CNS MONITORING

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CNS MONITORING
www.anaesthesia.co.in anaesthesia.co.in_at_gmail.co
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MAINLY CONSISTS OF

• Evoked potentials
• Electroencephalography and monitoring of
  anesthetic depth
• Monitoring intracranial pressure
• Monitoring the neuromuscular junction
• Specialized neurophysiological monitoring

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EVOKED POTENTIALS

• Useful in
• Evaluation of certain neurological disorders
• Monitoring functional integrity of sensory and
  motor pathways during many surgical procedures
• Preventing potential injury to the vital neural
  structures

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• Extremely small amplitude (microvolts) electrical
  potentials generated by nervous tissue in
  response to stimulation

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Source of evoked potentials

• Application of a stimulus to the nervous system
  is followed by the development of a neural signal
  which is transmitted along a specific pathway
• Represented as waveforms, voltage over time and
  are described in terms of amplitude, latency and
  morphology

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NEAR FIELD POTENTIALS

• Neuronal potentials created by depolarization are
  immediately below the recording electrode
• Large amplitude and exhibit marked changes in
  size and waveform with even small alterations in
  position of the recording electrode
• Initial positive wave

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FAR FIELD EVOKED POTENTIALS

• A depolarizing volley within the central nervous
  system white matter tracts travels toward the
  cortical mantle
• Produced by the deeper nuclei and tracts and are
  widely distributed
• Amplitude and morphology remain relatively
  constant despite changes in electrode position

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METHOD OF STIMULATION AND RECORDING

• STIMULATION
• RECORDING METHODS
• 10-20 electrode placement system
• Electrode, impedance,artifacts,deal with
  electrostatic and magnetic interference

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SOMATOSENSORY EVOKED POTENTIAL

• Electrical responses of brain or spinal cord to
  electrical stimulation of peripheral nerves
• Stimulus is a brief electric pulse delivered to
  the distal portion of the nerve
• Adjust the intensity of the stimulus so there is
  a small muscle twitch
• Activates low threshold myelinated nerve
  fibers-dorsal root- spinal column-gracile and
  cuneate nuclei-brainstem-thalamus-cortex
• Diagnosis of spinal cord diseases
• Intraoperative monitoring of some surgical
  procedures

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MEDIAN NERVE SSEP

• Useful in assessing the conduction in upper
  cervical cord and brain
• When used in conjunction with lower extremity
  SSEPs helps to recognize lesions between cauda
  equina and cervical spinal cord
• Stimulating electrode is at wrist
• Channel 1-contralateral central cortex
• Channel 2-c2/c7
• Channel 3-erbs point 1-EP2

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TIBIAL NERVE SSEP

• Obtained by stimulation of either the tibial or
  peroneal nerve
• Stimulating electrode-behind medial malleolus
• Channel 1-frontal midline region
• Channel 2-L1-T12
• Channel 3- popliteal fossa

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CONDITIONS PRODUCING CHANGES IN SSEP

• variable and complex effects on SSEPs and central
  conduction time
• Surgical anesthesia-prolonged latency and
  diminshed amplitude
• Premedication with atropine,morphine and diazepam
  can attenuate SSEP
• Thiopentone coma-dec ampl,inc lat
• Halogenated anesthetic-dose related method
• Nitrous oxide 50or more with fenanyl-dec
  ampl,inc latency
• Hypotension and hypothermia increase laency and
  dec amplitude
• Adjunct drugs,antibiotics and other cvs drugs
• Spinal surgery-narcotic/narcotic inhalational

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SSEP AND SPINAL CORD FUNCTION

• Spinal surgery
• Thoracic aortic surgery
• Spinal arteriography and therapeutic
  transvascualar embolization
• Extracranial carotid reconstruction, carotid
  endarterectomy( difficulty in detecting only
  motor tract ischemia ), cerebrovascualr surgery
  with induced hypotension and clippping and
  intraoperative localization for sensorimotor
  cortex
• Intracranial vascular surgery(median nerve for
  MCA
• Tibial nerve for ACA
• Basilar artery surgery

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• False negative SSEP-2-22
• False postive SSEP-2
• Changes in SSEPs indicate CNS ischemia
• Small areas of injury,motor area injury or
  lenticulostriate area injury may not be
  associated with changes

