Cerebral blood flow at constant cerebral perfusion pressure but changing arterial and intracranial pressure: relationship to autoregulation

Therapeutic agents for reducing raised intracranial pressure (ICP) may do so at the expense of reduced mean arterial pressure (MAP). As a consequence, cerebral perfusion pressure (CPP) = (MAP - ICP) may not improve. It is unknown whether the level of MAP alters cerebral blood flow (CBF) when MAP and ICP change in parallel so that CPP remains constant. This study investigates CBF at a constant CPP but varying levels of MAP and ICP in 12 anaesthetized cats. CBF was studied at three levels of CPP: 60 (n = 4), 50 (n = 4), and 40 mm Hg (n = 4) under conditions of both intact and impaired autoregulation. At CPP levels of 50 and 60 mm Hg, when autoregulation was intact, CBF remained unchanged. With loss of autoregulation, there was a trend for CBF to increase as MAP and ICP were increased in parallel at a CPP of 50 and 60 mm Hg, although the relationship did not achieve statistical significance. Absolute CBF levels were, however, significantly different between the autoregulating and nonautoregulating groups (p <0.001). At a CPP of 40 mm Hg, CBF showed a linear correlation with blood pressure (BP) (r = 0.57, p <0.05). These results demonstrate that when autoregulation is impaired, there is a functional difference between autoregulating and nonautoregulating cerebral vessels despite similar MAP and CPP. These results also show that at a CPP of 40 mm Hg when autoregulation is impaired, CBF depends more on arterial driving pressure than on CPP.