The cerebrovascular circulation is different from other vascular beds, reflecting the high metabolic demand of brain tissue and need for tight regulation of water homeostasis because small changes in brain volume can lead to large changes in intracranial pressure. Cerebral blood flow (CBF) remains relatively constant for cerebral perfusion pressure (intra-arterial pressure – intracranial pressure) and should be maintained above 40 mm Hg in children from birth to age 5 years and above 50 mm Hg in those 6 to 17 years old.1 Cerebral blood flow also varies inversely with hematocrit and increases dramatically as the partial pressure of oxygen (Po2) falls below 50 mm Hg through direct effects on vascular cells in the cerebral arteries and arterioles and by the increased production of nitric oxide, which promotes vasodilatation. Increases in the partial pressure of carbon dioxide (Pco2) also cause an increase in CBF by a direct effect of extracellular hydrogen ions on vascular smooth muscle and may be seen with other causes of acidosis.2 If cerebral blood flow is inadequate, the oxygen extraction increases. This is to prevent tissue hypoxemia that is poorly tolerated by brain tissue and quickly leads to ischemia and often irreversible infarction and neurological impairment.
John J. Strouse. Cerebral Oxygenation and Transfusion for Severe AnemiaSearching for Functional Outcomes to Guide Transfusion. JAMA Pediatr. 2016;170(10):936–937. doi:10.1001/jamapediatrics.2016.1730