Monitoring and Neurosurgery Flashcards
What are the two monitoring modalities?
nervous system blood flow and function
What are the three way to monitor blood flow?
cerebral oximetry, transcranial doppler, jugular bulb venous O2 saturation
What are the five ways to monitor nervous system function?
EEG, SSEP, BAEP, VEP, MEP
What is NIRS?
monitors CBF in relation to CMRO2
NIRS monitoring is based off of what Law?
Beer Lambert
What does NIRS monitor estimate?
brain tissue saturation
Which monitoring modality most closely resembles the pulse oximeter?
NIRS
If your patient is going to be monitored with NIRS what should you do before they go to sleep?
get a baseline reading before induction
What can decrease the accuracy of a NIRS monitor?
changes in BP, PaCO2, Hgb, regional BV, peripheral oxygenation of the scalp
The risk of cerebral ischemia is reduced if your NIRS monitor says within which range of baseline?
75% of baseline or greater
What is a normal value for NIRS?
60-80%
What is a limitation of NIRS?
only measures regional blood flow, not global, electrocautery interferes with reading
What is spatial resolution?
depth of tissue measured for the actual cerebral oxygenation is directly proportional to the distance between the light emitting diodes and the sensors
What monitor uses ultrasound waves to measure blood flow velocity of the brain?
Transcranial doppler
Which monitor assesses the integrity of the circle of willis?
transcranial doppler
Where is transcranial doppler placed?
temporal bone
transcranial doppler most likely monitors which circle of willis vessel?
middle cerebral artery
Does transcranial doppler measure blood flow or velocity?
blood flow velocity
Transcranial doppler provides information specifically about what measurements?
flow direction, peak systolic and end-diastolic flow velocity, flow acceleration time, intensity of pulsatile flow, and detection of microemboli
What does SjVO2 measure?
global cerebral O2 extraction
SjvO2 measures blood from which segments of the brain?
blood draining from both cerebral hemispheres
What vessel drains cortical blood, and subcortical blood?
dominant internal jugular vein drains cortical (usually right), non dominant drains subcortical (usually left)
The jugular bulb receives how much blood from the contralateral and ipsilateral sides?
70% ipsilateral hemisphere - RIJV 30% contralateral - LIJV
Is inter cranial mixing of blood complete or incomplete?
incomplete, therefore we only measure from one side and not both.
Where is a blood sample drawn from to measure SjvO2?
1 cm below and 1 cm anterior to the mastoid process
Where is SjvO2 monitoring useful?
cerebral ischemia with increased ICP
What value of SjvO2 is considered normal?
55-75%
What does a SjvO2 reading below 55% suggest?
inadequate O2 delivery to the cerebral tissues or increased consumption.
What does a SjvO2 reading above 75% suggest?
hyperemia or stroke
What does EEG measure?
difference of electrical potentials between groups of neurons
Does EEG measure cortical or subcortical structures or both?
cortical - outer 3mm
What are the three basic components of the EEG?
frequency, amplitude, and morphology
What is frequency of EEG?
rate and duration of impoulses
What is amplitude of EEG?
peak to peak measurements
What is morphology of EEG?
shape of the waveform due to amplitude and frequency of wave appearance.
What is the frequency of gamma waves?
32-100 Hz
What do gamma waves represent?
heightened perception, learning, problem solving tasks, cognitive processing
What is the frequency of beta waves?
15-30 Hz
What do beta waves represent?
awake, alert consciousness
What is the frequency of alpha waves?
10-15 Hz
What do alpha waves represent?
physically and mentally relaxed
What is the frequency of theta waves?
5-10 Hz
What do theta waves represent?
dreams, meditation, light sleep
What is the frequency of delta waves?
0.5-5 Hz
What do delta waves represent?
deep sleep
What is frequency of EEG waves?
speed of impulses
What is amplitude of EEG?
height of waveform
What is the progression of EEG waveforms as a result of ischemia?
