AANN Core Curriculum, 7th Edition, Ch. 4 Intracranial Dynamics: Digital Version-Chapter 4

Chapter 4

Pdf Summary

Intracranial dynamics describes how intracranial pressure (ICP), cerebral blood flow (CBF), and metabolism interact. Under the Monro–Kellie doctrine, the adult skull is a fixed-volume container holding brain tissue (about 80%), blood (10%), and cerebrospinal fluid (CSF, 10%). An increase in one component must be offset by a decrease in another, or ICP rises. Early compensation includes shifting venous blood and CSF and reducing CSF production. Once these reserves are exhausted, small additional volume increases can cause steep rises in ICP.<br /><br />Compliance is the change in volume accommodated per change in pressure; it falls as compensatory capacity is used up. Elastance is the inverse—pressure change per volume change—and reflects the brain’s resistance to expansion.<br /><br />CBF depends on cerebral perfusion pressure (CPP) and vascular resistance; CPP is approximately mean arterial pressure minus ICP. Vessel radius is a major determinant of flow, while increased blood viscosity reduces it. Typical adult CBF is about 50 mL/100 g/min (higher in gray than white matter). Flow below 18–20 mL/100 g/min is ischemic, below 10 risks tissue death, and above 55 may represent hyperemia. Normal adult CPP is about 80 mmHg; targets in traumatic brain injury are commonly 60–70 mmHg.<br /><br />Autoregulation normally keeps CBF relatively stable across CPPs of roughly 50–150 mmHg by adjusting vessel diameter. Flow also matches metabolic demand. Carbon dioxide strongly affects vessel tone: high PaCO₂ dilates vessels, while low PaCO₂ constricts them. Severe hypoxia causes vasodilation; neural influences play a smaller role. Autoregulation and CO₂ reactivity may be impaired after brain injury, making CBF vulnerable to pressure changes and mismatches with metabolic demand.<br /><br />The brain uses substantial oxygen and glucose, obtains most energy through aerobic glucose metabolism, and stores neither fuel nor oxygen. Ischemia disrupts cellular energy production and ion pumps, causing cytotoxic edema, excitatory neurotransmitter release, inflammation, free-radical injury, membrane breakdown, and ultimately cell death.

Keywords

intracranial pressure

cerebral blood flow

Monro-Kellie doctrine

cerebral perfusion pressure

autoregulation

compliance

elastance

carbon dioxide reactivity

ischemia

cytotoxic edema