Intracranial Pressure and Cerebral Perfusion

Intracranial Pressure

• Normal ICP is 10-15 mmHg (136-204 mm H2O).
• Normal Cerebral Blood Flow (CBF) is 55-60 ml/min/100gm brain tissue.

Cerebral Blood Flow

• In gray matter, CBF is 75 ml/min/100 gm brain tissue.
• In white matter, CBF is 45 ml/min/100 gm brain tissue.
• Cerebral Perfusion Pressure (CPP) = mean arterial pressure - ICP.
• CPP = (diastolic pressure + 1/3 of pulse pressure) - ICP.

Factors Affecting CBF

• Systemic BP
• CO2 concentration and H ions in arterial blood (most potent stimulus)
• O2 concentration

Cerebral Autoregulation

• Ability to maintain CBF at a constant level over a wide range of mean arterial pressure (50-160 mmHg).
• When mean arterial pressure is low, cerebral arterioles dilate to allow adequate flow, and vice versa.
• Cerebral autoregulatory system fails when mean arterial BP falls below 50 mmHg or above 160 mmHg.

Raised ICP

• Persistent elevation of ICP more than 20mmHg.
• Common causes:
  • Localized masses (hematoma, tumor, abscess, etc.)
  • Obstruction to CSF path (obstructive hydrocephaly)
  • Obstruction to major venous sinuses (cerebral venous thrombosis, depressed skull fracture over major venous sinus)
  • Diffused brain edema or swelling (diffused head injury, encephalitis, lead encephalopathy)
  • Idiopathic (pseudotumor cerebri)

Pathological Effects of Raised ICP

• Decrease CPP resulting in cerebral ischemia
• Distortion and herniation of the brain

Symptoms and Signs of Raised ICP

• Headache (most common)
• Vomiting
• Diplopia
• Ataxia
• Papilledema
• 6th nerve palsy (false localizing sign)
• Signs of brain herniation
• Hypertension, bradycardia, and respiratory irregularities (Cushing's response)

Treatment of Raised ICP

• Treatment of underlying cause (tumor resection, hematoma evacuation)
• Head elevation with neck straight
• Ventricular drainage (external or internal)
• Mannitol
• Hypertonic saline
• Loop diuretics (frusemide)
• Steroid (dexamethasone) for chronic raised ICP
• Barbiturate coma using short-acting barbiturate (thiopental)

ICP Monitoring

• Indications:
  • GCS < 8
  • CT findings (operative or non-operative)
  • Post-operative hematoma evacuation
  • High-risk patients (above 40 years, hypotension, patients who require ventilation)
• Methods:
  • Non-invasive (clinical deterioration, transcranial Doppler, manual feeling of craniotomy flap or skull defect)
  • Invasive (catheter connected to electrodes inserted intraventricular, subdural space, or intraparenchymal)