Definition:

• ICP: hydrostatic pressure within the cranial vault
• Normal value is 5-15 mmHg
• focal ischaemia when ICP > 20 mmHg
• global ischaemia when ICP > 50mmHg

Munro-Kellie Doctrine

• The rigid and closed cranial vault forms a fixed brain volume containing
  • Brain parenchyma (80%, 1400 g)
  • CSF (10%; 75 mL)
  • Cerebral blood and vessels (10%; 75 mL)
• Δ’s in volume of any components → Δ’s in others or and increase in ICP

CSF Production / Absorption

CSF Production:

• 60-70% of the CSF is formed by the choroid plexuses
• 30-40% by the cerebral vessels lining the ventricular walls
• Normal rate is 20mL/hour
• Formation independent of ventricular pressure
• Mechanism
  • From Coroid Plexus by net transport of Na+, K+, Cl-, HCO3- and water, from plasma to ventricles
  • Na down Conc grad
  • Others down electro chem grads

CSF Absorption:

• Absorbed through the arachnoid villi into the cerebral venous sinuses
  • Absorption by bulk flow, is proportional to ventricular pressure
  • If pressure < 7 cmH2O, CSF absorption ceases
  • Above 7cmCSF absorption is linear
  • At approximately 11 cmH2O, CSF Absorption = Formation

Control of ICP

• ICP is regulated via volume buffering
  • i.e. increase in volume of one intracranial component leads to compensatory decrease in volume of other intracranial components
• When volume buffering mechanism is exhausted → rapid increase in ICP (decompensation)
  • Movement of cerebral venous blood = rapid compensation, lower capacity
  • Movement of CSF = gradual compensation, larger capacity

Determinants of ICP:

• Brain
  • Age / Mass
  • Space occupying lesions
  • Cerebral Oedema
• CSF
  • CSF production
  • CSF Absorption
• Cerebral Blood Volume
  • Cerebral autoregulation: Flow-metabolic coupling
  • Cerebral metabolic rate
  • Increase in systemic blood pressure / flow
  • Venous Outflow obstruction
  • Vasoactive agents
• Monro-Kellie Doctrine
  • Loss of above – e.g. Fractures, surgery

Compensation for Elevated ICP (Intracranial Pressure)

Early compensation

• Δ CSF distribution and flow
• CSF is displaced to spinal subarachnoid space
• ↑’d resorption rate

Late compensation

• ↑ ICP → ↓ CBF → ↓ in cerebral blood volume → cerebral ischaemia

Decompensation

• ↑ICP → ↓ in cerebral tissue volume → brain herniation
• Cushing Reflex
  • Hypertension, bradycardia and abnormal breathing associated with raised ICP
• Mechanism:
  • Stage 1:
    • ↑ ICP → ↓ blood supply to vasomotor area → Local hypoxia/hypercarbia → ↑ SNS >> ↑ PSPS vasomotor stimulation
    • ↑ TPR → ↑ MAP
    • ↑ HR → ↑ CO
    • → compensatory ↑CBF
  • Stage 2:
    • ↑ CO → Baroreceptor stimulation → ↑ Vagus nerve stimulation → Bradycardia and ↓ contractility.

Gladwin / JC 2020

Methods of Measurement of ICP (Outline)

Invasive

Non-Invasive

Non-invasive methods like pupillometry, CT, MRI, TCD provide an adjunct to clinical examination of high ICP, but are not a surrogate for invasive ICP measurement.

Gladwin / JC 2020

2022A 10: 64% of candidates passed this question.
In the good answers to this question, and there were a number, the candidates included the volumes of the cranium and a correct description of the Monroe Kellie doctrine. A good answer should have included the compensations and consequences of increases in intra-cranial volumes; a discussion of all three components (brain tissue, blood, and CSF) and how they affect intracranial pressure; and then information on intra-ventricular and parenchymal devices in measuring ICP, briefly including their pros and cons. A common issue was writing quite a lot more than was needed on the relationship of cerebral blood flow to cerebral blood volume, and/or on the physiological consequences of raised ICP, which seemed to leave little time for discussion elsewhere. A few candidates did not provide any response for ICP measurement (worth 20% of the marks). Few candidates provided the intra-cranial elastance equation. A significant proportion of candidates missed out a part of the question, either the factors that affect CBV or ICP measurement.

2016B 18: 55% of candidates passed this question.
It was expected answers would include an explanation of the Monro-Kellie Doctrine. Many candidates gave insufficient details of compensatory mechanisms especially regarding decreased total cerebral blood volume (primarily venous) in response to increased intracranial pressure.
Most candidates had all the information but had difficulty synthesising the information to write a cohesive answer. Factors affecting ICP could be divided into factors affecting CBV, factors affecting CSF and factors affecting brain tissue. Under factors affecting CBV the effect of blood gases, autoregulation, temperature, metabolism, drugs and venous obstruction could have been detailed.

Guo 2021

2020B 07: 22% of candidates passed this question.
This question is ideally suited to a tabular format, where candidates are expected to highlight the significant similarities and differences as well as why a certain monitor may be chosen in preference to another rather than compile two lists written next to each other. To score well in this question, a statement of what could be measured (ICP: global vs local), a description of the measurement principles, along with other measurement related information like calibration and sources of error was required. Also sought was information regarding anatomical placement (e.g., lateral ventricle for EVD) and method of placement. Furthermore, a comparison with each other (e.g., higher infection/bleeding risk with EVD, greater risk of trauma due to size and insertion, expertise to insert, cost, therapeutic benefit, risk of blocking) was required for completion. Candidates who structured these elements into advantages and disadvantages were generally able to elucidate this information and score better.