Increased Beta -> amplitude increases -> theta -> delta waves
What does Delta waves on EEG likely represent?
increased ischemia
When delivered in equal doses, do IV or Inhalation anesthetics cause more EEG depression?
inhalation anesthetics
Inhalation agents have the most effect on which EEG waves?
Beta waves
How do gases change amplitude and frequency?
increase frequency decrease amplitude
Which IV anesthetics increase Beta wave frequency and decrease amplitude?
Propofol, Etomidate, and ketamine
At 1 MAC which waves are seen?
delta, and theta
As the patient enters stage 3 of anesthesia which waves are demonstrated on EEG?
low frequency, high amplitude
After giving Ketamine what types of EEG waves are expected?
high oscillation waves, no theta or delta
If you induce with high doses of induction agents what is commonly seen on EEG?
burst suppression
What is burst suppression?
pattern of high frequency activity, and periods of electrical suppression with variable duration
At what MAC levels will burst suppression be seen on EEG?
1.2-1.5
At what MAC does Iso, Sevo and Des cause burst suppression?
Iso 1.5 Sevo, Des 1.2
Which volatile does not cause burst suppression?
N2O
What volatile requires the highest MAC to cause burst suppression?
Iso
What does the BIS value signify?
anesthetic depth
To prevent recall you would titrate your anesthetic to a BIS value of what?
<60
If you are a good CRNA you would keep your BIS value above what level to prevent increased mortality?
40
If your BIS falls below 40, make a change within what time frame?
5 minutes
What does Ketamine do to BIS values?
falsely elevates them
What BIS value will be consistent with burst suppression on EEG?
20
What is an evoked potential?
electrical potential in response to a stimulus and are used to warn of current or impending neurological dysfunction and ischemia
What are the four types of evoked potentials?
motor, brainstem auditory, somatosensory, visual
What are the sensory evoked potentials?
BAEP, SSEP, VEP
Motor evoked potentials are derived from which area of the brain?
Precentral gyrus
Sensory evoked potentials are derived from which area of the brain?
Postcentral gyrus
What things affect evoked potentials?
anesthetics, hypothermia, hypotension, anemia, positioning
What is the amplitude of a Evoked Potential?
intensity of the evoked potential (height)
What is latency of evoked potentials?
period of time until evoked response is measured (time)
What changes in amplitude and latency are consistent with ischemia?
50% decrease in amplitude and 10% increase in latency
How do SSEP’s detect localized injury to specific areas of the neural axis?
assessing cortically generated waves
SSEPs are a non specific indicator of what?
cerebral O2 delivery
SSEP specifically monitors what part of the brain and spinal cord?
Faciculus Cuneatus and Gracillis tracts of the dorsal lemniscal system
If ischemia occurs what happens to SSEP impulses?
they are reduced
SSEPs monitor the integrity of which cortex?
somatosensory
What sensations does the dorsal lemniscal system transmit?
touch (fine, discrete), vibration, and proprioception
The Fasciculus gracilis is more ____ and transmits sensation from ___ and ___ regions?
medial, lumbar, sacral
The Fasciculus Cuneatus is more ____ and transmits sensation from ___ and ___ regions?
lateral, thoracic, cervical
Trace the dorsal lemniscal system pathway from first order to third order neurons.
First Order fibers enter from the dorsal horn into the fasciculus gracilis or cuneatus, ascending to the medulla, then synapse with their second order neurons in the nucleus gracilis or nucleolus cuneatus. Then traveling to the thalamus to synapse with a third order neuron.
Which nerves are most frequently stimulated with SSEP’s?
posterior tibial and median nerves
Where are stimulating and detecting SSEP electrodes placed?
stimulating placed peripherally detecting placed centrally
Where is an alternative SSEP detecting lead placed for the lower extremity?
common perineal nerve
Where is an alternative SSEP detecting lead placed for the upper extremity?
ulnar nerve
If we were to stimulate the left posterior tibial nerve. Where would the central electrode be placed on the brain?
postcentral gyrus, right side, midline
Where would you place a peripheral electrode on the lower extremity to ensure stimulation is adequate for SSEP?
iliac crest
Where would you place a peripheral electrode on the upper extremity to ensure stimulation is adequate for SSEP?
