Cerebral blood flow

Cerebral blood flow, or CBF, is the amount of bloodthat enters the brain. The adult human brainuses about 20% of the blood put out by the heart(Orlando Regional Healthcare, 2004), and blood normally takes up about 10% of the intracranial space(Sanders and McKenna, 2001; Walters, 1998).

CBF is tightly regulated to meet the brain's metabolicdemands, and on the average must be maintained at a flow of 50 milliliters(ml) of bloodper 100 grams(g) of brain tissue per minute (Orlando Regional Healthcare, 2004). It is important to maintain CBF within narrow limits because too much blood can raise intracranial pressure(ICP), which can compress and damage delicate brain tissue (Orlando Regional Healthcare, 2004), and too little blood causes ischemia, or inadequate blood supply. Ischemia results if blood flow to the brain is below 18 to 20 ml per 100 g per minute, and tissue death occurs if flow dips below 8 to 10 ml per 100 g per minute (Orlando Regional Healthcare, 2004). In brain tissue, a biochemical cascadeknown as the ischemic cascadeis triggered when the tissue becomes ischemic, potentially resulting in damage to and death of brain cells. Medical professionals must take steps to maintain proper CBF in patients with conditions like shock, stroke, and traumatic brain injury. Cerebral blood flow in excess of 55 to 60 ml per 100 g per minute, called hyperemia, is more than the brain needs and can contribute to an increase in intracranial pressure.

Cerebral blood flow is determined by a number of factors, such as viscosityof blood, how dilated blood vesselsare, and the net pressureof the flow of blood into the brain, known as cerebral perfusion pressure, which is determined by the body's blood pressureand intracranial pressure. Cerebral blood vessels are able to change the flow of blood through them by altering their diameters in a process called autoregulation; they constrict when systemic blood pressureis raised and dilate when it is lowered (Kandel, 2000 p.1305). Arterioles also constrict and dilate in response to different chemical concentrations. For example, they dilate in response to higher levels of carbon dioxidein the blood (Kandel, 2000 p.1305).

Functional magnetic resonance imagingand positron emission tomographyare neuroimagingtechniques that can both be used to measure CBF. These techniques are also used to measure regional CBF (rCBF) within a specific brain region.

See also

• Cerebral perfusion pressure
• Cerebral circulation

References

• Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., McGraw-Hill, New York.
• Orlando Regional Healthcare, Education and Development. 2004. "Overview of Adult Traumatic Brain Injuries."
• Sanders MJ and McKenna K. 2001. Mosby?s Paramedic Textbook, 2nd revised Ed. Chapter 22, "Head and Facial Trauma." Mosby.
• Shepherd S. 2004. "Head Trauma."Emedicine.com.
• Stock A and Singer L. 2004. "Head Trauma."Emedicine.com.
• Tolias C and Sgouros S. 2003. "Initial Evaluation and Management of CNS Injury."Emedicine.com
• Walters, FJM. 1998. "Intracranial Pressure and Cerebral Blood Flow."Physiology. Issue 8, Article 4.

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