Cerebral dynamics overview

Cerebral perfusion pressure (CPP) depends on mean systemic arterial pressure (MAP) and ICP by the following relationship:

As a result, CPP can be reduced from an increase in ICP, a decrease in blood pressure, or a combination of both factors. Through the normal regulatory process called pressure autoregulation, the brain is able to maintain a normal cerebral blood flow (CBF) with a CPP ranging from 50 to 150 mm Hg. At CPP values less than 50 mm Hg, the brain may not be able to compensate adequately, and CBF falls passively with CPP. After injury, the ability of the brain to pressure autoregulate may be absent or impaired, and even with a normal CPP, CBF can passively follow changes in CPP.

Causes of intracranial hypertension

• Intracranial (primary)
  • Brain tumor
  • Trauma (epidural and subdural hematoma, cerebral contusions)
  • Nontraumatic intracerebral hemorrhage
  • Ischemic stroke
  • Hydrocephalus
  • Idiopathic or benign intracranial hypertension
  • Other (eg, pseudotumor cerebri, pneumoencephalus, abscesses, cysts)
• Extracranial (secondary)
  • Airway obstruction
  • Hypoxia or hypercarbia (hypoventilation)
  • Hypertension (pain/cough) or hypotension (hypovolemia/sedation)
  • Posture (head rotation)
  • Hyperpyrexia
  • Seizures
  • Drug and metabolic (eg, tetracycline, rofecoxib, divalproex sodium, lead intoxication)
  • Others (eg, high-altitude cerebral edema, hepatic failure)
• Postoperative
  • Mass lesion (hematoma) Edema
  • Increased cerebral blood volume (vasodilation)
  • Disturbances of CSF

Intracranial hypertension secondary to traumatic brain injury

Special features should be considered in patients with traumatic brain injury (TBI), in which lesions may be heterogeneous, and several factors often contribute to increase the ICP 

1. Traumatically induced masses: epidural or subdural hematomas, hemorrhagic contusions, foreign body, and depressed skull fractures
2. Cerebral edema
3. Hyperemia owing to vasomotor paralysis or loss of autoregulation 
4. Hypoventilation that leads to hypercarbia with subsequent cerebral vasodilation
5. Hydrocephalus resulting from obstruction of the CSF pathways or its absorption
6. Increased intrathoracic or intra-abdominal pressure as a result of mechanical ventilation, posturing, agitation, or Valsalva maneuvers

Indications for ICP Monitoring

GCS Score: 3–8 (after resuscitation)

1. Abnormal Admission Head CT Scan
   1. Hematoma
   2. Contusion
   3. Edema
   4. Herniation
   5. Compressed basal cisterns
2. Normal Admission Head CT Scan PLUS 2 or more of the following
   1. Age > 40 years
   2. Motor posturing
   3. Systolic blood pressure < 90 mm Hg

Intracranial pressure treatment measures: brief summary of goals of therapy

1. Maintain ICP at less than 20 to 25 mm Hg.
2. Maintain CPP at greater than 60 mm Hg by maintaining adequate MAP.
3. Avoid factors that aggravate or precipitate elevated ICP.

Interventions to reduce Intracranial pressure

1-Decrease Brain parenchymal :

Osmotic therapy (eg, hypertonic saline, mannitol) to extract water volume ,.Brain parenchyma (80% of volume): Osmotic therapy decreases the parenchymal volume by creating an osmolar gradient that draws water out of edematous brain tissue. The most used agents are hypertonic saline  (administered via intermittent boluses or continuous infusion) and mannitol, ei (traumatic brain injury)

2-Decrease Cerebral blood volume :

-Head elevation
-Sedation to Decreased metabolic demand
-Hyperventilation to Decreased PaCO2, resulting in vasoconstriction

3-Decrease CSF volume :-

CSF removal (eg, external ventricular drain)

4-Increases Cranial volume:

- Decompressive craniectomy