Clinical Aspects of Cerebral Perfusion and ICP Flashcards
What percentage of cardiac output goes to the brain?
15%
What is the perfusion of the brain in ml/100g/min?
55-60ml/100g brain tissue per minute
How does grey matter and white matter blood flow differ?
Grey matter blood flow is 75ml/100g/min, white matter is around 45ml/100g/min
At what perfusion does ischaemia occur?
Ischaemia occurs at 20ml/100g/min
At what perfusion does permanent damage occur?
Permanent damage results when the blood flow drops below 10ml/100g/min
What is the most significant factor that determines cerebral blood flow?
Most significant factor that determines cerebral blood flow at any given time is cerebral perfusion pressure (CPP):
- Is the effective blood pressure gradient across the brain
What does CPP stand for?
Cerebral perfusion pressure
What formula describes cerebral perfusion pressure?
CPP = MAP – ICP
How does increased ICP impact CPP?
Increased ICP causes decreased CPP
What are some factors that regulate cerebral blood flow under physiological conditions?
- CPP
- Concentration of arterial CO2
- Arterial PO2
What is cerebral autoregulation?
Ability to maintain constant blood flow to the brain over a wide range of CPP (50-150mmHg)
Over what range of mmHg does cerebral autoregulation work?
50-150mmHg
What happens when CPP is low for cerebral autoregulation?
- Cerebral arterioles dilate to allow adequate flow at the decreased pressure
What happens when CPP is high for cerebral autoregulation?
- Cerebral arterioles constrict
Under certain pathological conditions, cerebral blood flow cannot always be autoregulated. What are some examples?
- Hypertension
- If CPP exceeds 150mmHg such as in hypertensive crises the autoregulatory system fails
- Exudation of fluid from the vascular system with resultant vasogenic oedema
- Toxins such as CO2 can cause diffuse cerebrovascular dilation and inhibit proper autoregulation
- During first 4 to 5 days of head trauma many patients experience a disruption in cerebral autoregulation
Cerebral oedema is a prominent cause of what?
Subacute to chronic intracranial hypertension
What is cerebral oedema?
State of increased brain volume as a result of an increase in water content
What are he different kinds of cerebral oedema?
- Vasogenic oedema (extracellular oedema)
- Cytotoxic oedema (intracellular oedema)
- Interstitial oedema
What is vasogenic oedema also called?
Extracellular oedema
What is cytotoxic oedema also called?
Intracellular oedema
Compare and contrast the pathogenesis of each type of cerebral oedema?
Compare and contrast the location of each type of cerebral oedema?
Compare and contrast the composition of oedema fluid of each type of cerebral oedema?
Compare and contrast the ECF (increasd/decreased) of each type of cerebral oedema?
Compare and contrast the effect of steroids on each type of cerebral oedema?
Compare and contrast the effect of mannitol on each type of cerebral oedema?
What does BBB stand for?
Blood brain barrier
What is the blood brain barrier?
A barrier composed of astrocytic foot process wrapping around a capillary endothelium composed of tight junctions
Can all substances carried in the blood reach the neural tissue?
No, the blood brain barrier stops them
What protects the brain from substances that can normally passively cross capillaries?
Endothelial tight junctions
What are mechanisms by which materials may be transported naturally across the endothelial cells of the blood brain barrier?
- Lipid soluble substances
- Can usually penetrate all capillary endothelial cell membranes in a passive manner
- Amino acids and sugars
- Transported across the capillary endothelium by specific carrier-mediated mechanisms
What is the total volume that makes up ICP and what is this composed of?
Wha is the Monro-Kellie Doctrine?
Describes the relationship between the contents of the cranium and intracranial pressure, in non-pathological states 3 components exist in equilibrium to maintain normal intracranial pressure, the brain tissue, the blood and the cerebrospinal fluid
What percentage of cranial volume is composed of brain tissue?
70%
What are the contents of the cranium?
Brain tissue
BLood
Cerebrospinal fluid
In line with Monro-Kellie Doctrine, what must happen when a new intracranial mass is introduced to the cranium?
Compensatory change in volume must occur through a reciprocal decrease in venous blood of CSF to keep the total intracranial volume constant
What is compliance?
Change in volume for a given change in pressure
dV/dP
What is elastance?
Inverse of compliance
Elastance = change in pressure for a given change in volume
dP/dV
Represents the accommodation to outward expansion of an intracranial mass
Describe the homeostatic mechanisms that means initial changes in cranium volume produce small changes in pressure?
Venous system collapses easily and squeezes venous blood out through the jugular veins or through the emissary and scalp veins
CSF can be displaced from the ventricular system through the foramina of Luschka and Magendie into the spinal subarachnoid space
What happens ones compensatory mechanism to keep cranium pressure change low with volume change have been exhausted?
Small changes in volume produce significant increases in pressure
What are the 3 distinct peaks of ICP waveforms?
- Percussion wave
- Tidal wave
- Dicrotic wave
What is A?
Percussion wave
What is B?
Tidal wave
What is C?
Dicrotic wave
What test is done to determine high or flow CSF?
ICP pressure monitoring
What are Lundberg waves?
Repetitive changes in intracranial pressure occurring at frequencies of 0.5 to 2 waves/min
What are the different kinds of ICP waveforms?
A waves:
- Abrupt elevation in ICP for 5 to 20 minutes followed by rapid fall in pressure to resting levels
- The amplitude may reach as high as 50 to 100mmHg
B waves:
- Frequency of 0.5 to 2 waves per minutes, are related to rhythmic variations in breathing
C waves:
- Rhythmic variations related to waves of systemic blood pressure and have small amplitude
What are A waves?
- Abrupt elevation in ICP for 5 to 20 minutes followed by rapid fall in pressure to resting levels
- The amplitude may reach as high as 50 to 100mmHg
What are B waves?
- Frequency of 0.5 to 2 waves per minutes, are related to rhythmic variations in breathing
What are C waves?
- Rhythmic variations related to waves of systemic blood pressure and have small amplitude
What is the frequency of B waves?
0.5 to 2 waves per minute
What can the amplitude of A waves reach?
50 to 100mmHg
What is Cushing’s reflex?
Physiological nervous system response to increased intracranial pressure that results in Cushing’s triad of increased blood pressure, irregular breathing and bradycardia
What does Cushing’s reflex cause?
- Compression of cerebral arterioles
- Decreased CBF, activation of autonomic nervous system
- Sympathetic response
- Alpha-1 adrenergic receptors causing hypertension and tachycardia
- Aortic baro-receptors stimulate vagus nerve causing bradycardia
- Bradycardia is also due to mechanical distortion of medulla
What is the management of increased ICP?
Head end elevation to facilitate venous return
Mannitol/hypertonic saline
Hyperventilation to decreased CBF (temporary measure)
Barbiturate coma to decrease cerebral metabolism and CBF
Surgical decompression
What are some newer concepts for the investigation of ICP?
Brain tissue oxygenation monitoring:
- Probe to monitor oxygenation of tissue
- Detect and treat low oxygenation, increasing CPP
Micro-dialysis:
- Investigate brain metabolism
- Implantation of specially designed catheters
- To collect small-molecular-weight substances to help measure and identify neurotransmitters, peptides and other substances
