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Cranial contents
  • Skull represents a closed, non-expandable unit containing three compartments (CBV, CSF, brain tissue) determining ICP
  • Any increase in volume of one of the cranial compartments must be compensated by a decrease in volume of another to maintain the pressure equilibrium (Monro-Kellie doctrine)
  • Small increases in intracranial volume will lead to large increases in ICP once this buffer mechanism is exhausted (Figure 101-1), compromising CBV
Blood and cerebral vasculature (cerebral blood volume, CBV)
  • Two carotid arteries (70% blood flow to the brain)
  • Two vertebral arteries (30% blood flow to the brain)
  • Arterial anastomosis through Willis circle and anastomosis with external carotid arteries through branches of facial and ophthalmic arteries
  • Venous return through cortical veins (superficial drainage), and basilar and ventricular veins (profound drainage) essentially into the IJ veins
Cerebro-spinal fluid (CSF)
  • Produced by choroid plexus, reabsorbed by granules of Pacchioni
  • Total volume of CSF in the adult: 140–270 mL
  • Production of 0.2–0.7 mL/min or 600–700 mL/day
Figure 101-1. ICP as a Function of IC Volume

Reproduced from Morgan GE, Mikhail MS, Murray MJ: Clinical Anesthesiology. 4th Edition. Figure 25-5. Available at: Copyright © The McGraw-Hill Companies, Inc. All rights reserved.

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Physiologic Background: Cerebral Perfusion and Autoregulation
Cerebral blood flow (CBF), cerebral metabolic requirement of oxygen (CMRO2), and cerebral perfusion pressure (CPP)
  • CBF (normal: 50 mL/min/100 g of brain) is coupled with CMRO2 and determined by cerebral autoregulation and cerebrovascular reactivity to CO2
  • Cerebral perfusion pressure (CPP) is the difference between mean arterial pressure (MAP) and ICP
  • Cerebral autoregulation is the capacity of the cerebral arterioles to maintain a relatively constant CBF by alteration of their vascular resistance (vasoconstriction over a wide range of blood pressures (between MAP of approximately 50–150 mm Hg). At the extreme limits of MAP or CPP (high or low), CBF is directly proportional to CPP (Figure 101-2)
  • CO2 is a potent vasodilator, showing a relationship between PaCO2 (30–80 mm Hg) and CBF that is nearly linear
  • Autoregulation and cerebrovascular reactivity to CO2 may be altered in various pathological states (TBI, severe focal ischemia, brain tumor), as well as by administration of anesthetics
Figure 101-2. Influence of PaO2, PaCO2, and MAP (Green Line) on Cerebral Blood Flow

Reproduced with permission from Shapiro HM. Intracranial hypertension: Therapeutic and anesthetic considerations. Anesthesiology. 1975;43:445.

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Influence of Anesthetic Agents on CBF, ICP, and CMRO2
MedicationCBF and ICPCMRO2
Nitrous oxide
Volatile anesthetics1
Opioids2=Slight ↓

1Do not exceed 0.8 MAC ...

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