Transcript
Chapter 15: Determinants and Assessment of Cerebral Perfusion
Chapter Summary
Cerebral arteries are thin and delicate, which makes them more susceptible to rupture. The carotid and vertebral arteries form the circle of Willis. The circle of Willis is the primary collateral pathway when major cerebral vessels become occluded. The cerebral venous circulation is valveless and drains by gravity, mostly through the internal jugular veins.
Cerebral oxygenation is maintained through a constant CBF through the process of autoregulation. Certain conditions affect CBF. When cerebral blood flow does not match cerebral metabolic needs, cerebral hypoxia or hyperemia can occur. Maintaining adequate cerebral oxygenation is of the utmost importance to support aerobic metabolism; episodes of hypoxia or hypotension must be avoided.
The Monro–Kellie hypothesis states that the cranial vault is rigid and fixed, and is made up of three compartments: the brain, the cerebral blood volume, and CSF. An increase in one component requires a decrease in another component or there will be an increase in ICP. Brain volume is controlled by the blood–brain barrier, and cerebral blood volume is controlled by CBF. CSF is displaced most easily and rapidly, and is the first reciprocal response to an increase in intracranial volume. CPP depends on MAP and ICP. CPP decreases when MAP is low or ICP is high.
Failure of intracranial mechanisms to compensate for increases in intracranial volume results in increased ICP and decreased intracranial adaptive capacity, and impairs cerebral tissue perfusion and oxygenation. Space-occupying lesions and cerebral edema increase brain volume and result in herniation, which carries a grave prognosis.
Deterioration in function of the cerebral hemispheres results in changes in level of consciousness, as evidenced by a decrease in GCS score, changes in pupillary and oculomotor responses, and changes in vital signs. Assessment of these changes helps determine the extent of cerebral dysfunction. Early intervention at the sign of any of these changes may prevent further neurologic damage.
A variety of invasive and noninvasive diagnostic tests are available to assess cerebral perfusion, and include CT scan, MRI, transcranial Doppler, cerebral angiography, and others.
ICP monitoring is an extremely useful adjunct in the care of the patient with a critical brain injury. It requires diligent high-acuity nursing care. Various ICP monitoring devices and systems are available. Clinicians must be familiar with the advantages, disadvantages, and complications specific to each device and system. The nurse must have a working knowledge of each system and be able to recognize system inaccuracies.
