Cerebral perfusion pressure is calculated as MAP minus ICP. Brain ischemia occurs when CPP falls below what value?

• A. 20 mmHg
• B. 40 mmHg
• C. 60 mmHg
• D. 80 mmHg

Answer: (B)

Cerebral perfusion depends on the pressure gradient driving blood through the brain, which is CPP. The brain is protected by autoregulation across a range of CPP, but when this pressure drops below a critical level, blood flow cannot meet metabolic needs and brain tissue becomes ischemic. CPP is calculated as mean arterial pressure minus intracranial pressure, so increases in ICP or decreases in MAP reduce CPP. The commonly cited lower limit where ischemia begins is about 40 mmHg, so when CPP falls below roughly 40 mmHg, brain tissue is at risk of ischemia. For example, if MAP is around 90 and ICP rises toward 50, CPP would be about 40, placing the brain at the ischemic threshold. Values substantially above this threshold generally maintain adequate perfusion, while much lower values markedly increase ischemia risk. Therefore, 40 mmHg is the threshold at which brain ischemia becomes likely.