Importantly, cerebral perfusion pressure (mean arterial pressure - the greater of ICP or CVP) is only a surrogate marker for cerebral blood flow. The function of hypotension as a useful clinical variable is dependent on two factors - first, its correlation with the true variable of interest (cerebral blood flow) and second, the ability of clinicians to manipulate the underlying variable of interest (cerebral blood flow) based on the surrogate marker (cerebral perfusion pressure).