Loss of cerebral blood flow autoregulation and assessment of the critical cerebral perfusion pressure at high ICP (674.14)

Objective: To study the effect of increased intracranial pressure (ICP) on the critical cerebral perfusion pressure (CPP) and its relationship to the loss of cerebral blood flow (CBF) autoregulation. Background: Earlier studies showed that the critical cerebral perfusion pressure (CPP) fell from 60 to 30 mmHg as determined by decreasing arterial pressure as opposed to increasing intracranial pressure (ICP), respectively. We hypothesized that the decrease in the critical CPP at high ICP was due to microvascular shunting resulting in a pathological higher CBF at a lower CPP. Methods: Using two‐photon laser scanning microscopy (2PLSM) we measured microvascular (3‐25 m diam) flow velocity in rat cerebral cortex by increasing ICP or decreasing arterial blood pressure. In separate studies we also increased ICP progressively while monitoring the capillary (CAP) to microvascular shunt (MVS) ratio to study the effect of increasing ICP on the development of microvascular shunts. Results: The reduction in critical CPP with increasing ICP was due to increased microvascular shunting resulting in a sustained CBF at a lower critical CPP, which therefore decreased the critical CPP from 60 to 30 mmHg and associated with increased microvascular shunting. A progressive increase in ICP resulted in a parallel increase in the ratio of MVS/CAP ratio. Conclusion: The decrease in critical CPP obtained when ICP is increased as opposed to decreasing arterial pressure is due to the opening of microvascular shunts thereby creating low resistance microvascular conduits through the brain. The progressive increase in MVS with increasing ICP leads to the loss of CBF autoregulation, which differs from that observed at high CO 2 . Grant Funding Source : NIH RO3 NS051639;AHA 2BGIA11730011