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AUDITORY EVOKED POTENTIALS

• Assessment of peripheral auditory function and
  the integrity of central auditor pathways
• Clicks generated by applying a brief square wave
  by tone pips (brief tone bursts)to activate
  restricted portion of the membrane of the cochlea
• Electrodes are placed athe vertex and on the ear
  lobes
• Age, sex, body temperature and hearing can alter

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• Anesthetic agents have dose related effects
• Thio and propofol may have an effect
• Hypotension and hypothermia may produce changes
• Monitoring in surgery of CPA tumours, posterior
  fossa, cavernous sinus and brainstem

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VISUAL EVOKED POTENTIALS

• Useful outside the OT in diagnosis of disease
  optic nerve and pathways
• Difficult in OT
• Used in surgery involving optic chiasma,
  pituitary gland
• anesthetic agents change them greatly

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MOTOR EVOKED POTENTIALS

• Detect damage to motor cortex or
• Motor pathway from motor cortex to muscle
• We use NMBA, recording from epidural space has
  been recommended
• Not popular because of complexity and time
  involved in setting up monitors, interpretation
  of evoked responses.

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SPECIALIZED NEUROPHYSIOLOGIC MONITORING

• TRANSCRANIAL DOPPLER USG
• JUGULAR BULB OXIMETRY
• NEAR INFRARED SPECTROSCOPY
• BRAIN PARENCHYMAL OXYGEN TENSION

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• IDEAL MONITOR FOR CEREBRAL ISCHEMA WOULD BE
  NONINVASIVE, SIMPLE , AT BEDSIDE OR OT, PROVIDE
  CONTINUOUS DETECTION OF GLOBAL , REGIONAL AND
  GLOBAL ISCHEMIA

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JUGULAR BULB OXIMETRY

• GLOBAL MEASURE , FAILS TO DETECT REGIONAL
  ISCHEMIA
• NIRS AND POP ARE LOVALIED
• TCD USG IS CAPABLE OF MONITORING A LARGE
  PROPORTION OF CBP BUT IS NOT IN ALL SITUATIONS

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TRANSCRANIAL DOPPLER USG

• CAROTID ENDARTERECTOMY
• -DECISION TO SHUNT
• DETECTION OF EMBOLI
• POSTOPERATIVE HYPOPERFUSION
• -POSTOPERATIVE OCCLUSION
• CARDIAC SURGERY
• SUBARACHNOID HEMORRHAGE
• HEAD INJURY
• AUTOREGULATION
• VASOSPASM
• BRAIN DEATH

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JUGULAR BULB OXIMETRY

• CHOICE OF SIDE
• EQUIPMENT AND TECHNIQUE
• COMPLICATIONS AND CONTRINDICATIONS

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CLINICAL APPLICATIONS

• TRAUMATIC BRAIN INJURYINTERPRETATION OF jbo
  SATURATION
• NEUROSURGICAL ANESTHESIA
• CPB
• LIMITATIONS

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NEAR INFRARED SPECTROSCOPY

• EQUIPMENT AND TECHNIQUE
• CLINICAL APPLICATIONS
• LIMITATIONS

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BRAIN PARENCHYMAL OXYGEN TENSION

• EQUIPMENT AND TECHNIQUE
• CLINICAL APPLICATIONS
• LIMITATIONS

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EEG

• MONITORING ADEQUATE BLOOD PRESSURE AND OXYGEN
  SATURATION WILL NOT PREVENT CEREBRAL ISCHEMIA
• OCCURRENCE OF SEIZURES
• DEGREE OF BARBITURATE BURST SUPPRESSION
• LEVEL OF SEDATION PRODUCED BY DRUGS

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• EEG is not the product of propagated action
  potentials
• Myelinization of axons tends to limit spread of
  ionic current to a few hundred micrometers,
  making it impossible to record axon potentials on
  the scalp
• EEG is derived from summation of nearly
  synchronous depolarization of cell bodies and
  dendrites,
• scalp tends to act as an averager of positive
  and negative voltages

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methods

• Place 21 electrodes on the scalp at locations
  standardized by the international 10-20 system
• Place in pairs for bipolar recording or
  individually using a common reference
• Electrode-lowest impedance and highest quality
  signal
• Metallic cups
• Needle electrode

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PROBLEMS

• Extremely susceptible to noise
• High electrode impedance-improper site
  preparation, poor adhesion, mechanical disruption
  desiccation, oxidation or broken contacts, proper
  electrode application
• Accurate placement

