A sudden arterial blood pressure decrease is compensated by an increase in intracranial blood volume

A sudden decrease in arterial blood pressure (ABP) will cause the intracranial blood volume (IBV) to rise, despite the fact that arterial cerebral blood flow decreases. The aim of this study was to test the hypothesis that the increase in IBV is caused by a relative decrease of intracranial venous outflow.

In 10 healthy volunteers we studied cerebral autoregulation (CA) by causing an ABP drop with bilaterally deflating leg cuffs. Blood flow velocities (BFV) in the middle cerebral artery and the straight sinus were monitored continuously with transcranial Doppler ultrasound, and the ABP with a non-invasive photoplethysmographic method. After transforming all variables in relative changes, the arterio-venous BFV difference was calculated. Allowing for diameter changes of the intracranial vessels of up to 10 %, and assuming a resting averaged cerebral blood flow of 55 to 60 ml per 100 g brain tissue per minute, an IBV increase of 9 to 10 ml could be calculated.

In intact CA, a steep decrease of ABP results in an increase of intracranial blood volume. The transformation of our IBV data by means of the human intracranial pressure-volume relationship results in an excellent agreement with previously reported ICP increases of 10 mmHg. The increase in intracranial blood volume might be of clinical relevance in orthostatic dysregulation by increasing the ischemic tolerance of the brain before cerebral autoregulation becomes effective.