Erbs point
SSEP latency increases how much for every 2 degrees Celsius decrease in temperature?
3ms
Hyperthermia suppresses SSEP amplitude by how much?
15%
In general how do anesthetics affect latency and amplitude?
increase latency and decrease amplitude of SSEP
How does etomidate and ketamine affect SSEP amplitude?
increase amplitude 200-600%
How can we optimize SSEP readings?
not making changes to gas or IV drugs
What nerve does BAEP monitor?
vestibulocochlear (CN8)
What procedures are BAEP’s monitored?
inner ear and auditory cortex
Which evoked potential monitor is least sensitive to anesthetics?
BAEP
How does hypothermia affect BAEPs?
increases latency
While monitoring BAEP’s what happens to amplitude as temperature increases?
amplitude decreases
What does electromyography monitor?
the facial nerve also 3, 4, 10, 11, 12
What is the standard of care monitor for acoustic tumor surgery?
electromyography
Does electromyography monitor ischemia?
no. only mechanical and thermal damage
What is electromyography very sensitive to?
muscle relaxation. Avoid
What monitor is used for the visual pathway for everything from the retina to the occipital cortex?
VEP
Where are VEP electrodes placed?
visual cortex
What do MEPs monitor?
motor cortex and descending tracts (corticospinal tracts)
Where are detecting and stimulating electrodes placed in MEP’s?
detecting electrodes peripherally stimulating electrons centrally
What suppresses MEP’s?
inhalation agents and NMB
What is the goal MAC when using MEP’s?
0.5 MAC
What is the most appropriate way to give NMB’s during MEPs?
infusion
What is the order of sensitivity to anesthetic gases of evoked potentials?
VEP>MEP>SSEP>BAEP Visual very, Motor moderately, Sensory somewhat, Brain barely
What is the first compartment to decrease in response o increased ICP?
CSF
What begins to occur as volume compensation mechanisms reach exhaustion?
local and focal ischemia
ICP has a direct relationship with what?
herniation risk, mechanical injury, & ischemia
ICP has a indirect relationship with what?
CPP
Sustained elevations in ICP lead to what?
catastrophic herniation of the brain
What are the four locations herniation can occur?
- the cingulate gyrus under the falx cerebri
- uncinate gyrus through the tentorium cerebelli
- cerebellar tonsils through the foramen magnum
- any area beneath a defect in the skull
Name the Herniations
- Subfalcine 2. Transtentorial (uncal) 3. Tonsilar 4. Transcalvarial
Pressure on the brainstem, rostral to caudal. altered consciousness, ocular and sight reflex issues, respiratory and cardiac dysfunction are symptoms of which type of herniation?
transtentorial
What are symptoms of uncal herniation?
pupillary dilation, ptosis, deviation of ipsilateral eye
What causes a tonsilar herniation?
increased infratentorial pressure -> compression on the medulla
What does a tonsilar herniation lead to?
cardio-respiratory instability
What is the gold standard for ICP monitoring?
intraventricular catheter
What is a major advantage of the ventriculostomy?
allows CSF drainage
Where is a ventriculostomy zeroed?
jugular foramen (tragus)
Draining CSF in a patient with brain tumor can lead to what?
tonsilar herniation
what is the most common complication of ICP monitoring?
infection
What is the most effective and rapid way of lowering ICP?
ventriculostomy, intraventricular catheter or lumbar subarachnoid
What do the ICP waveforms signify?
P1: percussion, highest, arterial compliance P2: tidal wave, cerebral compliance P3 dicrotic wave, aortic valve closure, the dicrotic notch
What does a P2 wave is greater than P1 wave signify?
intercranial hypertension
What maintains ICP?
inter cranial elastance
What is elastance the inverse of?
compliance
What is a plateau ICP wave called?
