However, if ICP remains constant, the mean arterial pressure can change across its relatively wide range of normal without dramatically decreasing or increasing CPP. The mean arterial pressure is much more likely to change during day-to-day activities such as exercise, rest, and stress. The normal range of MAP is 70 to 100 mm Hg. MAP is the average blood pressure during one cardiac cycle and can be directly measured through invasive hemodynamic monitoring or calculated as the systolic blood pressure, plus two times the diastolic blood pressure, divided by three. ĬPP and MAP: As ICP in normal ranges is a relatively small number, the CPP is much more dependent on the mean arterial pressure. Too much alteration outside of normal CBF ranges can lead to brain ischemia and injury. When blood pressure increases, auto-regulation causes cerebral vasoconstriction and a decrease in CBF, resulting in a decrease in cerebral blood volume and maintaining ICP and CPP. When blood pressure decreases, auto-regulation causes cerebral vasodilation and an increase in CBF and cerebral blood volume, thus maintaining ICP and CPP.
Cerebral auto-regulation ensures a steady flow of blood to the brain over a wide range of physiologic changes and disturbances. As volume increases (a growing space-occupying lesion, brain tissue edema, or blood), this mechanism becomes overwhelmed, and ICP increases sharply.Ĭerebral blood flow (CBF) is also a critical factor in ICP homeostasis. As volume adds to the intracranial space, CSF can move into the spinal subarachnoid space, leaving the ICP relatively unchanged. Several mechanisms ensure that ICP remains in the normal range for as long as possible during periods of changing intracranial volume and compliance. As the ICP increases (or intracranial compliance decreases), CPP will decrease. As the skull is a fixed and rigid anatomic space, ICP will increase as intracranial volume increases and intracranial compliance decreases. Intracranial compliance is the relationship between ICP and volume in the intracranial cavity, including cerebrospinal fluid (CSF), brain tissue, and arterial and venous blood volume. Physiologically, ICP is a function of intracranial compliance. ICP is usually directly measured via intracranial pressure transduction. Under normal conditions, the ICP is between 5 and 10 mm Hg and thus has less of an impact on CPP than MAP for clinical situations not involving intracranial pathology. CPP and ICP: The CPP, at its most basic, is dependent on the ICP and mean arterial pressure, and its normal range is 60 to 80 mm Hg.
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