Neurophysiology

Question 1

What is ICP

Answer 1

hydrostatic pressure within cranium
normally 5-15mmHg
ICP = MAP - CPP

Question 2

What is the Monro-Kellie Doctrine

Answer 2

states that the cranium is fixed volume contain brain parenchyma (85), CSF (10%) and, CBV (5%) and that an increase in any volume results in a compensatory decrease in the other volume/s. If this compensatory mechanism is exhausted, ICP will rapidly increase.

Question 3

Draw the elastane curve for ICP

Answer 3

X axis - volume; Y - axis - ICP

Question 4

Factors affecting ICP

Answer 4

Brain tissue, CSF, CBF

Question 5

Factors affecting CBF

Answer 5

Hagen Pouseille equation
Metabolic - CO2, O2, temperature
Myogenic - auto regulation MAP 50-150mmHg
Others - posture, viscosity

Question 6

What is CSF

Answer 6

specialised extracellular fluid
total volume - 150mls (75mls in cranium, 75mls in spinal cord)
acts as a buffer by translocating CSF to extracranial subarachnoid space

Question 7

Describe CSF production and absorption

Answer 7

Production:

• Produced at 20ml/hr or 500ml/day
• Choroid plexus (70%), ependymal walls of ventricles (30%)
• mainly determined by choroid plexus flow (so depends on CBF and esp CPP)
• if CPP >70 - production rate unchanged
• if CPP <70 - production rate decreases

Absorption

• Arachnoid villi (90%) and cerebral venues (10%)
• absorption increases linearly with increasing ICP (>7mmHg to 22.5mmHg)