A patient with head injury has a blood pressure of 120/60 mm Hg and an intracranial pressure (ICP) of 24 mm Hg. The cerebral perfusion pressure (CPP) is 56 mm Hg. This CPP most clearly indicates which of the following?

• A. Normal intracranial pressure
• B. High blood flow to the brain
• C. Impaired brain blood flow
• D. Adequate cerebral perfusion

Answer: (C)

Cerebral perfusion depends on the pressure driving blood through the brain, which is the difference between mean arterial pressure and intracranial pressure. Here, the mean arterial pressure is about 80 mm Hg (MAP ≈ [Systolic + 2×Diastolic]/3 = [120 + 2×60]/3 = 80). The intracranial pressure is 24 mm Hg. So the cerebral perfusion pressure is 80 − 24 = 56 mm Hg. Normal CPP is roughly 60–100 mm Hg, so 56 is just below normal. This means cerebral perfusion is impaired—the elevated ICP is reducing the pressure gradient needed to drive blood flow to brain tissue, increasing the risk of ischemia despite a seemingly adequate blood pressure.