Autoregulation of cerebral blood flow (CBF) ensures that a fall in perfusion pressure is counterbalanced by vasodilatation of cerebral arterioles. Cerebral vasomotor reactivity (VMR) means an ability of arterioles to dilate (or constrict) to maintain an adequate CBF, which can be estimated by measuring the change in CBF in response to the vasodilatory stimuli. Hemodynamically compromised tissue is supplied by maximally dilated arterioles, that are therefore unable to dilate much further in response to additional vasodilatory stimuli. VMR can be evaluated with transcranial Doppler sonography (TCD), by measuring the changes of flow velocities induced by CO2 or acetazolamide. Several methods have been used to increase the CO2 concentration in the blood: inhalation of 5% CO2, alternative CO2 inhalation and hyperventilation, or breath-holding (for 30 sec). VMR measured with these methods is well correlated with the degree of stenosis in carotid disease. Among those, the breath-holding method is quick and easy to apply and CO2 equipments are not necessary. VMR measured with this method is comparable to CO2 inhalation or intravenous acetazolamide method. Rebreathing within the closed air bag is another method to increase CO2 concentration. Modified semi-closed method is more comfortable to patients and possible to achieve the maximum plateau flow velocity. In patients with stenosis of middle cerebral artery, VMR decrement measured by this method is well correlated with the degree of the stenosis. Bed-side TCD measurement of VMR is feasible and it may provide useful information about hemodynamic status and the risk of stroke in steno-occlusive disease of extracranial or intracranial arteries. Standardization and further internal/external validation of the methods are necessary for the wide clinical application.

Journal of Neurosonology 3(1):1-6, 2011
Key Words: Transcranial Doppler sonography, Vasomotor reactivity, Stroke, Carotid stenosis, Middle cerebral artery stenosis