Question 14

Describe the factors that influence intracranial pressure.

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College Answer

A structure approached works well for “describe the factors …” questions. Better answers 
provided a definition of ICP, explained the Monro-Kellie doctrine and then detailed the factors 
which affect the volume of each of the components - cerebro spinal fluid (CSF), cerebral blood 
flow and brain parenchyma. Some candidates focused only on factors which cause intracranial 
hypertension and were thus unable to score full marks. Many candidates stated that CSF 
production was ICP dependant which is incorrect.


CSF production is CPP-dependent, people. 

  • Intracranial pressure (ICP) is the pressure within the intracranial space relative to atmospheric pressure
    • It is determined by the three components of the Monro-Kellie relationship, which states that an increase in the volume of one intracranial compartment will lead to a rise in ICP unless it is matched by an equal reduction in the volume of another compartment
    • These compartments are:
      • Brain tissue: 1400ml on average
      • Cerebral blood volume: 150ml
      • Cerebrospinal fluid: 150ml
  • Factors that affect ICP include:
    • Factors affecting the CSF volume
      • CSF efflux: ability to drain the CSF
        • Into the spinal canal (eg. hydrocephalus)
        • Out of the CNS (eg. EVD, VP shunt, inadvertent dural tear)
        • Into the venous circulation (arachnoid granulations, blocked by meningitis and subarachnoid haemorrhage)
      • CSF production
        • Acetazolamide (decreases CSF production by chroid plexus)
        • Volume status of the patient, eg. dehydration 
    • Factors affecting the brain tissue
      • Age (decreased mass)
      • Space occupying lesions (eg. tumour, abscess)
      • Cerebral oedema 
    • Factors affecting the Monro-Kellie doctrine
      • Continuity of the cranial vault (i.e. decompressive craniectomy)
    • Factors affecting the cerebral blood volume
      • Cerebral arterial blood volume
        • Cerebral metabolic rate
          • Drugs, eg. sedatives
          • Temperature (eg. hypothermia and hyperthermia)
          • Pathological increased activity, eg. seizures
        • Cerebral arterial vasoactive agents:
          • CO2, hypoxia, 
        • Systemic increases in blood flow or blood pressure
          • Noxious stimuli, eg. pain, anxiety
      • Cerebral venous blood volume
        • Venous outflow obstruction:
          • C-spine collar, jugular or sinus venous thrombosis
          • Raised intrathoracic pressure, eg. Valsalva, cough, childbirth 
          • Poor RV compliance, eg. right heart failure
          • Muscle contraction, eg. shivering
        • Venous reflux
          • Head-down position
        • Venous efflux
          • Upright posture


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