Cerebral Blood Flow & ICP Flashcards
- Cerebral Blood flow and its determinants - Cerebral Oedema - ICP & Compliance - ICP Monitoring - Cushing's (Vasopressor) reflex - Cerebral Herniation
What is the normal rate of cerebral blood flow and what constitutes ischaemia?
Units: ml/g of tissue/min
- ~60ml/100g tissue/min
Ischaemia is considered to be when it hits 20ml/100g/min
What are 4 ways that the brain regulates cerebral blood flow?
What is the equation/formulae that shows the relationship between MAP, ICP and Cerebral Perfusion Pressure (CPP)?
- Autoregulation via myogenic mechanism
- PaCO2 & PaO2
- Cerebral metabolism
- Neurohumeral factors
Cerebral Perfusion Pressure (MAP - ICP)
How do arterial O2 & CO2 affect cerebral blood flow? [2]
What is the relationship between vessel radius in the brain and cerebral blood flow?
High PaCO2 causing cerebral arteries to dilate
High PaO2 causes cerebral arteries to constrict
Larger vessel diameter, increased cerebral blood flow
Describe the physiology of cerebral auto regulation or myogenic mechanism.
What happens when high CPP and what happens when low CPP
2 functions of the myogenic mechanism
Cerebral Autoregulation
High CPP -> Arteriolar Constriction
Low CPP -> Arteriolar Dilation
This enables the body to maintain constant CBF over CPPs of 50-150mmHg [1] and prevents vascular hemorrhage [1] as it prevents high MAP from reaching smaller blood brain vessels
In what cases does Cerebral Autoregulation fail? [3]
What happens when this mechanism is impaired? [2]
Stroke, SAH, hypertensive crises
When impaired the cerebrovascular system becomes pressure dependent [1] so increase in MAP leads to increase in CBF [1]
What is the Monro-kelly doctrine [2]
It states that since the cranium is a rigid cage, any increase in intracranial volume [1] (e.g. bleed) will cause an increase in pressure [1]
How does the brain compensate for increases in intracranial volume so the pressure doesnt increase?
By forcing out venous blood and IJV and CSF into thecal sac.
This is called “Compliance”
What do we call the point at which cerebral compliance fails?
The Critical Volume
What are the 3 types of cerebral oedema?
- Vasogenic
- Cytotoxic
- Interstitial
Describe vasogenic cerebral oedema [2]
Local breakdown of the BBB, usually traumatic.
Allows fluid to pass into extracellular spaces of the brain
Describe Cytotoxic Cerebral Oedema [2]
Damage to cells in the brain or altered metabolisms causing them to retain water
–> Intracellular Oedema
E.g. during infarction
Describe Interstitial Oedema? [2]
Disruption of the CSF-brain barrier allowing CSF to flow into the interstitial spaces of the brain
What would we see when monitoring ICP? [1]
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? [2]
3 peaks of decreasing size [1] with roughly equal distances between [1]
Describe an abnormal ICP waveform of increased arterial pulsation? [2]
3 peaks of decreasing size [1]
The 1st is much bigger than the other two [1]
Describe an abnormal ICP waveform of a non-compliant system? [2] What would this clinically suggest? [1]
3 Peaks [1]
P2 is taller than P1 [1]
Indicates compliance has failed due to added mass or congestion [1]
What are A, B & C waves? [3]
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? [2]
What is the triad that it presents with? [3]
A MEDICAL EMERGENCY
An end stage response to raised ICP [1] when ICP exceeds MAP [1]
It results in a triad of Hypertension, Bradycardia & Irregular Breathing
How does Cushing’s Reflex occur? [5]
Explain the vicious cycle that this engenders [3]
When ICP exceeds MAP
- > Compresses the cerebral arteries, CBF drops
- > Sympathetic system activated
- > Aortic baroreceptors detect rising BP, MAP
- -> This stimulates parasympathetic Vagus fibres
- > Reflex Bradycardia occurs
- -> Compression of brainstem (respiratory centers in medulla) causes irregular breathing
The MAP rise > to ICP further increasing > further drop in CPP
What are the main types of brain herniation? [4]
- 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? [6]
- Elevate Head 30 degrees
- Mannitol or Hypertonic Saline
- Hyperventilation
- Barbiturate Coma
- Surgical interventions
- Brain tissue Oxygenation Monitoring
- Micro-Dialysis
Function of mannitol & Hypertonic saline? [4]
Both increase blood volume (decrease viscosity) [1] to increase cerebral blood flow [1] without decreasing blood tonicity [1] and so exacerbating cerebral oedema as pure fluids would [1]
How does hyperventilation help ICP? [4]
Prevents Hypercapnia [1] which causes cerebral arteries to constrict [1]
This decreases CBF [1] lowering the blood in the cranial vault and so the ICP [1]
How does a baribiturate coma help raised ICP? [2]
Lowers brain metabolism [1] thus decreasing blood flow [1]
How does brain tissue oxygen monitoring work?
Probes are inserted to measure oxygenation of the tissue directly
How does micro-dialysis work? [2]
What does it monitor? [1]
Collecting molecular size peptides [1] etc from the brain through special catheter [1]
Allows you to monitor brain metabolism [1]
If decreased MAP and increased ICP co-exist, what are the possible sequelae on CPP and the brain [2]
Significantly reduced CPP
Risk of brain ischemia
Describe cerebral metabolism and neurohumeral factors as a mechanism that regulates CBF [5]
Cerebral metabolism: detection of substances in blood [1] passing through the BBB [1]
Neurohumeral factors: sympathetic tone on cerebral arteries [1] lead to mild tonic vasoconstriction [1] allowing for higher limits on auto regulation curve [1]
Surgical interventions for raised ICP [3]
Craniotomy, evacuation of clot
Eternal ventricular drainage
Decompressive craniotomy
Intracranial hypertension: approach to management - avoid pyrexia - reasons [2]
Avoid pyrexia - increases ICP and is an independent predictor of poor outcome after severe head injury
Intracranial hypertension: approach to management - manage seizures - why and how, who
Why? Contribute to raised ICP
How? manage aggressively using standard anticonvulsant, consider prophylactic mx
Who? Children with significant head injury and neuroinfections are at increased risk