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EEG WAVEFORMS
BAND FREQUENCY RANGE
DELTA lt3.5 OR 4 Hz(cerebral ischemia, severe depression)
THETA 3.5-7.5 or 8Hz(cortical depression)
ALPHA 7.5-13 Hz( awake, calm)
BETA gt13 Hz( excitation)
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COMPRESSED SPECTRAL ARRAYS

• SIMPLIFIED DISPLAY OF AMPLITUDES AND FREQUENCIES
• Conversion of EEG signal from amplitude as a
  function of time to amplitude as a function of
  frequency time domain to frequency domain
• Makes it easier to see effects
• better assesses
• individual spectra are computed every few
  seconds and then stacked or compressed into an
  array CSA for easy comparison

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DENSITY MODULATED SPECTRAL ARRAYS
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INDICATIONS FOR EEG

• Assures cerebral well being
• Shunt placement during carotid endarterectomy
• Cardiac surgery
• For regional sensitivity-16-32 channels
• Acute monitoring of periopertive changes-7-8
  channels
• Hemispheric differnce-2 channels
• Ischemia and cerebral protection by barbiturate
  therapy
• Detection of seizure
• Progress and prognosis in coma

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• Sensitivity is high
• Specificity can be affected by anesthetic
  technique and analysis methods
• not to be used as isolated guide

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BIS

• INDICATIONS
• DIFFERENTIATES THE NEED FOR DEEPER HYPNOSIS,
  MORE ANALGESIA OR DIRECT AUTONOMIC CONTROL
• DIRECT MEASUREMENT OF THE FUNCTIONAL EFFECTS OF
  THE DRUGS ON THE BRAIN ALLOWS INDIVIDUALIZED
  PHARMACOLOGICAL TT.
• MONITORING OF SEDATION IN ICU
• DOSE OF BARBITURATES FOR CEREBRAL PROTECTION
  THERAPY

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INDIVIDUALIZED DOSING

• CLINICALLY STANDARD EFFECTIVE DOSE
• TARGET CONTROLLED INFUSION SYSTEMS
• DEFINITIONS OF MAC
• DIFFERENT GOALS FOR DIFFERENT PTS
• DIFF ANESTHETIC APPROACHES BY DIFFERENT
  CLINICIANS
• DIFF GOALS AT DIFFERENT TIMES IN THE INDIVIDUAL
  CSE

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Components of anesthetic depth

• Many monometric parameters from EEG have been
  investigated
• Median frequency
• Spectral edge frequency
• Various power bands
• Magic no. to describe anesthetic effects on the
  EEG has eluded definition

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STATISTICALLY DERIVED MULTIPARAMETRIC INDICES ARE

• BIS
• MIDLATENCY AUDITORY EVOKED RESPONSE INDEX
• AUTOREGRESSIVE MODELS OF Mac-narcograph

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• initially used prediction of movement
• Now improved artifact detection and rejection
• Recognizes and eliminated ECG contamination
• EMG contamination is problematic(temporalis and
  frontalis muscles picks up signals in the range
  of beta rhythm

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• BIS was statistically derived to correlate with
  response to command and not specifically to
  recall, the variance between drugs is wider for
  recall but nonetheless falls to very low below 80

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WHAT IT IS AND WHT IT ISNOT

• NOT A MACMETER
• DOES NOT PREDICT LIKELIHOOD OF MOVEMENT IN
  RESPONSE TO AND INCISON
• NOT A PREDICTOR OF ANY FUTURE BEHAVIOR BUT AN
  INDICATOR OF THE LEVEL OF SEDATION OVER THE LAST
  MINUTE

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• CEREBRAL ISCHEMIA CAN OCCUR WITHOUT A NOTABLE
  DECREASE IN BIS

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ANESTHETIC MGT WITH BIS

• USE DRUG TILL AVERAGE BIS IS 50
• USE ANALGESIC TILL MINIMUM VARIATION OVER A FEW
  MTS IS LESS THAN 10
• IF UNEXPLAINED TACHYCARDIA OR HYPERTENSION ,
  TRIAL OF ANALGESIC BEFORE TITRATING AUTONOMIC
  RESPONSE

