This lecture deals with:
- Cerebral Blood flow and its determinents
- Cerebral Oedema
- ICP & Compliance
- ICP Monitoring
- Cushing’s (Vasopressor) reflex
- Cerebral Herniation
- Management of Raised ICP
What is the normal rate of cerebral blood flow and what constitutes ischaemia?
- ~60ml/100gtissuemin
Ischaemia is considered to be when it hits 20ml/100g/min
What factors affect Cerebral blood flow?
- Cerebral Perfusion Pressure (MAP - ICP)
- PaCO2 & PaO2
How do arterial O2 & CO2 affect cerebral blood flow?
High PaCO2 causing cerebral arteries to dilate
High PaO2 causes cerebral arteries to constrict
Since cerebral perfusion pressure can vary how does the body ensure the brains blood supply remains fairly constant?
Cerebral Autoregulation
High CPP -> Arteriolar Constriction
Low CPP -> Arteriolar Dilation
This enables the body to maintan a constant Cerebral blood flow over CPPs of 50-150mmHg
In what cases does Cerebral Autoregulation fail?
When the capacity for autoregulation fails e.g. stroke
When the CPP because too high or low e.g. hypertensive crisis
What is the monro-kelly doctrine
It states that since the cranium is a rigid cage, any increase in intracranial volume (e.g. bleed) will will cause an increase in pressure.
How does the brain compensate for increases in intracranial volume so the pressure doesnt increase?
By Forcing out venous blood and CSF into the Jugulars and spinal subarachnoid place.
This is called “Compliance”
What does a high vs low compliance mean?
High compliance means an increase in intracranial volume wont increase ICP much.
As intracranial volume increases compensatory mechanisms are used up and Compliance falls until it fails
What do we call the point at which cerebral compliance fails?
The Critical Volume
What are the types of cerebral oedema?
- Vasogenic
- Cytotoxic
- Interstitial
Describe vasogenic cerebral oedema?
Local breakdown of the BBB, usualy traumatic.
Allows fluid to pass into hte extracellular spaces of the brain
Describe Cytotoxic Cerebral Oedema?
Damage to cells in the brain or altered metabolisms causing them to retain water
–> Intracellular Oedema
E.g. during infarction
Describe Interstitial Oedema?
Disruption of the CSF-brain Barrier allowing CSF to flow into the interstitial spaces of the brain
What would we see when monitoring ICP?
A three peaked Waveform
What are the 3 peaks in an ICP waveform?
P1 - Percussive Wave - Arterial pulsation
P2 - Tidal Wave - Intracranial Compliance (Lower is more compliant)
P3 - Dicrotic Wave - Venous Pulsation
Describe a normal ICP waveform?
3 peaks of decreasing size with roughly equal distances between
Describe an abnormal ICP waveform of increased arterial pulsation?
3 peaks of decreasing size
The 1st is much bigger than the other two
Describe an abnormal ICP waveform of a non-compliant system?
3 Peaks
P2 is taller than P1
Indicates compliance has failed due to added mass or congestion
What are A, B & C waves?
Types of abnormal ICP waveforms.
A waves are abrupt elevation of the whole waveform lasting minutes to hours
B waves are the same lasting for a just minutes
C waves are individual raised waves
What is Cushing’s Reflex?
An end stage response to raised ICP, when ICP exceeds MAP
It results in a triad of Hypertension, Bradycardia & Irregular Breathing
How does the Cushing’s Reflex occur?
ICP exceeds MAP
- > Compresses the cerebral arteries
- > Cerebral blood flow drops
- > Sympathetic system activated
- > Alpha-1 receptors trigger increased contraction strength and HR
- > Aortic baroreceptors detect rising BP and stimulate parasympathetic Vagus fibres
- > Bradycardia
Hence the Hypertension, bradycardia and irregular breathing
What are the main types of brain herniation?
- Sub-falcine (Cingulate gyrus herniates under falx)
- Uncal (Temporal herniates over tentorium pressing down on it)
- Tonsilar herniation through foramen magnum
- Central or Transtentorial (Herniates through tentorium)
How doe we manage Intracranial Hypertension?
- Elevate Head
- Mannitol or Hypertonic Saline
- Hyperventilation
- Barbiturate Coma
- Surgical Decompression
- Brain tissue Oxygenation Monitoring
- Micro-Dialysis
Function of mannitol & Hypertonic saline?
Both increase blood volume (decrease viscosity) to increase cerebral blood flow without decreasing blood tonicity and so exacerbating cerebral oedema as pure fluids would
How does hyperventilation help ICP?
Prevents Hypercapnia, causing cerebral arteries to constrict
This decreases CBF, lowering the blood in the cranial vault and so the ICP
How does a baribiturate coma help raised ICP?
Lowers brain metabolism thus decreasing blood flow
How does brain tissue oxygen monitoring work?
A probes inserted to measure oxygenation of the tissue directly
How does micro-dialysis work?
Collecting molecular size peptides etc from the brain through special catheter
Allows you to monitor brain metabolism