Lundberg A wave
How long does a Lundberg A wave last and what is the range of ICP?
20 minutes ICP of 20-100
What does Lundberg A waves tell you about brain elastace and compliance?
increased elastance or decreased compliance
Cushings Triad is common with Lundberg A waves? What is cushings triad?
HTN, tachycardia, irregular respirations
How long to Lundberg B Waves last and what is the typical ICP?
.5-2 minutes ICP 20-50
What does Lundberg B waves tell you about elastance and compliance?
increased elastance decrease compliance
Intracranial hypertension occurs with a sustained increase in ICP above what?
20
What are signs of increased intracranial pressure?
HEADACHE, nausea, vomiting, papilledema, blurred vision, neurological deficits, ventilatory deficits, decreasing consciousness, seizures, and coma
What happens to CBF when ICP exceeds 30?
CBF decreases -> brain edema -> increased ICP ->ischemia
What happens if increased ICP is not corrected?
herniation
What does expanding tissue mass, fluid mass, CSF absorption interference, excessive CBF, or systemic disturbances promoting brain edema lead to?
inter cranial HTN
How do brain tumors increase ICP?
size, edema formation, obstruction of CSF flow
What causes aqueductal stenosis?
obstructive hydrocephalous
How are the third and fourth ventricle affected by aqueductal stenosis?
enlarged 3rd ventricle, normal 4th ventricle
How is aqueductal stenosis treated?
ventricular shunt
Lack of cerebral lesions, and increased ICP with no known cause is called what?
benign intercranial HTN
Benign intracranial HTN is most common in which population?
obese women with autoimmune irregularities
What are potential symptoms of benign intracranial HTN?
Headache and bilateral visual disturbances
What is the treatment for benign intracranial HTN?
removal of CSF and administration of acetazolamide and corticosteroids
What can acetazolamide cause?
acidemia d/t hydrogen ion secretion by renal tubules
what is the anesthetic management of benign intracranial HTN?
avoid hypoxia and hypercarbia.
What is the safest type of anesthetic for someone with benign intracranial HTN?
general
What type of anesthesia should be avoided in someone with benign intracranial HTN?
Epidural
Which of the 4 intracranial sub compartments can anesthetists control?
CSF, fluid and blood. We can’t alter cellular compartment
It is not recommended to remove CSF in patients with increased risk of which types of herniation?
transtentorial (uncal) or tonsillar herniation
Which intracranial subcompartment can anesthesia rapidly alter?
blood
Does the venous or arterial system more likely increase ICP?
venous due to obstruction of drainage or increased intrathoracic pressure
How do volatile anesthetics affect ICP, CBV and CBF?
increase ICP, CBV, CBF by decreasing cerebrovascular dilation
What affect does gases have on CPP?
decreases CPP by decreasing MAP
What affect does gases have on auto regulation and CMRO2?
CMRO2 decreased, impaired autoregulation
CMRO2-CBF coupling becomes impaired at what MAC level?
0.6-1 MAC
Which gases cause the greatest increase in CBF and ICP?
Des > Sevo > Iso
Which gas increases absorption of CSF?
Iso
Are volatile agents beneficial in focal or global ischemia?
global. Focal causes steal
With a MAC above 1.5 which gas causes the greatest increase in CBF?
Iso
Gases cause activation of ATP-dependent potassium channels, up-regulation of nitric oxide synthase, reduction of excitotoxic stressors and CMR, augmentation of peri-ischemic CBF, and up-regulation of anti-apoptosis factors leading to what?
neuroprotection during ischemic insults
Does N2O affect auto regulation?
not on its own, but it does in combination with other gases
How does N20 affect CBF and CMRO2?
increase both
What happens to CBF at 0.5 MAC?
doubles
How does N2O affect ICP?
increase due to cerebral vasodilation