Neuro Red Memorize Material Flashcards
CBF normal
CBF-50ml/100 gm/ min (750 ml/min)
CBF % of CO
15-20% of cardiac output
CBF impaired values
CBF <20-25
cerebral impairment/slowing EEG
CBF < 15-20
isoelectric EEG
CBF <10
irreversible brain damage
Cerebral Autoregulation & CBF
The brain normally tolerates wide swings in blood pressures with little change in blood flow…like the heart /kidneys
Changes in flow result in vasodilatation & vasoconstriction to maintain flow
*In the normal brain
CBF remains with MAP 50 - 150
CBF & PaCO2 tensions
CBF is directly proportionate to PaCO2 between tensions of 20 - 80 mmHg
CBF changes per PaCO2 changes
Blood flow changes approximately 1-2 ml/100g/min per mm hg change in PaCO2
This effect is immediate & is thought to be secondary to changes in pH of CSF & cerebral tissue
critical oxygen tension & CBF
when O2 falls below 50 mmHg, rapid increase in CBF and arterial blood volume
Brain Tissue Oxygen Monitoring Systems
Critical Thresholds:
Goal: above 20-25 mmHg
Brain Oxygen <20 mmHg = cerebral ischemia
Brain Oxygen >50 mmHg = Luxury Perfusion
Local area of cerebral hyperemia or increased cerebral blood flow
Hyperemia may contribute to brain swelling & we can adjust therapies…
CPP and ICP
CPP normal = 70-100 mmHg
Since ICP is normally <10 mmHg, CPP is largely dependent on MAP
However, moderate to severe increases in ICP (** > 30 mmHg**) can significantly compromise CPP & CBF even in the presence of normal MAP
CPP low values and EEG
CPP < 50 mmHg - Slowing of EEG
CPP 25-40 mmHg - Flat EEG
CPP < 25 mmHg - Irreversible brain damage
CPP autoreg
CPP = the pressure gradient driving cerebral blood flow (CBF)
hence oxygen and metabolite delivery
The NORMAL brain autoregulates its blood flow to provide a constant flow regardless of blood pressure by altering the resistance of cerebral blood vessels
CPP and brain injury
These homeostatic mechanisms are often lost
CVR is usually increased
The brain becomes susceptible to changes in b/p!
Ischemic brain regions or those at risk of ischemia are critically dependent on adequate cerebral blood flow thus CPP
CPP is a…
Maintaining CPP is a cornerstone of modern brain injury therapy
CPP and mortality
Mortality increases approximately 20% for each 10mmHg loss of CPP
In those studies where CPP is maintained above 70mmHg:
The reduction in mortality is as much as 35% for those with severe head injury
CPP & MAP
CPP may be maintained by raising the MAP or by lowering the ICP.
In practice ICP is usually controlled to within normal limits (<20mmHg) and MAP is raised therapeutically
It is unknown whether ICP control is necessary providing CPP is maintained above the critical threshold
CBF values (again)
CBF 50 ml/100g/min - Normal
CBF <20-25 - Cerebral Impairment/Slowing of EEG
CBF < 20 - Isoelectric EEG/Irreversible brain damage
CPP abnormal values
CPP < 50 mmHg - Slowing of EEG
CPP <25-40 mmHg - Flat EEG
CPP < 25 mmHg - Irreversible
A - ICP
B – PaCO2
C – CPP
D – PaO2
Monro Kellie Hypothesis
INCREASE in the volume of any ONE requires a corresponding DECREASE in the other TWO components
Cushings Triad
- HTN
- Bradycardia
- Resp disturbances
CSF volume flow and photo
Lateral Ventricle
Foramina of Monro
Third Ventricle
Cerebral aqueduct of Sylvius
Fourth Ventricle
Foramen of Magendie
Foramen of Luschka
Cisterna Magna
SA Circulation
Absorbed in the arachnoid granulations over the cerebral hemispheres
CSF Dynamics - how much in body, produced amnt, where produced, and eliminated
100-160cc in the body
500cc produced q 24 hours
Production: Choroid plexus
**Elimination/Reabsorbed: ** Arachnoid villi
Effects of drugs: Enflurane/Lasix
Hyperventilation
In some patients, hyperventilation actually increased brain oxygen deficit.
Presumably was a result of vasoconstriction, which augmented ischemic states
Jugular venous hemoglobin oxygen saturation monitoring has become widely applied in the intensive care unit
***Remains impractical in most operative settings
Inverse Steal or Robin Hood Phenomenon
Back to hyperventilation:
Decreased PCO2 constricts normal vessels but not the ischemic areas (d/t vasomotor paralysis).
This is one reason we do hyperventilate patients with intracranial tumors and ICP
VA and Coupling
VA alter the normal coupling of CBF & CMR
The combination of a in neuronal metabolic demand with an increase in cerebral blood flow (metabolic supply) is termed luxury perfusion
May only be desirable during induced hypotension & it supports the use of a VA, particularly Iso, during this technique