Neurophysiology & Pathophysiology Flashcards
Cerebral metabolic O2 consumption (CMRO2) is normally…..
3.5 mL/100 g/min. (~50mL/min)
The brain uses what percentage of total body O2 consumption?
20%
What part of the brain has the highest CMRO2?
Gray matter
CMRO2 usually parallels what?
Glucose consumption
What parts of the brain are most sensitive to hypoxia injury?
Hippocampus and cerebellum
The average CBF is what percentage of the cardiac output?
15-20%
What is the average value for CBF?
50 mL/100 g/min. (~750mL/min)
What is CPP?
Cerebral perfusion pressure.
CPP = MAP - ICP (or CVP)
What are the CPP values for normal pts, isoelectric EEG and irreversible brain damage?
Normal CPP = 80-100 mmHg
Isoelectric EEG = 25-40 mmHg
Brain damage = <25 mmHg
What is a normal ICP?
Less the 10 mmHg
In normal patients, CBF remains constant between MAP of …..?
50-150 mmHg
What happens to CBF when the MAP falls outside of the normal range (150 mmHg)?
It becomes more pressure dependent
150 usually represents edema
What effect does chronic arterial HTN have on the cerebral auto regulation curve?
Shifts is to the Right.
How is CBF related to PaCO2?
CBF is directly proportional to PaCO2 between 20-80 mmHg
For every 1 mmHg change in PaCO2, how is the CBF affected?
CBF increases 1-2 mL/100 g/min
Why does PaCO2 have such a profound influence in CBF?
CO2 can readily cross the blood brain barrier, but H+ ions do not
What effect does PaO2 have on CBF?
Only severe hypoxemia (PaO2 < 50 mmHg) significantly increase CBF
What is the blood brain barrier?
A lipid barrier that lets the lipid-soluble substances pass, but restricts ionized substances or those with large molecular weight
Passage through the blood brain barrier depends on what four characteristics?
Size
Charge
Lipid solubility
Degree of protein binding in the blood
What is the function of cerebrospinal fluid?
Protects CNS from trauma
What are the normal values for production rate and total volume of CSF?
Production rate: 0.3-0.4 mL/min (~500mL/day)
Total Volume: 150mL
Where is CSF produced?
CSF is formed by the choroid plexuses
Which drugs decrease CSF production?
Corticosteroids
Diuretics
Vasoconstrictors
The cranial vault is a rigid structure with a fixed total volume consisting of what three parts?
Brain (80%)
Blood (12%)
CSF (8%)
What is ICP?
Intracranial pressure is the supratentorial CSF pressure measured in lateral ventricles or over the cerebral cortex
What is the normal range for ICP?
5 to 10 mmHg
What are some compensatory mechanisms for decreasing ICP?
- Displacement of CSF (brain -> spinal cord)
- Increased CSF absorption
- Decreased CSF production
- Decrease in CBV
How is intracranial hypertension defined?
Sustained ICP > 15 mmHg
What are some possible causes of intracranial hypertension?
- Expanding tissue or fluid mass
- Depressed skull fracture
- CSF absorption abnormality
- Excessive CBF
- Systemic disturbances resulting in brain edema
What are some of the signs and symptoms associated with intracranial hypertension?
Headache N/V Papilledema Mental status changes Visual changes Cushing reflex (HTN & Bradycardia) Fixed, dilated pupils Seizures Altered breathing pattern
How is intracranial hypertension treated?
- Treat underlying cause
- Fluid restriction
- Decrease CSF volume (drain or diuretics)
- Decrease CBF (hyperventilation)
- Decrease brain volume (decadron, mannitol)
What is intracranial compliance?
Change in ICP in response to change in intracranial volume
Arnold-Chiari malformation is associated with what type of brain herniation?
The cerebellar tonsils through the foramen magnum
What is the worst site for a brain herniation?
Cerebellar tonsils through the foramen magnum
Which inhalational agents produce the greatest and least effect on cerebral metabolic rate?
Isoflurane = greatest depression Halothane = least effect
Which inhalational agents is best for a patient with intracranial hypertension?
Isoflurane
Has little to no effect on ICP
What phenomenon is possible with volatile anesthetics in the setting of focal ischemia?
Circulatory steal
What is the circulatory steal phenomenon?
Increasing blood flow in normal areas of the brain, but not in ischemic areas where arterioles are already maximally vasodilated.
end result = redistribution of blood flow away from ischemic -> normal areas
All intravenous agents either have little effect on or reduced CMR and CBF with the exception of which drug?
Ketamine
Ketamine is the only intravenous anesthetic that does what to the cerebral vasculature?
Dilates cerebral vasculature thus causing an increase in CBF (50-60%)
What are the most commonly used monitors for neurosurgical procedures?
EEG
Evoked potentials
EEG activation (As with light anesthesia and surgical stimulation) Shows what type of activity?
A shift to predominantly high-frequency and low-voltage activity