Clinical Aspects of Cerebral Perfusion and ICP Flashcards
What percentage of CO does the cerebral blood flow account for?
15%
What is the normal cerebral blood flow?
Normal cerebral blood flow averages 55 to 60 mL/100 g brain tissue per minute
What is the normal blood flow of grey matter?
Grey matter the blood flow is 75 mL/100 g/ minute
What is the normal blood flow of white matter?
White matter it is around 45 mL/100 g/ minute
When does ischaemia occur?
Ischemia at 20 mL/100 g/minute
When does permanent damage usually occur?
Permanent damage usually results when the blood flow drops below 10 mL/100 g/minute
What is the most significant factor determining cerebral blood flow at any given time?
Most significant factor that determines cerebral blood flow at any given time is the cerebral perfusion pressure (CPP)
What is the CPP?
CPP is the effective blood pressure gradient across the brain
What is CPP equal to?
CPP = MAP − ICP
How does the ICP affect the cerebral perfusion?
Increased ICP causes the cerebral perfusion pressure to decrease
What factors regulate cerebral blood low under physiological conditions?
- CPP
- Concentration of arterial CO2
- Arterial PO2
What is cerebral autoregulation?
The ability to maintain constant blood flow to the brain over a wide range of CPP (50-150 mm Hg) is calledcerebral autoregulation
What is the cerebral autoregulatory response to low CPP?
The cerebral arterioles dilate to allow adequate flow at the decreased pressure
What is the cerebral autoregulatory response to high CPP?
The cerebral arterioles constrict
When does the cerebral autoregulatory system fail?
Under certain pathological conditions cerebral blood flow cannot always be autoregulated
- CPP exceeds 150 mm Hg, such as in hypertensive crisis, the autoregulatory system fails
- Exudation of fluid from the vascular system with resultant vasogenic edema
- Certain toxins such as carbon dioxide can cause diffuse cerebrovascular dilatation and inhibit proper autoregulation
- During the first 4 to 5 days of head trauma, many patients can experience a disruption in cerebral autoregulation
What is cerebral oedema a prominent cause of?
Cerebral edema is a prominent cause of subacute to chronic intracranial hypertension
What is intracranial hypertension?
State of increased brain volume as a result of an increase in water content
What type of cerebral oedema are steroids effective in?
Extracellular oedema
What type of cerebral oedema is mannitol effective in?
- Extracellular oedema
- Intracellular oedema
- ? Interstitial oedema
What is the blood brain barrier?
- A barrier composed of astrocytic foot process wrapping around a capillary endothelium composed of tight junctions
- This means not all substances that are carried in the blood can reach the neural tissue
What do endothelial tight junctions present barrier to in the brain?
Endothelial tight junctions are the barrier to the passive movement of many substances in order to protect the sensitive neural tissue from toxic materials
By what mechanisms can material be transported naturally across endothelial cells?
- Lipid-soluble substances can usually penetrate all capillary endothelial cell membranes in a passive manner
- Amino acids and sugars are transported across the capillary endothelium by specific carrier-mediated mechanisms
What is the Monro-kellu doctrine?
The craniums is a rigid structure:
-When a new intracranial mass is introduced, a compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant
What is complicance?
- Change in volume observed for a given change in pressure
- dV / dP
