The control mechanism involved in post-subarachnoid hemorrhage vasospasm

In the present work the main relationships among cerebral blood volume (CBV), cerebrospinal fluid (CSF) dynamics, cerebral blood flow (CBF) and cerebrovascular reactivity following subarachnoid hemorrhage are critically examined and discussed. It is hypothesized that, following the rupture of an aneurysm, antagonistic mechanism which regulate CBF (through a vasodilatation of the arteriolar vessels) and CBV (through a constriction of basal intracranial arteries) are activated, due to the initial increase in intracranial pressure (ICP) the time pattern of ICP and cerebral hemodynamics in the following days can be largely different depending on the state of CSF dynamics. When the CSF outflow is not altered by blood in the subarachnoid space ICP suddenly returns to the basal value, and a normal cerebral hemodynamics is rapidly restored. By contrast, in conditions in which the normal CSF dynamics is impaired, the opposite action of mechanisms regulating CBF and CBV may lead to instability of the cerebrovascular bed, with the result of a maximal dilatation of pial vessels and a strong constriction of basal arteries (spasm). In our opinion the phenomenon of vasospasm can be better understood if the reactivity of basal intracranial arteries is analyzed as a part of the complex physiological system of cerebrovascular regulation.