Describe the flow of CSF through the brain.
produced by the choroid plexus in the 2 lateral ventricles -> through the foramen of Monro into the third ventricle -> through the aqueduct of Sylvius and into the fourth ventricle -> through foramen of Magendi and the 2 foramina of Luschka into the cistern magna -> into the subarachnoid space and spinal column -> absorbed in the subarachnoid space by arachnoid villi of venous system
What is the average CBF?
50 mL / 100 gm / min.
(total 750 mL / min.)
What is luxury perfusion?
perfusion in excess of metabolic needs.
can be beneficial in healthy brains, but may cause "steal phenomenon" in brains which have ischemic areas
What is steal phenomenon?
increased pCO2 or VAA globally "steal" blood flwo from ischemic areas of the brain by causing vasodilatation in healthy areas of the brain. ischemic brain tissue, which already has maximally dilated vessels due to released vasodilator substances, loses luxury perfusion benefit due to global shunting of blood flow.
What is inverse steal, or robin hood phenomenon?
decreased pCO2 constricts normal vessels but not necessarily in ischemic areas due to vasomotor paralysis. this is one rationale for hyperventilating patients with intracranial tumors associated with increased ICP especially when administering VAAs which cause vasodilatation.
What arteries are a part of the Circle of Willis?
How does PaCO2 effect CBF?
linear relationship 1mm Hg increase PaCO2 = 1-2ml /100Gm/min. increase CBF.
How and when does PaO2 effect CBF?
Profound increase in CBF only at PaO2 < 50 mm Hg
How do you calculate CPP?
CPP = MAP-ICP or CVP whichever is higher
What is normal CPP?
What occurs when CPP is < 50 mmHg? How about 25-40 mmHg? Or a CPP of < 25 mmHg?
< 50 = EEG slowing
25-40 = EEG flat
< 25 = permanent brain damage
What is the average CMRO2?
3 - 3.8mL O2 / 100gm / min.
Coupling is the direct linear relationship of _____ to _____.
CMRO2 to CPP.
as CMRO2 increases, CPP will correspondingly increase
VAAs decrease ___1___ while increasing ___2___, disrupting the relationship between ___1____ to ____2____.
1 - CMRO2
2 - CPP
Glucose is the main energy substrate used by the brain. What is the average glucose consumption of the brain?
5mg / 100gm / min
For each 1 degree Celsius decrease in temperature, there is a corresponding ___-___% decrease in CMRO2.
(at 20 degrees Celsius, an EEG is flat showing no brain activity)
Do seizures increase or decrease CMRO2?
Rehashing the concept of coupling.... as metabolic demands increase, will the cerebral blood flow increase or decrease?
What is a normal value for ICP?
(temporary elevation of ICP occurs during coughing, Valsalva maneuvers, or hypertensive episodes)
Sustained elevated ICP > 15 mmHg decreases CPP and increases the risk of what?
(severely increased ICP may lead to brainstem herniation through the foramen magnum)
How does hyperventilation decrease CBF?
by causing cerebral vascular vasoconstriction which decreases CBF
Decreasing CBV does what to ICP?
Diuretics decrease brain tissue water content, doing what to ICP?
Ventriculostomy and intrathecal catheters allow CSF to drain, which does what to ICP?
What occurs as a part of the Cushing reflex?
hypertension and bradycardia
If the Cushing reflex cannot adequately compensate for the worsening increases in ICP, then the Cushing's triad appears. What is the triad?
hypertension, bradycardia, and irregular respirations
(these reflect severe increases in ICP and severe cerebral ischemia and impending herniation of the brain stem down through the foramen magnum)
What area of the brain do supratentorial tumors occupy?
midbrain and cerebral cortex
What area of the brain do infratentorial tumors occupy?
the vital centers of the cerebellum and brainstem
What is a risk of being in the sitting position?
venous air embolism
What is a risk of being in the lateral oblique position?
brachial plexus injury