Clinical Aspects of Cerebral Perfusion and ICP Flashcards
What is normal cerebral blood flow?
Averages around 55-60 ml/100g brain tissue per minute
What blood rate and flow does ischaemia and parmanent damage usually occur?
Ischaemia - 20 ml/100g per minute
Permanent damage - 10 ml/100g per minute
What is the largest factor in determining the cerebral blood flow?
Cerebral perfusion pressure
How do we calculate the cerebral perfusino pressure? (CPP)
MAP - ICP
What is the result of an increased ICP on cerebral perfusion pressure?
Increased ICP causes cerebral perfusino pressure to decrease
What are the factors that regulate cerebral blood flow inder physiological conditions?
CPP
Concentration of arterial CO2
Arterial PO2
What is meant by cerebral autoregulation?
The ability to maintain constant blood flow to the brain over a wide range of CPP (50-150 mmHg)
How is cerebral autoregulation achieved when there is low CPP?
Cerebral arterioles dilate to allow adequate flow at decreased pressure
Under what corcumstance would cerebral arterioles constrict?
When the CPP is high
When does the autoregulatory system fail?
When the CPP exceeds 150 mm Hg - such as in the hypertensive crisis
During the first 4-5 days of head trauma
Exudation of fluid from the vascular system with resultant vasogenic oedema
Toxins such as carbon dioxide causing cerebrovascular dilation and inhibit proper vasoregulation
What is the pathology associated with cerebral oedema?
Prominent cause of subacute to chronic intracranial hypertension
Why does cerebral oedema cause increased brain volume?
Because of an increase in water content
Increased capillary permeability, the exudate is plasma filtrate containing plasma proteins and is mainly located in the white matter.
Extracellular fluid is increased
What are the possible pathogenic lesions that can cause vasogenic oedema?
Tumour (primary or metastatic)
Absecess
Infarction (later stages)
Trauma
What is the structure of the blood brain barrier?
A barrier composed of astrocytic foot process wrapping around a capillary endothelium composed of tight junctions
(Endothelial tight junctions are the barrier to the passive movement of many substances in order to protect the sensitive neural tissue from toxic materials)
How do lipid soluble substances cross the endothelial cells?
Can penetrate all capillary endothelial cell membranes in a passive manner
How do amino acids and sugars pass across the capillary endothelium?
By specific carrier mediated mechanisms
What are the different compositions within the skull?
Brain (70%) and interstitial fluid (10%) - 1400 ml
Blood (10%) - 150 ml
Cerebrospinal fluid (10%) - 150 ml
Total 1700ml
What is the monro - kelly doctrine?
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 meant by compliance?
Change in volume observed for a given change in pressure
dv/dp
What is elastance?
The inverse of compliance, change in pressure observed for a chanfe in volume
dp / dv
It represents the accomdation to outward expansion of an intracranial mass
How does venous blood get displaced when the intracranial pressure increases?
Venous system collapses easily
Squeezes out venous blood through the emissary and scalp veins or through the jugular veins
How is CSF displaced?
CSF is displaced from the ventricular system through the foramina luschka and magendie into the spinal subarachnoid space
What is the effect of an increasing pressure when the compensatory mechanisms have been exhausted?
Small changes in volume produce significant increase in pressure
Displacement of CSF and venous blood keeps compliance flat until a critical volume is reached
After this critical volume, small volumetric changes result in precipitous increases in pressurem and intracranial hypertension naturall ensues
What are the P1, P2 and P3 waves indicative of?
P1 - Percussion wave
P2- Tidal wave
(Dicriotic notch of the trough between p2 and p3)
P3- Dicrotic wave
What are the different types of Lundberg waves?
A waves - abrupt elevation in ICP for 5 - 20 minutes followed by a rapid fall in the pressure to resting levels
B waves - Frequency of 0.5 to 2 waves per minute, are related to rhythmic variations in breathing
C waves - Rhythmic variations related to waves of systemic blood pressure and have smaller amplitude
What does the cushings reflex combat?
ICP
What are the three main feeatures of cushings reflex?
Hypertension, irregular breathing and bradycardia
What is the physiological response to raised ICP?
Compression of cerebral arteries
Decreased cerebral blood flow and therefore activation of the autonomic nervous system
Sympathetic response: Alpha - 1 adrenergic receptors - results in hypertension and bradycardia
Aortic baro-receptors stimulate vagus nerve - bradycardia
Bradycardia also due to mechanical distortion of the medulla
What is management of increased ICP?
Head end elevation - facilitates venous return
Mannitol / hypertonic saline (In the case of vasogenic oedema steroids and mannitol are both effective in reducing intracranial pressure)
Hyperventilation: Decrease CBF (temporary measure)
Barbituate coma: decrease cerebral metabolism
Surgical decompression
What are the newer concepts in terms of managing intracranial pressure?
Brain tissue oxygen monitoring -
- Probe to monitor oxygenation of tissue
- Detect and treat low oxygenation, increasing CPP
Micro-dialysis:
- Investigate brain metabolism
- Implantation of specially designes catheters
- To collect small - molecular weight substances to help measure and identify neurotransmitters, peptides and other substances
