Neurological Monitoring

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Neurological Monitoring

• Augusto Torres, MD
• Department of Anesthesiology
• MetroHealth Medical Center
• April 2009

www.anaesthesia.co.in anaesthesia.co.in_at_gmail.co
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Outline

• EEG
• SSEP
• MEP
• Transcranial Doppler
• Cerebral Oximetry

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EEG

• Electroencephalogram surface recordings of the
  summation of excitatory and inhibitory
  postsynaptic potentials generated by pyramidal
  cells in cerebral cortex
• EEG
• Measures electrical function of brain
• Indirectly measures blood flow
• Measures anesthetic effects

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EEG

• Three uses perioperatively
• Identify inadequate blood flow to cerebral cortex
  caused by surgical/anesthetic-induced reduction
  in flow
• Guide reduction of cerebral metabolism prior to
  induced reduction of blood flow
• Predict neurologic outcome after brain insult
• Other uses identify consciousness,
  unconsciousness, seizure activity, stages of
  sleep, coma

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EEG

• Electrodes placed so that mapping system relates
  surface head anatomy to underlying brain cortical
  regions

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EEG

• 3 parameters of the signal
• Amplitude size or voltage of signal
• Frequency number of times signal oscillates
• Time duration of the sampling of the signal
• Normal EEG characteristic frequency (beta, then
  alpha) with symmetrical signals

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EEG

• Abnormal EEG
• Regional problems - asymmetry in frequency,
  amplitude or unpredicted patterns of such
• Epilepsy high voltage spike with slow waves
• Ischemia slowing frequency with preservation of
  amplitude or loss of amplitude (severe)
• Global problems affects entire brain, symmetric
  abnormalities
• Anesthetic agents induce global changes similar
  to global ischemia or hypoxemia (control of
  anesthetic technique is important)

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Abnormal EEG
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EEG

• The gold standard for intra-op EEG monitoring
  continuous visual inspection of a 16- to
  32-channel analog EEG by experienced
  electroencephalographer
• Processed EEG methods of converting raw EEG to
  a plot showing voltage, frequency, and time
• Monitors fewer channels, less experience required
• Reasonable results obtained

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Anesthetic Agents and EEG

• Anesthetic drugs affect frequency and amplitude
  of EEG waveforms
• Subanesthetic doses of IV and inhaled anesthetics
  (0.3 MAC)
• Increases frontal beta activity (low voltage,
  high frequency)
• Light anesthesia (0.5 MAC)
• Larger voltage, slower frequency

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Anesthetic Agents and EEG

• General anesthesia (1 MAC)
• Irregular slow activity
• Deeper anesthesia (1.25 MAC)
• Alternating activity
• Very deep anesthesia (1.6 MAC)
• Burst suppression ? eventually isoelectric

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Anesthetic Agents and EEG

• Some agents totally suppress EEG activity (e.g.
  isoflurane)
• Some agents never produce burst suppression or an
  isoelectric EEG
• Incapable (e.g. benzodiazipines)
• Toxicity (e.g. halothane) prevents giving large
  enough dose

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Anesthetic Agents and EEG

• Barbiturates, propofol, etomidate
• Initial activation, then dose-related depression,
  results in EEG silence
• Thiopental increasing doses will reduce oxygen
  requirements from neuronal activity
• Basal requirements (metabolic activity) reduced
  by hypothermia
• Epileptiform activity with methohexital and
  etomidate in subhypnotic doses

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Anesthetic Agents and EEG

• Ketamine
• Activates EEG at low doses (1mg/kg), slowing at
  higher doses
• Cannot achieve electrocortical silence
• Also associated with epileptiform activity in
  patients with epilepsy
• Benzodiazepines
• Produce typical EEG pattern
• No burst suppression or isoelectric EEG

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Anesthetic Agents and EEG

• Opioids
• Slowing of EEG
• No burst suppression
• High dose epileptiform activity
• Normeperidine
• Nitrous oxide
• Minor changes, decrease in amplitude and frontal
  high-frequency activity
• No burst suppression

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Anesthetic Agents and EEG

• Isoflurane, sevoflurane, desflurane
• EEG activation at low concentrations slowing,
  eventually electrical silence at higher
  concentrations
• Isoflurane
• Periods of suppression at 1.5 MAC
• Electrical silence at 2 2.5 MAC

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Anesthetic Agents and EEG

• Enflurane
• Seizure activity with hyperventilation and high
  concentrations (gt1.5 MAC)
• Halothane
• 3-4 MAC necessary for burst suppression
• Cardiovascular collapse

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Non-anesthetic Factors Affecting EEG Miller et al.

