the human brain weighing only 2% of the total body mass, it receives 15% of the cardiac output at rest and consumes 20% of the body’s oxygen. We also describe the control of cerebral blood flow in autonomic disorders and functional syndromes.Ĭerebral blood flow (CBF) is normally 50–60 ml/min per 100 grams of brain tissue. We review clinical standards for the acquisition and analysis of TCD signals in the autonomic laboratory and the multiple methods available to assess cerebral autoregulation. Syncope occurs when hypotension is severe and susceptibility increases with hyperventilation, hypocapnia and cerebral vasoconstriction. Cerebral autoregulatory mechanisms help defend the brain against hypoperfusion when perfusion pressure falls on standing. TCD can be very useful in the work up of a patient with recurrent syncope. The transcranial Doppler (TCD) is a non-invasive technique that provides real-time measurements of cerebral blood flow velocity. This involves the interplay between myogenic and metabolic mechanisms, input from noradrenergic and cholinergic neurons, and the release of vasoactive substrates including adenosine from astrocytes and nitric oxide from the endothelium. The cerebral circulation has its own complex finely tuned autoregulatory mechanisms to ensure blood supply to the brain can meet the high metabolic demands of the underlying neuronal tissue. When cerebral blood flow falls below a critical limit, syncope occurs and if prolonged ischemia leads to neuronal death. ![]()
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