2. Classification of monitoring techniques:
The brain can be monitored in terms of:
Function
Cerebral blood flow (CBF) & intracranial pressure
(ICP)
Brain oxygenation and metabolism
3. Monitoring of FUNCTION:
Electroencephalograms (EEG)
Raw EEG
Computerized Processed EEG: Compressed spectral array, Density spectral
array, Aperiodic analysis, Bispectral analysis (BIS)
Evoked Potential
Sensory EP:
Somatosensory EP
Visual EP
Brain stem auditory EP
Motor EP:
- Transcranial magnetic MEP
- Transcranial electric MEP
- Direct spinal cord stimulation
EMG
- Cranial nerve function (V, VII, IX, X, XI, XII)
5. 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
6. 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
7. EEG
Electrodes placed so that
mapping system relates
surface head anatomy to
underlying brain cortical
regions
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
8. EEG
EEG Waves :
Beta: high freq, low amp
(awake state)
Alpha: med freq, high amp
(eyes closed while awake)
Theta: Low freq (not
predominant)
Delta: very low freq high
amp (depressed
functions/deep coma
9. 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
10. Anesthetic agents and EEG
Subanesthetic doses of inhaled anesthetics (0.3 MAC):
Increases frontal beta activity (low voltage, high frequency)
Light anesthesia (0.5 MAC):
Larger voltage, slower frequency
General anesthesia (1 MAC):
Irregular slow activity
Deeper anesthesia (1.25 MAC):
Alternating activity
Very deep anesthesia (1.6 MAC):
Burst suppression eventually isoelectric
11.
12. Non-anesthetic Factors Affecting EEG
Surgical
1. Cardiopulmonary bypass
2. Occlusion of major cerebral
vessel (carotid cross-clamping,
aneurysm clipping)
3. Retraction on cerebral cortex
4. Surgically induced emboli to
brain
Pathophysiologic
Factors
1. Hypoxemia
2. Hypotension
3. Hypothermia
4. Hypercarbia and hypocarbia
13. Uses of EEG
1. Carotid endarterectomy
2. Cerebral aneurysm surgery when temporary clipping is
used.
3. Cardiopulmonary bypass procedure
4. Extracranial-intracranial bypass procedure
5. Deliberate metabolic supression for cerebral protection.
Surgery that place the brain at risk (difficulties: restricted
access)
Seizure monitoring in ICU
14. Processed 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.
The common processing techniques used are time
domain analysis and frequency domain analysis.
15. Time domain analysis
EEG is split into small epochs
of a given duration, usually
about 1-4 sec.
The frequency and/or
amplitude information
contained in each epoch is
depicted graphically.
A change in the value of the
variables derived form this
display is expected to represent
a change in the raw EEG.
16. Frequency domain analysis
The EEG is split into small
epochs.
Each epoch is further resolved
into its component sine waves
and reconstructed as frequency
Vs power plot by using Fourier
Analysis.
Compressed Spectral Array
(CSA) and Density Modulated
Spectral Array (DSA)
20. Sensory Evoked Potential
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
21. SEP
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
22. SSEP
Time-locked, event related,
pathway specific EEG in
respones of peripheral
stimulus
Monitor integrity of the
pathway from periphery to
the cortex
Electrical stimulator placed at
median, ulnar, or posterior tibial
nerves
23. Indications for SSEP
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
24. 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
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
25. Limitations
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
SSEP’s generally correlate well with spinal
column surgery
26. • Visual Evoked Potential (VEP)
Using LED goggles to create stimulus
Difficult to perform
• Brainstem Auditory Evoked Potential (BAEP)
Repetitive clicks delivered to the ear
Reflects the VIII nerve & brainstem “well-being”
33. Motor Evoked Potentials
Motor EP:
- Transcranial magnetic MEP
- Transcranial electric MEP
- Direct spinal cord stimulation
34. 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
35. 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
36. EMG
Early detection of surgically
induced nerve damage and
assessment of level of nerve
function intra-operatively.
Active or passive.
Uses:
1. Facial nerve monitoring
2. Trigeminal nerve monitoring
3. Spinal Accessory nerve
39. Intra-cranial Pressure
The pressure inside the lateral ventricles/lumbar
subarachnoid space in supine position.
The normal value of ICP is 10-15 mm Hg in adults.
40. Indications for ICP monitoring
1. Head Injury
2. Brain Tumors
3. Subarachnoid Heamorrhage
4. Hydrocephalus
5. Neuromedical conditions
42. ICP waveforms
ICP shows a pulsatile recording with slow
respiratory component superimposed on a biphasic
recording synchronous with cardiac cycle.
Normally, respiratory oscillations are greater than
the cardiac oscillations, but when ICP increases,
arterial pulsations also assume greater amplitude
43. Abnormalities of ICP waveforms
A WAVES: plateau waves
indicate ICP above 40mmHg
and are sustained for 5-
20min.
B WAVES: Amplitude of
20mmHg and occur at the
rate of 1-2/min. Occur
synchronus with cheyne-
stokes breathing
C WAVES: no pathological
significance
44. Transcranial Doppler
Measures the blood flow velocity in major cerebral
blood vessles.
Examination carried out through the temporal
window, orbital foramen or foramen magnum.
