RE5: Chapter 18: Clinical Monitoring III Flashcards
What is ICP directly r/t?
Intrathoracic pressure and changes dramatically with coughing or increased intrathoracic pressures.
What is a normal ICP range?
1-15 mmHg
A jugular venous oxygenation (SjvO2) that ranges between ____% to ___% has been found to be a reasonable predictor of positive outcomes for traumatic brain injury.
55% to 75%
What is an electrogencephalogram (EEG)?
A measurement of differences in electrical potentials in groups of neurons between brain regions rather than the brain emitting electrical waves.
It measures how “awake” or metabolically active the brain is during surgery.
What are the basic parameters of an EEG?
Frequency
Amplitude
Shape
Time
What are the 4 common types of brainwaves noted on an EEG?
Alpha
Beta
Delta
Theta
What are common variants or subgroups of waves noted during specific activities?
Gamma
My
Lambda
What types of waves are typically seen with high order activity such as problem solving or analytical thinking?
Gamma waves
The amplitude of the mu wave is about _____ that of the beta wave and is seen more frequently over the motor areas of the brain.
1/2
What types of waves occur in the awake patient and are usually present when staring, reading, or looking at objects for long periods?
Lambda waves
During the awake state, describe the type of waves commonly seen.
High-frequency and low-amplitude beta waves
What can cause depression of EEG activity?
Reduction in cerebral blood flow, oxygen or glucose delivery
With increased ischemia or hypoxia, what waves start to disappear and what waves start to appear?
Beta waves start to disappear and low-amplitude delta waves start to appear on the EEG.
True or False. The EEG can provide information about the cerebral cortex fxn, sub cortical brain, spinal cord and the cranial/peripheral nerves.
FALSE. EEG only provides information about the cerebral cortex function!
What effects do etomidate and propofol cause on the EEG?
They increase the frequency and decrease the amplitude of beta waves. This beta-rhythm EEG correlates with the patient losing consciousness.
Is myoclonus caused by etomidate seen on EEG?
No. EEG frequency decreases as serum levels rise, leading to burst suppression.
What is burst suppression?
An alternated high-frequency activity with 0.5 to several-second periods of electrical suppression.
When is burst suppression typically seen?
With a decrease in cerebral circulation and oxygenation as well as with hypothermia, particularly during CPB.
Why is burst suppression sometimes desirable?
To reduce cerebral oxygen requirements and provide neuroprotective properties. This may be desirable during manipulation of the brain tissues.
What anesthetics can cause burst suppression?
Etomidate
Propofol
Inhalation agents
What is unilateral burst suppression usually indicative of?
Ischemia or injury to the brain
What are CSA and DSA?
Compressed spectral array
Density spectral array
These methods further analyze the EEG
What is the Fourier analysis?
Compressed view of EEG waveforms presented in a 2 or 3 dimensional graph
How does GA affect EEG?
Reduction in high frequency waves and an increase in low-frequency amplitudes
What are the two major variables preventing exact correlation between EEG and anesthetic depth?
- Combination of many different drugs and dosages affecting the EEG in different ways
- Environmental factors and manipulation of the brain intraoperatively
What is the BIS?
Bispectral index monitor. It assesses anesthetic depth based on EEG electrical signals and processes them displaying a number from 0 (no brain activity) to 100 (awake).
What BIS values suggest adequate general anesthesia for surgery?
40-60
What device is the intraoperative gold standard for EEG monitoring?
16- to 32-channel analog EEG monitored by an experienced tech
What is NIRS?
Near-infrared spectroscopy
It is a noninvasive physiologic interpretation of oxygenation by evaluating the transmission and absorption of infrared light in the Hgb in brain tissue in real time.
What three things can NIRS evaluate during anesthesia?
Oxygenation
Blood volume
Motor function monitoring
What are evoked potentials?
Electrical potentials that are measured in response to some type of stimulus.
What can injury to neural structures be caused by in surgery that can be detected by evoked potentials?
Heat (electrocautery)
Mechanical stress (retraction)
Ischemia (ligation and vessel damage)
Loss of functional integrity (transection)
What does the nerve perineurium protect against?
Longitudinal retraction
What does the epineurium protect against?
Retraction (elastic limit of nerves is usually 20%, meaning stretching a nerve farther may produce irreversible damage)
What can EPs be affected by?
Hypothermia
Hypotension
Positioning
Anesthetic Agents
Is there a standard for the amt of change in latency or amplitude of an EP that necessitates warning the surgeon?
NO - however the rule of thumb is that 50% decrease in amplitude or 10% decrease in latency should exist before warning
What is the goal when monitoring EPs during anesthesia?
Adequately anesthetizing the patient while optimizing conditions that monitor neurologic structures and preserve them.
What agents interfere with neural membrane conduction and subcortical conduction?
Lipophilic agents (increase both inter peak latency and control conduction time)
Do anesthetic agents that interfere with EEG also interfere with EP?
Yes (the component frequencies of EP are the same as EEG)
What has a bigger effect on EPs, IV anesthetics or inhaled anesthetics?
Inhaled anesthetics
What are SSEPs used to monitor?
