![]() ![]() ![]() Emphasis was given to articles published within the last ten years. ![]() We here review the available literature, using a PubMed search with the following keywords: cerebral blood flow, transcranial Doppler, syncope, orthostatic hypotension, autonomic testing and autonomic failure. We focus on the transcranial Doppler (TCD) as a non-invasive method to evaluate cerebral hemodynamics and its usefulness in the outpatient autonomic clinic. The elderly population, in particular those with autonomic or cardiovascular disorders, are at a greater risk for dementia, stroke, long-term disability and death. The failure of cerebral autoregulation can occur at any age. This adaptability ensures that the delivery of oxygen and nutrients can meet the high metabolic demands of the neuronal tissue in different regions of the brain. This interplay occurs at the level of the arterioles in the cerebrovasculature and at the neurovascular unit, and over multiple time scales from seconds to hours. The CBF autoregulation involves integrative interactions between brain tissue metabolism, systemic blood pressure, arterial blood gases, as well as neurogenic input from the central autonomic network. The inbuilt capacity of the cerebral circulation to regulate its own flow to remain constant in the face of changes in perfusion pressure is known as cerebrovascular autoregulation. When blood flow falls below the critical limit, even for a few seconds, syncope (i.e., a reversible loss of consciousness with no neurological sequelae) occurs. Brain tissue has high metabolic demands and, in order to maintain consciousness, it must receive an adequate supply of blood flow to ensure that energy and oxygen demands are met. 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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