[Physiopathology of the cerebral arterial circulation. Application to cerebro arterial surgery].

Autoregulation of cerebral arterial circulation corresponds to a hemodynamic reserve enabling normal cerebral blood flow to be assured when arterial blood pressure drops. However, the capacity of this autoregulatory system is exceeded when pressure drops below 60 +/- 10 mmHg, at which time metabolic disturbances appear followed by the arrest of electric activity and the death or neurons. These ischemic phenomena involve more or less extended regions: cerebral or choroidal arterioles or arteries, or even those of the entire brain. They can occur in series as, for example, the embolism of a cerebral arteriole or artery in a brain already globally hypoperfused downstream from a carotid occlusion or narrow stenosis. The association of these phenomena accounts for the different clinicopathologic cerebrovascular accidents. These disturbances of cerebral blood flow can be explored clinically by isotopic techniques. A cerebral perfusion blood pressure drop can be suspected, when nonhemispheric symptoms appear, on the basis of ophthalmodynamometry, oculoplethysmography or changes in intracranial arterial velocity (transcranial Doppler examination) during the Diamox (Acetazolamide) test. After release of carotid clamping or correction of a "hemodynamically significant" stenosis, increased cerebral blood flow can be observed, with sometimes a "cerebral hyperperfusion" syndrome similar to that of hypertensive events which exceed the capacities of cerebral arterial autoregulation.