Cerebral perfusion and ICP Flashcards
What % of the cardiac output does the brain receive?
15%
Normal cerebral blood flow averages 55 to 60 mL/100 g brain tissue per minute
Does Grey or White matter receive more blood?
Grey
Factors that regulate cerebral blood flow under physiological conditions (3)
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 cerebral perfusion pressure (CPP) (50-150 mm Hg)
What is CPP
The net pressure gradient that drives oxygen delivery to cerebral tissue. It is the difference between the mean arterial pressure (MAP) and the Intracranial Pressure (ICP)
What happens to the cerebral arterioles when CPP is low?
they dilate to allow adequate flow at decreased pressure
Trauma/stroke can have what effect on autoregulation?
Autoregulation mechanism stops working – little bit of change in pressure has bad effects
If CPP exceeds 150mmHg such as in hypertensive crisis what can happen?
autoregulatory system fails
What is Vasogenic cerebral oedema? (extracellular)
Type of cerebral oedema in which the blood brain barrier (BBB) is disrupted
Increased capillary permeability- intravascular proteins and fluid to penetrate into the extracellular space
Which toxin can cause diffuse cerebrovascular dilatation and inhibit proper autoregulation?
CO2
Effects of cerebral oedema
prominant cause of subacute to chronic intracranial hypertension
What are the 4 types of cerebral oedema
Vasogenic
Cytotoxic
Osmotic
Interstitial
What is Cytotoxic oedema (intracellular)
BBB remains intact but a disruption in cellular metabolism impairs functioning of the Na+ and K+ pump in the glial cell membrane, leading to cellular retention of sodium and water.
oedema in G and W matter
Neuronal, glial and endothelial cells swell
Causes of cytotoxic oedema?
early stages of infarction, water intoxication, stroke/brain injury
Where are the tight junctions situated?
In the endothelium of blood vessels - capillary network
What mechanisms exist by which materials can be transported across the endothelial cells: discuss lipid-soluble substances and amino acids and sugars
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 happens when a new intracranial mass is introduced within the cranial cavity?
A compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant
For example a tumour adds on to brain vol. The vol of CSF decreases or less blood comes to brain as a result. If tumour mass continues to increase - more venous blood or CSF leave and pressures will change
Define elastance
Inverse of compliance
Change in pressure observed for a given change in volume
It represents the accommodation to outward expansion of an intracranial mass
Define compliance
Change in volume observed for a given change in pressure
Homeostatic mechanism in venous system?
venous system collapses easily and squeezes venous blood out through jugular veins or through emissary and scalp veins
Homeostatic mechanism relating to CSF displacement
CSF can be displaced from the ventricular system through the foramina of Luschka and Magendie into the spinal subarachnoid space
How is it that tumours can go undetected until the brain can no longer accommodate for them?
The innate homeostatic pressure-buffering mechanism offered by displacement of CSF and venous blood keeps compliance flat until a “critical volume” is reached. This mechanism carries on in the background as the tumour grows but at some point the changes in vol and pressure are too much for the mechanism to accommodate.
After this critical volume, small volumetric changes result in dangerously high increases in pressure, and intracranial hypertension naturally ensues
What is the Monro-Kelly Doctrine?
The cranium is a rigid structure containing the brain, blood and CSF
The Monro-Kellie doctrine states the sum of volumes of brain, CSF and intracerebral blood is constant.
So when a new intracranial mass is introduced there must be a compensatory change in vol
Describe the ICP waveform
It has 3 distinct peaks. P1 and P2 = most important.
P1 - percussion - arterial pulse transmitted through the choroid plexus into the CSF
P2 - tidal wave - cerebral compliance, it can be thought of as a “reflection” of the arterial pulse wave bouncing off the springy brain parenchyma.
P3 - dicrotic wave - closure of the aortic valve
