Applied cerebrovascular physiology

Abstract

The understanding and manipulation of cerebrovascular physiology is essential in the management of head injuries and anaesthesia for neurosurgery. The high metabolic requirements (20% of basal oxygen and 25% of basal glucose consumption) and blood flow (15% of the cardiac output) emphasize the need to ensure adequate substrate delivery for the production of energy to transmit electrical impulses and maintain ionic gradients across cell membranes. The cerebral microenvironment is responsible for a blood-brain barrier with unique properties affecting ionic and fluid distribution, active transport mechanisms and drug distributions. The rigid cranium with its contents of parenchyma, CSF, blood and interstitial fluid creates pressure and volume relationships with implications in pathological processes as well as targets for therapy. Physiological determinants of cerebral blood flow and volume include flow-metabolism coupling, autoregulation, carbon dioxide and oxygen arterial tensions, temperature, haematocrit, venous pressures, and the autonomic nervous system. Local metabolic control of regional cerebral blood flow is mediated by actions on vascular tone (vasodilators and vasoconstrictors). The roles of nitric oxide, prostaglandins, adenosine, cations (potassium and calcium) and endothelin are noteworthy. Estimations of cerebral blood flow and oxygenation are essential to assess and to manipulate cerebrovascular physiology. Global and local techniques are briefly discussed including the Kety-Schmidt technique, xenon-133 washout, imaging (dynamic CT, SPECT, PET, and fMRI) jugular venous oximetry, transcranial Doppler, brain tissue oxygenation and intracerebral microdialysis.