• Surgical
• Cardiopulmonary bypass
• Occlusion of major cerebral vessel (carotid
  cross-clamping, aneurysm clipping)
• Retraction on cerebral cortex
• Surgically induced emboli to brain

• Pathophysiologic Factors
• Hypoxemia
• Hypotension
• Hypothermia
• Hypercarbia and hypocarbia

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Intraoperative Use of EEG

• EEG used to monitor for ischemia
• Avoid during critical periods of the case
• Changing anesthetic technique
• Changing gas levels
• Administering boluses of medications that affect
  EEG

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Intraoperative Use of EEG

• Cardiopulmonary bypass
• Theoretically beneficial
• Embolic events with cannulation
• Increased risk in patients with carotid disease
• Difficult to interpret EEG changes
• Alteration of arterial carbon dioxide tension
• Changes in blood pressure
• Hypothermia
• Hemodilution (anemia)

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Intraoperative Use of EEG

• Carotid endarterectomy
• Well-established
• 20 of patients with major EEG changes awaken
  with neurological deficits
• Normal cerebral blood flow 50mL/100g/min
• Cellular survival threatened 12mL/100g/min
• EEG changes seen at 20mL/100g/min
• With isoflurane EEG changes not seen until
  10mL/100g/min
• If EEG changes noted, intervene
• Shunting
• Increase CBF

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Intraoperative Use of EEG

• Limitations to EEG for CEA
• Need for experienced technician to monitor
• Strokes still occur despite normal intra-op EEG
• Subcortical events not monitored by EEG
• Not proven to reduce incidence of stroke
• False positives

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Intraoperative Use of EEG

• What to do if EEG technician indicates a possible
  problem?
• Check to see if anesthetic milieu is stable
• Rule out hypoxemia, hypotension, hypothermia,
  hypercarbia and hypocarbia
• Raise the MAP, obtain ABG
• See if there is a surgical reason

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Evoked Potentials

• Definition electrical activity generated in
  response to sensory or motor stimulus
• Stimulus given, then neural response is recorded
  at different points along pathway
• Sensory evoked potential
• Latency time from stimulus to onset of SER
• Amplitude voltage of recorded response

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Sensory Evoked Potential

• Sensory evoked potentials
• Somatosensory (SSEP)
• Auditory (BAEP)
• Visual (VEP)
• SSEP produced by electrically stimulating a
  cranial or peripheral nerve
• If peripheral n. stimulated can record
  proximally along entire tract (peripheral n.,
  spinal cord, brainstem, thalamus, cerebral
  cortex)
• As opposed to EEG, records subcortically

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Sensory Evoked Potential

• Responds to injury by increased latency,
  decreased amplitude, ultimately disappearance
• Problem is response non-specific
• Surgical injury
• Hypoperfusion/ischemia
• Changes in anesthetic drugs
• Temperature changes

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Sensory Evoked Potentials

• Signals easily disrupted by background electrical
  activity (ECG, EMG activity of muscle movement,
  etc)
• Baseline is essential to subsequent interpretation

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SSEPs

• Stimulation with fine needle electrodes
• Stimulate median nerve signal travels
  anterograde causing muscle twitch, also travels
  retrograde up sensory pathways along dorsal
  columns all the way to brain cortex

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SSEPs

• Can measure the electrophysiologic response to
  nerve stimulation all the way up this pathway
• Monitor many waves (representing different nerves
  along pathway) and localization of where the
  neural pathway is interrupted is possible

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Intraoperative SSEPs

• Neurologic pathway must be at risk and
  intervention must be available
• Indications
• Scoliosis correction
• Spinal cord decompression and stabilization after
  acute injury
• Brachial plexus exploration
• Resection of spinal cord tumor
• Resection of intracranial lesions involving
  sensory cortex
• Clipping of intracranial aneurysms
• Carotid endarterectomy
• Thoracic aortic aneurysm repair

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Intraoperative SSEPs

• Scoliosis surgery well established
• Lessen degree of spine straightening
• False-negatives rare, false positives more common
• Motor tracts not directly monitored
• Posterior spinal arteries supply dorsal columns
• Anterior spinal arteries supply anterior (motor)
  tracts
• Possible to have significant motor deficit
  postoperatively despite normal SSEPs
• SSEPs generally correlate well with spinal
  column surgery
• Poor correlation in thoracic aortic surgery

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Intraoperative SSEPs

• Carotid endarterectomy
• Similar sensitivity has been found between SSEP
  and EEG
• SSEP has advantage of monitoring subcortical
  ischemia
• SSEP disadvantage do not monitor anterior
  portions - frontal or temporal lobes

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Intraoperative SSEPs

• Cerebral Aneurysm
• SSEP can gauge adequacy of blood flow to anterior
  cerebral circulation
• Evaluate effects of temporary clipping and
  identify unintended occlusion of perforating
  vessels supplying internal capsule in the
  aneurysm clip

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Other SEPs

• Auditory (BAEP) rapid clicks elicit responses
• CN VIII, cochlear nucleus, rostral brainstem,
  inferior colliculus, auditory cortex
• Procedures near auditory pathway and posterior
  fossa
• Decompression of CN VII, resection of acoustic
  neuroma, sectioning CNVIII for intractable
  tinnitus
• Resistant to anesthetic drugs

