VALLEY: CNS MONITORING Flashcards
4 indications for EEG monitoring during anesthesia
- carotid endartercetomy
- CPB procedures
- Cerebrovascular surgery
- When burst suppression is desirable for cerebral protection
2 indications for EEG monitoring in ICU
Barbiturate coma for patients with TBI
When subclinical seizures are suspected.
How many mV are generated by EEG signals
0.1 mV which is 100 microVolts
EEG waves are
Alpha
Beta
Delta
Theta
EEG waves are categorized based on
Frequency and amplitude
Lowest frequency wave and range
Delta waves (0-4Hz)
Greatest amplitude waves in the EEG
Delta waves
Slighly lower amplitude than delta waves
Theta waves
Theta waves frequency
Theta 4-7 Hz
Alpha waves are typically recorded over the
posterior aspect of the head during awake, alert, but relaxed activities.
Alpha waves amplitude
have an intermediate amplitude
Alpha < Delta =Theta
Alpha > Beta
Alpha Waves frequency
frequency range of 8- 12 Hz.
Waves are the highest frequency
beta (> 12 Hz),
Lowest amplitude waveforms
Beta
Are recorded predominantly over the frontal areas of the head
Beta
Delta waves are seen when?
Delta waves (0-4 Hz) are seen in the sleeping adult, but are considered abnormal in the awake adult.
Delta waves seen which pathology
Delta waves are also seen in encephalopathy, deep
coma, and deep anesthesia.
Theta waves are seen in
Theta waves (4-7 Hz) are seen in sleep and in deep anesthesia.
Prominent alpha wave activity (7-12 Hz) is characteristic of
awake, alert, but relaxed activities.
An ‘eyes closed’ resting alpha pattern is the
baseline awake pattern used when anesthetic effects on the EEG are described
Are characteristic of aroused, attentive, active thinking
Beta waves (> 12 Hz)
What happens to the electroencephalography (EEG) waveforms as anesthetic depth increases?
Increasing depth of anesthesia from the awake state is characterized by increased amplitude and synchrony in the EEG waveforms
As anesthetic depth increases, what happens ?
periods of electrical silence occupy greater proportions of the electroencephalogram (EEG).
What is Electrical silence in the EEG
period of electrical silence in an EEG is called an isoelectric EEG pattern.
What MAC correlates with an isoelectric EEG pattern?
An isoelectric pattern dominates the EEG in the range of 1.5 to 2.0 MAC.
During certain surgical procedures, maximal suppression of cerebral metabolic rate is desirable why?
to protect the brain during an ischemic insult
During certain surgical procedures, maximal suppression of cerebral metabolic rate is desirable to protect the brain
during an ischemic insult. Under such circumstances, the anesthetic agent can be
titrated against the EEG until the desired effect is achieved
Typically Typically, instead of an isoelectric EEG, the goal is a state called
called burst suppression.
Characterize “burst suppression” on the electroencephalogram
(EEG) burst suppression is characterized by periods
of isoelectric EEG punctuated by “bursts” of EEG activity. The “burst” is high-frequency activity and the “suppression” is O.S· to several-second periods of isoelectric activity
The electroencephalogram {EEG) is occasionally used during cerebrovascular surgery to confirm
adequate cerebral oxygenation
Identify four conditions o agents that can produce EEG changes mimicking cerebral ischemia (HEMA)
(1) hypothermia,
(2) electrolyte disturbances,
(3) marked hypocapnia, and
(4) anesthetic agents
When would you use sensory evoked potential monitoring?
The primary application of sensory evoked potential monitoring (somatosensory, brainstem auditory, visual) is to assess continually the function and integrity of neural pathways
Monitor the spinal cord or or cranial nerve II {visual).
somatosensory
Monitor cranial nerve VIII
(brainstem auditory),
Evoked potentials are used during
resection of spinal cord tumors, corrective surgery of the spine and cranial tumor resection.
How are somatosensory evoked potentials elicited?
by electrically stimulating tibial, ulnar or radial nerves.
SSEP neurons excited are
Low threshold sensory neurons (those that carry touch and pressure sensations) are excited
What is the value of somatosensory evoked potential monitoring?
monitor the integrity of the posterior (dorsal) spinal cord where the sensory tracts (cuneatus and gracilis) of the dorsal-lemniscal system are located.
