[Cerebral reserve. A method for evaluating cerebro-vascular hemodynamic risk].

Together with embolic risk, hemodynamic risk tends to play an important role in cerebrovascular pathology. Studies of cerebral reserve have resulted from the difficulty of accurately estimating the repercussions of arterial lesions. The vascular reserve of a given region is the ratio between maximum and baseline flow. Maximum cerebral flow is obtained by inhalation of a gaseous mixture containing CO2 5%. An excellent correlation was found between lesion size and a decrease in the reserve. Three stages could be defined by positron emission transaxial tomography: a zone of hemodynamic reserve without modification of oxygen extraction; a zone of extraction reserve in which inflow insufficiency is compensated by increased oxygen extraction (this is the critical zone requiring action); and a zone of actual ischemia in which the extraction reserve is not exhausted. However, this technique is cumbersome and difficult to apply. Protocols are now being developed using the transcranial Doppler examination to define "a limit of autoregulation reserve", in which case it will be possible to assess the effects of arterial lesions. In fact, it would appear that cerebral irrigation depends on a very large number of factors, many of which are variable, whereas only a "snapshot" of the vascular situation can be obtained. Nevertheless, hemodynamic risk can be better assessed through an association of different explorations.