Using 2MHz probe.
MCA commonly used.
Change in velocity is proportional to change in flow
considering the vessel diameter is constant.
46. Clinical applications ofTCD
1. It is useful as a noninvasive monitor of CBF.
2. It is helpful to diagnose cerebral vasospasm and monitor response to
therapy in patients with subarachnoid haemorrhage and head injury.
3. It is used to study autoregulation of CBF and cerebral vascular
response to carbon dioxide.
4. It can be used to assess intracranial circulatory status in raised ICP.
5. It can be a useful tool to identify intraoperative cerebral
embolisation during surgery on carotid artery and cardiopulmonary
bypass procedures.
6. It can be used to optimise CPP and hyperventilationin patients with
head injury.
47. Intravascular tracer compounds
Method originally described by Kety and Schmidt.
Administration of radioactive isotope of xenon-133
Measurement of radioactivity washout with gamma
detectors.
Disadvantages: 1.Exposure to radioactivity
2.Cumbersome detector equipment
3.Focal areas of hypoperfusion missed
4.Snapshot of CBF not continuous monitor.
48. Thermal diffusion cerebral blood flow
monitoring
The rate at which heat dissipates in a
tissue depends on the tissue’s thermal
conductive properties and the blood
flow in that area.
Measurement is automatically
suspended if the passive thermistor
measures a brain temperature of 39.1°
C.
The inability to monitor during a febrile
episode may constitute a true limitation
of the technique
50. Monitoring of cerebral oxygenation and
metabloism
Brain tissue oxygenation
Jugular bulb venous oximetry monitoring
Microdialysis catheter
Near Infrared Spectroscopy (NIRS)
51. Jugular venous oximetry :principle
(A-V)DO2 x CBF = CMRO2
When CMRO2 is constant, any change in CBF is
associated with a reciprocal change in the cerebral
arteriovenous oxygen difference.
Based on the principle of reflectance oximetry.
52. Jugular venous oximetry
Continuous monitoring of
jugular venous oxygen
saturation (SjVO2 ) is carried
out by a catheter placed
retrograde through the
internal jugular vein intothe
jugular bulb.
For accurate measurement,
the tip of the catheter must
be within 1 cm of the jugular
bulb.
53. Indices obtained from SjVO2
1. Jugular venous oxygen
saturation (SjVO2 )
2. Cerebral arteriovenous
oxygen difference (A-VDO2 )
(the difference between
arterial and jugularvenous
oxygen content) and
3. Cerebral oxygen
extraction(CEO2 ) (the
difference between SaO2
and SjVO2 ).
54. Interpretation of SjVO2
Interpretation of jugular venous oxygen saturation (SjvO2)
Increased values: >90% indicates absolute/relative
hyperemia
Reduced metabolic need comatose/brain death
Excessive flove sever hypercapnia
AVM
Normal Values: 60-70% focal ischemia?
Decreased Values: <50% increased O2 extraction,
indicates a potential risk of ischemia injury
Increased demand: seizure / fever
Decreased supply: decreased flow, decreased hematocrit
As ischemiaprogress to infarction: O2 consumption
decreases
55. Near Infra-red Spectroscopy NIRS
The principle of absorption of near-infrared light by
chromophores in the body like
oxyhaemoglobin,deoxyhaemoglobin and
cytochrome aa3.
Light in the near-infrared region (70-1000 nm) is
very minimally absorbed by body tissues. It can
penetrate tissues upto 8 cm.
Measure regional cerebral blood flow, cerebral
blood volume, cerebral oxygen saturation and
cerebral metabolism.
56. NIRS limitations
Inability to assess the contribution of extracranial tissue
to the signal changes.
Presence of intracranial blood in the form of
haematomas and contusions can interfere with the
measurements.
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?)
57. Tissue partial pressure oxygen
monitoring:
Based on an oxygen-sensitive electrode originally described by
Clark.
The diffusion of oxygen molecules through an oxygen-
permeable membrane into an electrolyte solution causes an
electric current that is proportional to Po2.
The catheter is placed into the brain tissue through a twist
drill hole into the subcortical white matter.
Normal values for brain tissue oxygen tension are 20-40
mmHg.
In patients with cerebral ischaemia the values are 10 ± 5
mmHg as against 37 ± 12 mmHg in normal individuals
58. Cerebral Microdialysis
Small catheter inserted with ICP/tissue PO2
monitor
Artificial cerebrospinal fluid,equilibrates with
extracellular fluid,chemical composition analysis
Markers:
○ Lactate/pyruvate ratio : onset of ischemia
○ High level glycerol: inadequate energy to maintain
cellular integrity- membrane breakdown
○ Glutamate: neuronal injury and a factor in its
exacerbation
59. Catheter placement is
usually in ‘high risk’ tissue.
Uses: 1.Ischemia/trauma
2.epilepsy
3.Tumor chemistry
60.
61. References
Millers anesthesia 8th edition
Neurological monitoring. Dr. G S Rao IJA 2002;46(4)
Advances in neuroanesthesia monitoring Dr.
Pramod Bithal AIIMS new delhi. 2006 ISACON
GE-Datex Ohmeda Entropy monitor manual
Coviden BIS monitor users manual
