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58 Cards in this Set
- Front
- Back
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What is autoregulation?
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The automatic alteration in the diameter of cerebral blood vessels to maintain constant blood flow with changes in systolic pressure
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Define cerebral blood flow?
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The amount of blood in milliliters passing through 100g pf brain tissue in 1 minute
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What is the cerebral blood flow of white matter?
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25ml/min
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What is the cerebral blood flow of gray matter?
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75 ml/min
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What is the global cerebral blood flow?
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50 ml/min
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What % of the body's oxygen is used by the brain?
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20%
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What % of the body's glucose is used by the brain?
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25%
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What is MAP?
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Mean arterial pressure
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What is the calculation to determine MAP?
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DBP + 1/3 (SBP - DBP) = MAP
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At what MAP does cerebral blood flow decrease and symptoms of cerebral ischemia occur?
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50 mm Hg
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At what MAP does cerebral autoregulation fail to compensate for increased pressure?
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150 mm Hg
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1)How does the brain compensate for increased pressure?
2)Decreased pressure? |
1) Vasoconstriction
2) Vasodilation |
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What is CPP?
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Cerebral perfusion pressure. The pressure needed to ensure blood flow to the brain
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How is CPP calculated?
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CPP = MAP - ICP
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What does CPP not consider in it's calculation of the pressure needed to obtain cerebral blood flow?
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The effect of systemic vascular resistance
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What generates cerebral vascular resistance?
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The arterioles within the cranium
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Where can the ICP be measured?
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In the ventricles, subarachnoid space, subdural space, epidural space, or brain parenchymal tissue
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What is the normal ICP range?
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0-15 mmHg
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What devices are used to measure ICP?
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A water manometer or a pressure transducer
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What are the 7 reasons ICP would become elevated?
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Head trauma, stroke, subarachnoid hemorrhage, brain tumor, inflammation, hydrocephalus, or brain tissue damage from other causes
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What are the 3 goals for nursing management of elevated ICP?
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Preservation of cerebral perfusion, early identification of neurologic changes, and prevention of complications
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What is the normal CPP?
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70 to 100 mmHg
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What level of CPP is needed to maintain adequate cerebral perfusion?
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50-60 mmHg
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What happens if the CPP falls below 50 mmHg?
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Ischemia and neuronal death
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What happens if the CPP falls below 30 mmHg?
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Cellular ischemia and actual death
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What is the purpose of autoregulation of cerebral blood flow?
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To ensure a consitent cerebral blood flow to provide for the metabolic needs of the brain and to maintain CPP within normal limits
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Why is it crucial to maintain MAP when there is increased ICP?
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Because CPP does not reflect perfusion pressure in all parts of the brain. So if there is inflammation and/or compression a higher MAP is needed to increase CPP (CPP= MAP -ICP)to prevent tissue damage
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How is the relationship of ICP to intracranial volume measured?
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The pressure-volume curve
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What effects the pressure-volume curve?
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Elastance and Compliance
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What is Elastance [of the brain]?
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Elastance is a measure of the tendency of a hollow organ to recoil toward its original dimensions upon removal of a distending or compressing force. It is the reciprocal of compliance.
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What is Compliance [of the brain]?
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The expandability of the brain, it is the inverse of elastance
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How is the pressure-volume curve used?
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To represent the stages of increased ICP
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How many stages are there to increased ICP?
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4 Stages
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What happens in the first stage of increased ICP?
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There is high compliance and low elastance. The brain has total compliance d/t accomadation and autoregulation and there is no increase in ICP
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What happens in the second stage of ICP?
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Compliance is lower and elastance is increasing placing the patient at risk for increased ICP with an increase in volume
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What happens at the 3rd stage of increased ICP?
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There is high elastance and low compliance. Any small addition of volume causes a great increase in pressure
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What happens in stage 4 of increased ICP?
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ICP raises to terminal levels with little increase in volume
herniation occurs as brain tissues shift from areas of greater to areas of lesser pressure |
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At what stage of increased ICP does the patient mainfest signs and symptoms? What are they?
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The third stage. Changes in mentation, LOC, pupillary responsiveness, or HA
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What is Cushings Triad?
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Systolic hypertension (widening pulse pressure)bradycardia with full and bounding pulse, and irregular respiratory rate
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What factors affect cerebral vessel tone?
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CO2, O2, and H+ concentration
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What effect does increased PaCO2 have on cerebral blood flow?
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An increase in PaCo2 relaxes smooth muscle, dilates cerebral blood vessels, decreases cerebral vascular resistance and increases CBF
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What effect does decreased PaCo2 have on CBF?
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Constricts smooth muscle and cerebral blood vessels, increases cerebrovascular resistance and decreases CBF
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What affect does cerebral O2 tension below 50mmHg have on CBF?
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Cerebral vascular dilation, decreased cerebral vascular resistance, increases CBF, and raises O2 tension
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What happens if decreased intracerebral PaO2 is not corrected?
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Anaerobic metabolism begins = increased lactic acid = increased # H+ ions & acidic environment = cerebral vasodilation where autoregulation is lost and compensatory mechanisms fail to meet tissue demands
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Define cerebral edema
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Increased accumulation of fluid in the extravascular spaces of the brain.
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What is the result of cerebral edema?
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An increase in tissue volume that carries the potential for increased ICP
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Define Increased ICP
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Life threatening situation tht result from an increase in any or all of the 3 compartments of the skull (brain tissue, blood, CSF)
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What determines the degree of cerebral edema?
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The extent and severity of the original insult to the 3 components of the skull
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What are the 3 types of cerebral edema?
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Vasogenic, cytotoxic, and interstitial edema
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Define Vasogenic cerebral edema
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Most common type of cerebral edema where changes in the endothelial lining of cerebral capillaries cause macromolecules to leak from capillaries into surrounding tissue resulting in a change in osmotic gradient where fluid moves from vascular space to tissues
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What are some causes of the increase in permeability of the blood vessels /blood brain barrier in vasogenic cerebral edema?
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Brain tumors, abcesses, ingestion of toxins, and other brain insults
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What factors affect the speed and extent of edema in vasogenic cerebral edema?
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Systemic BP, site of brain injury, and extent of blood-brain barrier defect
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Where does vasogenic cerebral edema mainly occur?
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It mainly occurs in the white matter of the brain
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Cytotoxic Cerebral Edema
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Cerebral edema that occurs when fluid and proteins shift from extracellular space directly into the brain cells causing swelling of cells and loss of cellular function.
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What is the cause of cytotoxic cerebral edema?
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A local disruption of the function or shape of cell membranes from destructive lesions or trauma to brain tissue, causing hypoxia or anoxia, sodium depletion, and syndrome of inappropriate ADH which causes fluid to shift from extra to intracellular spaces
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Interstitial Cerebral Edema?
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Fluid moves into cells d/t hypoosmotic interstitial tissue conditions caused by sytemic water excess (hyponatremia) or periventricular diffusion of ventricular CSF (hydrocephalus)resulting in increased ICP unless compensation is adequate(osmosis causes increased fluid around cells to enter cells which have greater concentration of molecules)
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What is the clinical determination of increased ICP?
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ICP above 20 mmHg
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Why is elevated ICP so dangerous?
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It diminishes CPP (cerebral perfusion pressure,)increases the risk for brain ischemia and infarction, and is assoc. with poor prognosis
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