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
What is elastance?
- Inverse of compliance
- Change in pressure observed for a given change in volume
- dP / dV
What does elastance represent?
It represents the accommodation to outward expansion of an intracranial mass
How can CSF be displaced from the ventricular system?
CSF can be displaced from the ventricular system through the foramina of Luschka and Magendie into the spinal subarachnoid space
What happens when the venous system collapses?
The venous system collapses easily and squeezes venous blood out through the jugular veins or through the emissary and scalp veins
What happens when the homeostatic compensatory mechanisms fail to resolve an increase in volume?
When the compensatory mechanisms have been exhausted, small changes in volume produce significant increase in pressure
What keeps compliance flat in increasing cerebral volume?
The innate homeostatic pressure-buffering mechanism offered by displacement of CSF and venous blood keeps compliance flat until a “critical volume” is reached
What happens once the ‘critical volume’ is reached?
After this critical volume, small volumetric changes result in precipitous increases in pressure, and intracranial hypertension naturally ensues
What are A Lundberg waves?
Abrupt elevation in ICP for 5 to 20 minutes followed by a rapid fall in the pressure to resting levels
The amplitude may reach as high as 50 to 100 mm Hg
What are B Lundberg waves?
Frequency of 0.5 to 2 waves per minute, are related to rhythmic variations in breathing
What are C Lundberg waves?
Rhythmic variations related to waves of systemic blood pressure and have smaller amplitude
What is Cushing’s reflex?
Vasopressor response
What is the Cushing’s reflex triad?
- Hypertension
- Irregular breathing
- Bradycardia
What is the physiology behind Cushing’s triad?
- Increased ICP more than MAP
- Compression of cerebral arterioles
- Decreased CBF, activation autonomic nervous system
- Sympathetic response: alpha-1 adrenergic receptors -> Hypertension and tachycardia
- Aortic baro-receptors stimulate vagus nerve -> Bradycardia
- Bradycardia also due to mechanical distortion of medulla
How should raised ICP be managed?
- Head end elevation: facilitate venous return
- Mannitol/ Hypertonic saline
- Hyperventilation: decrease CBF (temporary measure)
- Barbiturate coma: decrease cerebral metabolism, CBF
- Surgical decompression
What is brain tissue oxygenation monitoring?
- Probe to monitor oxygenation of tissue
- Detect and treat low oxygenation, increasing CPP
What is 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
What is the composition of oedema fluid in intracellular oedema?
Increased intracellular water and sodium due to failure of membrane transport
What is the composition of oedema fluid in interstitial oedema?
CSF
What is the extracellular fluid level in extracellular oedema?
Increased
What is the extracellular fluid level in intracellular oedema?
Decreased
What is the extracellular fluid level in interstitial oedema??
Increased
What is the pathological lesion causing oedema in extracellular oedema?
- Primary or metastatic tumour
- Abscess
- Late stages of infarction
- Trauma
What is the pathological lesion causing oedema in extracellular oedema?
- Early stages of infarction
- Water intoxication
What is the pathological lesion causing oedema in extracellular oedema?
Obstructive or communicating hydrocephalus
