Figure 2.

Expected effects of subarachnoid hemorrhage on autoregulation. Altered vascular reactivity in subarachnoid hemorrhage (SAH) can have three hypothetical effects on cerebral autoregulation. Note that in this figure, cerebral autoregulation is plotted as the relationship between cerebral blood flow (CBF) and cerebral perfusion pressure (i.e., the difference between mean arterial pressure and intracranial pressure): this was done because intracranial pressure varies in SAH patients, thereby adding a variable to consider when relating CBF to mean arterial pressure in this pathological setting. The autoregulation curve in black represents the normal, non-pathological situation, while the red and yellow lines represent altered autoregulation. In (A), augmented myogenic reactivity (i.e., microvascular constriction) reduces perfusion, shifts the upper limit of autoregulation leftward and narrows the autoregulatory range. In severe cases, cerebral blood flow drops below the critical perfusion limit. In (B), upstream large artery constriction (i.e., angiographic vasospasm) reduces the perfusion pressure entering the microcirculation. This stimulates a right-ward shift in the autoregulatory curve, but perfusion deficits do not occur until the new lower limit is reached. In (C), both microvascular and larger artery constriction occur, creating a hybrid of (A) and (B).