A number of neural structures along both the peripheral and central somatosensory pathways. Stimulations for SSEPs are created by stimulating peripheral nerves electrically. *Remember, peripheral nerves contain both sensory and motor component combining to provide a mixed signal!
What nerves are usually monitored in SSEPs?
Lower extremity: Posterior Tibial
Upper extremity: Median
(If unable, then common peroneal in popliteal fossa and ulnar nerve)
Where are recording electrodes used for SSEPs placed at the spinal cord?
C2, C3, C4
C7 is also used if monitoring subcortical SSEPs
Where is Erb’s point?
Side of the neck 2-3cm above the clavicle and in front of the transverse process of the 6th cervical vertebra.
What does pressure over the Erb’s point elicit?
Duchenne-Erb paralysis
What is meant by SSEPs being processed signals?
They are processed as an average, with electrical filters to remove background noise, instead of providing real-time electrical waveforms
How do anesthetic agents effect SSEPs?
Almost all anesthetic agents produce change in latency or amplitude, with exception of ketamine, etomidate and opiates!
What may be considered the best technique for anesthesia when monitoring SSEPs?
Narcotic based anesthetics, less than 1 MAC and nitrous oxide
Can paralytic agents be used with SSEPs?
Yes
What are some surgeries where SSEPs can be helpful?
Vascular surgery, such as carotid endarterectomy to determine need for shunting intraoperatively. If changes are immediate, SSEPs can indicate high risk or neurologic injury during aortic cross-clamping.
Also, during cerebral aneurysm surgery, changes can indicate occlusion of parental vessels, directing positioning of important aneurismal vascular clips.
What do BAEPs monitor?
Entire auditory pathway from the distal auditory nerve to the midbrain, inadvertently allowing monitoring of basic brainstem function.
What is the BAEP decibel?
The stimulus is a repeating click with repetition around 10Hz and intensity around 65 to 70 decibels ABOVE the click threshold.
How many peaks are represented on BAEPs?
I, II, III, IV, V
What are BAEPs affected by?
They are resistant to alternation by anything other than structural pathology in the brainstem.
Inhalational agents can mildly affect BAEP latency and amplitude.
What are BAEPs not significantly affected by?
Barbiturates, benzos, ketamine, nitrous oxide, propofol, and muscle relaxants.
How does hypothermia affect BAEPs?
Decreased latency and prolonged inter peak intervals
What can exaggerate BAEPs?
BAEPs can be exaggerated by an increase in latency with low PCO2 seen during hyperventilation.
What nerve is being monitoring as an auditory nerve compound action potential (AN-CAP)
Auditory nerve
When the 8th CN is involved, what is the AN-CAP is referred to?
eighth nerve potential (8NP)
What is electrocochleography (ECochG) used for?
To provide information about the cochlea and distal section of the auditory nerve. Typically used to evaluate and/or verify blood supply to the cochlea. The measure itself is called the ECochG-CAP.
What are otoacoustic emissions (OAEs) used for?
It is NOT an EP because a stimulus is not used, there is only a recording device to record sounds transmissions to assess auditory hair cell function and not internal structure of the ear.
What do MEPs monitor?
The functional integrity of motor tracts, particularly in the corticospinal tract.
What can be used for sites of stimulation for MEPs?
Motor cortex
Spinal cord
What two types of stimulation can be used for MEPs?
Magnetic stimulation of motor cortex
Electrical stimulation
What patients is contact with magnetic stimulation contraindicated in?
Patient with pacemakers, spinal or bladder stimulators, epilepsy, metallic foreign body, or previous craniotomy.
What anesthetic agents effect MEPs?
The choice of anesthesia is essentially unrestricted except for paralytic agents!
What are some indications for MEPs?
See Box 18-2
What is considered the most comprehensive approach for assessing the functional integrity of the spinal cord motor tracts?
Muscle MEPs and D wave recordings
What is the gold standard for monitoring the motor or anterior pathway?
Sensory or dorsal pathway?
MEPs - motor / anterior pathway
SSEPs - sensory / dorsal pathway
What are visual evoked potentials (VEPs) used to monitor?
Function of the visual pathway, which comprises the retina to the occipital cortex and everything in between including optic nerve and optic chiasm.
What are two major classes of VEPs?
Patterned and unpatterned (luminance)
Awake tests consist of pattern
What are the two most common pattern stimuli?
Pattern reversal and pattern onset/offset stimuli
What is used as stimulation in conjunction with anesthesia or sedation?
Stroboscopic flash (luminescence) stimulus
When monocular stimulation is used, what must be done to the unstimulated eye?
Monocular stimulation is typically used to avoid masking of unilateral conduction abnormality. Care should be taken to ensure that no light enters the eye not be stimulated by using a patch or tape to block light.
What should be assessed for preop for VEPs?
Eyes preoperatively since this may effect recorded outcomes.
Examples: extreme pupil size or anisocoria (inequality of pupil diameter)
Do pupils need to be dilated for VEPs?
No. Mydriatics and miotics should not be used with awake tests.
For flash VEPs, what peaks are the most robust components?
N2 and P2
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