Somatosensory evoked potential (SSEP} monitoring assesses the integrity of which region of spinal cord: dorsal, lateral, or ventral?
SSEP monitoring assesses the integrity of the dorsal cord.
Where are recording electrodes placed for somatosensory evoked potential monitoring if the tibial nerves are stimulated bilaterally?
recorded from electrodes placed between the ears on the scalp. The crucial electrode is placed midway
between each ear.
For SSEPs , where is the crucial electrode placed
Midway between each ear.
What happens to the somatosensory evoked potential (SSEP) when the patient undergoing laminectomy is paralyzed with a muscle relaxant?
Nothing. Neuromuscular blockade will not alter transmission of action potentials in the sensory tracts.
SSEPs: All anesthetic agents except muscle relaxants
depress SSEPs to a varying degree.
How are evoked potentials altered by volatile anesthetics?
Volatile anesthetics, especially high concentrations (> 1 MAC), produce dose-dependent increases in latency and decreases in amplitude of evoked potentials.
Which volatile inhalational agent LEAST depresses the amplitude and increases the latency of the somatosensory evoked potential (SSEP)?
Halothane
Which volatile inhalational agent MOST depresses the
amplitude and increases the latency of the SSEP?
Enflurane
How does nitrous oxide affect latency and amplitude of the somatosensory evoked potential?
Nitrous oxide causes a decrease in amplitude without a change in latency of the somatosensory evoked potential when used alone or when added to opioid or volatile anesthetics.
List 5 physiologic factors that may alter sensory evoked potentials (SEPs).
(l) temperature. (2) hypotension, (3) hypoxia, (4) hypocarbia, and (5) • isovolemic hemodilution.
Which physiologic factor has the greatest effect on sensory evoked potentials?
altered temperature affects SEPs the most, whereas
Which physiologic factor has the least effect on sensory evoked potentials?
hemodilution affects SEPs the least
SEP : Hypothermia will
increase latency and decrease amplitude of SEPs (latency
increases by 1 ms for each 1 ° C decrease in temperature)
SEP: Hyperthermia will
decrease amplitude by up to 15%.
SEPs are lost at which temperature
42C.
MAP and SEP
With a mean arterial pressure less than 40 mm-Hg. there
is a progressive decrease in amplitude ofSEPs.
A decrease in SEP amplitude is seen with hypoxia, probably due to
alterations in blood flow
With an ETC02 S 25 mm-Hg, SEP.
latency increases
SEP latency does not increase until the hematocrit is
less than 15%
SEP amplitude does not decrease until the hematocrit is
less than 7%.
Flow through what spinal arterial vessel(s) is monitored by somatosensory evoked potentials (SSEPs)?
SSEPs monitor flow through the posterior spinal arteries
Arterial blood is delivered to the spinal cord via
- one anterior spinal artery
- two posterior spinal arteries and small segmental (radicular} arteries that augment flow to the anterior and posterior arteries
The major source of blood for the spinal cord is
the anterior and posterior arteries.
SSEPs monitor sensory action potentials, which ascend in tracts located in the, so
posterior cord
Posterior cord ischemia resulting from hypoperfusion of the posterior spinal arteries would result in _____latency and ______amplitude of the SSEP
increased latency and decreased amplitude of the SSEP
Brainstem auditory evoked potential monitoring is useful during operations involving what cranial nerve?
Cranial nerve VIII.
Is brainstem auditory evoked potential monitoring appropriate for surgery for acoustic neuroma? Why or why not?
Yes it is appropriate, since cranial nerve VIII carries acoustic messages.
Monitoring of what evoked potential may be useful during pituitary surgery?
Visual evoked potential monitoring may be used during pituitary surgery to assess direct compression of, or compromise of the blood supply to, the optic nerve and optic chiasm.
What evoked potential (somatosensory, brainstem auditory, or visual) is monitored during transsphenoidal surgery?
Visual evoked potential monitoring may be employed for transsphenoidal surgery if the tumor is large and involves the optic nerves (cranial nerve II)
Multiple drugs used in the perioperative period can influence the ability to accurately monitor
sensory-evoked responses (SER; e.g. somatosensory,
visual, and brainstem (auditory) evoked potentials).