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Other SEPs

• VEP flash stimulation of retina assess pathway
  from optic n. to occipital cortex
• Procedures near optic chiasm
• Very sensitive to anesthetic drugs and variable
  signals - unreliable

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Anesthetic Agents and SEPs

• Most anesthetic drugs increase latency and
  decrease amplitude
• Volatile agents increase latency, decrease
  amplitude
• Barbituates increase in latency, decrease
  amplitude
• Exceptions
• Nitrous oxide latency stable, decrease amplitude
• Etomidate increases latency, increase in
  amplitude
• Ketamine increases amplitude
• Opiods no clinically significant changes
• Muscle relaxants no changes

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Physiologic Factors and SEPs

• All of these affect SSEPs
• Hypotension
• Hyperthermia and hypothermia
• Mild hypothermia (35-36 degrees) minimal effect
• Hypoxemia
• Hypercapnea
• Significant anemia (HCT lt15)
• Technical factor poor electode-to skin-contact
  and high electrical impedence (eg electrocautery)

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Anesthetic ManagementSchubert Clinical
Neuroanesthesia

• Stable, constant anesthetic level, especially
  during critical periods
• Response to poor signal
• Rule out technical factors
• Electrode impedance, radio frequency interference
• Cortical vs. subcortical changes

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Anesthetic ManagementSchubert Clinical
Neuroanesthesia

• Rule out systemic factors
• KEY improve neural tissue blood flow and
  nutrient delivery
• Intravascular volume and cardiac performance
  optimized (crystalloid/colloid or blood) to
  increase oxygen-carrying capacity optimal HCT
  30 or higher
• Elevate MAP
• Blood gas assure oxygenation, normocarbia to
  help improve collateral blood supply if
  hypocarbic
• Consider steroids (shown to work with traumatic
  spinal cord injury)
• Mannitol improve microcirculatory flow and
  reducing interstitial cord edema

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Anesthetic ManagementSchubert Clinical
Neuroanesthesia

• Rule out neurological factors
• Brain and spinal cord ischemia
• Pneumocephalus
• Peripheral n. ischemia and compression

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Motor Evoked Potentials

• Transcranial electrical MEP monitoring
• Stimulating electrodes placed on scalp overlying
  motor cortex
• Application of electrical current produces MEP
• Stimulus propagated through descending motor
  pathways

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Motor Evoked Potentials

• Evoked responses may be recorded
• Spinal cord, peripheral n., muscle itself

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Motor Evoked Potentials

• MEPs very sensitive to anesthetic agents
• Possibly due to anesthetic depression of anterior
  horn cells in spinal cord
• Intravenous agents produce significantly less
  depression
• TIVA often used
• No muscle relaxant

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Transcranial Doppler

• Direct, noninvasive measurement of CBF
• Sound waves transmitted through thin temporal
  bone, contact blood, are reflected, and detected
• Most easily monitor middle cerebral artery

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Transcranial Doppler

• Does not measure actual blood flow but velocity
• Velocity often closely related to flow but two
  are not equivalent
• Surgical field may limit probe placement and
  maintenance of proper position
• Carotid endarterectomy
• Measure adequacy of CBF during clamping
• Technically difficult in 20
• Useful for detecting embolic events How much
  emboli is harmful?

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Transcranial Doppler

• CPB
• Detect air or particulate emboli during
  cannulation, during bypass, weaning from bypass,
  decannulation
• Significant data pending
• Detection of vasospasm (well-established)
• Smaller area increase in velocity (gt120cm/s)

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Cerebral Oximetry (Near infrared spectroscopy)

• Measures oxygen saturation in the vascular bed of
  the cerebral cortex
• Interrogates arterial, venous, capillary blood
  within field
• Derived saturation represents a tissue oxygen
  saturation measured from these three compartments
• Unlike pulse oximetry (requires pulsatile blood),
  NIRS assess the hemoglobin saturation of venous
  blood, which along with capillary blood, composes
  approximately 90 of the blood volume in tissues
• Believed to reflect the oxygen saturation of
  hemoglobin in the post extraction compartment of
  any particular tissue
• Measures tissue oxygen saturation

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Cerebral Oximetry (Near infrared spectroscopy)

• Concerns
• Measures small portion of frontal cortex,
  contributions from non-brain sources
• Temperature changes affect NIR absorption water
  spectrum
• Degree of contamination of the signal by
  chromophores in the skin can be appreciable and
  are variable
• Not validated threshold for regional oxygen
  saturation not known (20 reduction from
  baseline?)
• High intersubject variability
• Low specificity
• Rigamonti et al. (J Clin Anesth 200517426)
• Compared EEG to rSO2 in CEA in terms of
  predicting need to place shunt 44 sens 84 spec

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Conclusion

• EEG is a useful modality for measuring
  intraoperative cerebral perfusion
• SSEP offers the additional advantage of measuring
  subcortical adverse events
• New techniques for neurological monitoring are
  being developed which need to be further
  evaluated and validated

www.anaesthesia.co.in anaesthesia.co.in_at_gmail.co
m