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IF HPOTENSION OCCURS

• DECREASE DOSE OF ANALGESIC
• ASSESS THE VOLUME STATUS AND TT WITH SYMPATHETIC
  AGONISTS
• DURING CLOSURE, DECREASE SEDATIVE DOSE TO
  INCREASE TO 65
• IF ABOVE 70, TOO SOON , ADD SHORT ACTING
  SEDATIVE( INHALATION, IDOCAINE, PROPOFOL

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• MAKE PACO2 RISE TO 40, PATIENT STARTS BREATHING
• IF RR IS ABOVE NORMAL SUPPLEMENT ANALGESIA
  DURING EMERGENCE
• CONTINUED RISE IN BIS WILL ASSURE A RAPID
  EMERGENCE WITHOUT THE NEED TO WITHHOLD NARCOTICS
• SEDATION AND ANALGESIA CONTROLLED, REMAINING
  HEMODYNAMIC RESPONSES CAN BE CONTROLLED USING
  DRUGS

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LIMITATIONS

• SPURIOUS READINGS WITH CONTAMINATION (HIGH
  IMPEDENCE OR POOR CONTACT
• EMG ACTIVITY FROM FRONTALIS MUSCLE DIRECTLY
  BENEATH BIS ELECTRODES( WHICH OCCCURS IN ALL
  FREQUENCIES TILL BETA RHYTHM
• GIVE SMALL DOSE OF NMBA TO SEE IF VALUES RETURN
• IF DOESNT DECREASE, SMALL DOSE OF SEDATIVE
  SHOULD BE TRIED(PT NEEDED MORE SEDATION)

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PARADOXICAL AROUSAL RESPONSE

• Sudden drop to low values seen at light
  anesthetic level with minimal analgesia provoked
  by strong noxious stimulus with very sudden onset
  and resolution
• Treat it with narcotics
• Large swings in BIS (gt10) over 1 to 2 mts suggest
  the need for increased analgesia

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ENTROPY

• Shannon in 1948
• Johnson and Shore in the year 1984.
• The wave forms of entropy value of zero or near
  zero are predictable and
• those with very high entropy value (for eg.100)
  are totally unpredictable.

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• is independent of absolute scales such as
  amplitude or the frequency of the signal.
• A sine wave
• When the patient is awake, EEG is highly
  irregular and the amount of entropy is very high.
  
• As the patient goes into deeper planes of
  anaesthesia, EEG will have more regular pattern
  of wave forms which brings down entropy can be
  quite regular at different frequencies

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• Approximate (ApEn) or Shannon entropy in time
  domain
• Spectral entropy in frequency domain
• cross-approximate entropy (C-ApEn) measures the
  statistical dissimilarity or independence of two
  concurrent biologic signals viz. spatial and
  temporal, - suitable indicator of the state of
  consciousness or depth of sedation, than EEG
  index based on temporal properties alone i.e.
  ApEn.

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SE- state entropy

• SE is computed between the frequency ranges of
  0.8Hz and 32Hz, which corresponds mainly to the
  EEG dominated part of the spectrum and is the
  primary reflection of the cortical activity of
  the patient with a potential small amount of
  frontal electromyography activity (FEMG).
• The SE and RE are scaled to the ranges
• of 0 to 91

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RESPONSE ENTROPYRE

• is computed between 0.8Hz and 47Hz
• includes higher EMG dominated frequencies from
  fast muscle activity of frontal muscle.
• The RE is scaled to the range of 0-100
• main differences between the two is related to
  the contributions from frequencies between 32 and
  47Hz.
• These high frequency signals are analysed every
  1.92 sec, thus giving an immediate indication of
• frontal EMG activity.

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• During light levels of anaesthesia and sedation
  FEMG activity is present.
• Neuromuscular blocking agents do not abolish
  totally the ability of facial muscles to react to
  noxious stimuli when used in clinically practical
• amounts.
• Inadequate anaesthesia or arousal at the end of
  anaesthesia is associated with an abrupt increase
  in FEMG activity

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• often preceded by a more gradual, predictive
  rise. This feature of FEMG has been utilized in
  the development of Entropy algorithm.
• In addition, for accuracy of the displayed
  entropy value on the monitor, the raw EEG signal
  is treated for detection and
• removal of artifacts arising from electrocautery,
  ECG, pacemakers, EMG eye blinks/movements and
  other movement artifacts.

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• The studies have shown that the entropy indices
  decrease progressively with increasing levels of
  propofol sedation
• loss of consciousness with nitrous oxide is not
  associated with change in entropy indices.

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