Inhalational agents, including nitrous oxide, generally have
more depressant effect on evoked
potentials than equipotent doses of intravenous agents.
Propofol and thiopental
attenuate the amplitude of virtually all evoked potential modalities but do not obliterate them.
Opioids and benzodiazepines on evoked potentials
Opioids and benzodiazepines have negligible effects on the recording of all evoked potentials
2 drugs that have been reported to enhance the quality of signals in patients with weak baseline somatosensory evoked potential (SSEP) signals.
Ketamine and etomidate
Rank the three major sensory-evoke responses-somatosensory (SSEP). visual (YEP), and brainstem/auditory
(BAPE)-based upon sensitivity to anesthetic agents
Visual are Very
Somatosensory are Somewhat, and
Brainstem are Barely sensitive.
Brainstem auditory evoked potentials (BAEP) are generally very resistant to alteration by anything other than structural pathology in the brainstem. What operating room environmental variable will decrease latency and prolong interpeak intervals in BAEPs?
Mild hypothermia has been associated with decreased latency and prolonged interpeak intervals during brainstem auditory evoked potentials
Where are motor evoked potentials stimulated?
Motor evoked potentials (MEPs) are stimulated over or in the motor region of the cerebral cortex.
MEPs, How are they stimulated?
by transcranial electrical stimulation (teMEP), transcranial magnetic stimulation (tmMEP) or by direct stimulation of the motor cortex with an electrode
Where may motor evoked potentials be monitored?
(1) the spinal cord, (2) a peripheral nerve, or (3) the involved muscle.
T/F Neither sensory evoked potentials (SEPs} nor motor evoked potentials (MEPs) can be recorded at the site of stimulation.
True
What is the wake-up test, and how is it performed?
The wake-up test is used to assess integrity of the spinal motor pathways, which are found in the anterior (ventral) cord. It is performed by lightening the anesthetic depth. The patient is asked to squeeze a hand and move
his/her feet and toes. After these maneuvers are performed, anesthesia is quickly deepened
Where is the spinal motor pathway found
Anterior (ventral) cord
The wake-up test monitors what region of spinal cord {dorsal, lateral, or ventral}?
The wake-up test monitors ventral cord, which is supplied by the anterior spinal artery
During the wake up test, When the patient is awakened, he or she is asked to
squeeze the anesthetists hand and move his or her feet and toes.
Motor tracts are found in the____Cord.
ventral cord
Recall that the somatosensory evoked potential
monitors the _____cord.
dorsal
What complications can occur during the wake-up test?
Extubation in the prone position,
Recall of intraoperative events (incidence, 0-20%), Myocardial ischemia, self- injury,
Dislodgement of instrumentation, and air embolus from open sinuses if the patient is breathing spontaneously and inhales vigorously.
What agents will not alter bispectral index (BIS} monitoring?
the BIS is not affected by opioids or analgesics. Nitrous oxide alone will have no effect on BIS.
Agent that may affect BIS
Ketamine has minimal effect on BIS, and may slightly increase BIS transiently
What is cerebral oximetry?
Cerebral oximetry is a noninvasive monitor used to measure regional blood hemoglobin saturation (rS02) via near-infrared optical spectroscopy (NIRS).
Cerebral oximetry used for patients
Technology is similar to that of pulse oximetry and can be used for patients at risk for stroke
Briefly describe the technique of cerebral oximetry via near-infrared optical spectroscopy (NIRS).
To measure regional hemoglobin oxygen saturation (rS02) via trans cranial near-infrared spectroscopy (NIRS). an infrared light source contained in a self-adhesive patch is affixed to the forehead.Because the human skull is
translucent to infrared light, photons are transmitted through the skull and underlying tissues to the outer layers of the cerebral cortex. Adjacent sensors separate photons reflected from the skin, muscle, skull, and dura from those
of the brain tissue. NIRS measures all hemoglobin, pulsatile and nonpulsatile, in a mixed microvascular bed.
NIRS measures
all hemoglobin, pulsatile and nonpulsatile, in a mixed microvascular bed.
Cerebral oximetry has the advantage that it may be used
during
nonpulsatile CPB and circulatory arrest
