The document discusses central nervous system monitoring during anesthesia, detailing various techniques for assessing both metabolic and functional integrity, including evoked potentials and EEG. It highlights the importance of monitoring cerebral blood flow and the risks associated with awareness under anesthesia, emphasizing factors that can lead to inadequate anesthesia. Additionally, it outlines methods of measurement and prevention strategies to mitigate the risk of intraoperative awareness.

1. CENTRAL NERVOUS SYSTEM
MONITORING : AWARENESS UNDER
ANESTHESIA
Chairperson: Dr HV Airani
Moderator : Dr Anupama
Presenter: Dr. Shreya Shetty
1

2. CENTRAL NERVOUS SYSTEM MONITORING:
- Normal cerebral blood flow is 50ml/100g/min.
About 15-20% of CO.
- EEG becomes to become abnormal with CBF of
.
- Cellular survival is not threatened until CBF
decreases to
2

3. -Neurologic monitoring during anesthesia care for a patient spans a wide
spectrum of techniques, diverse procedures, and various intraoperative or even
postoperative settings.
-Techniques for monitoring fall into two broad categories:
1. To assess metabolic integrity of the nervous system, which determines the
blood flow or oxygenation
2. To assess functional integrity
3

4. CNS MONITORING
4
FOR BLOOD FLOW FOR NEUROLOGICAL FUNCTIONS
INVASIVE NON INVASIVE
-Intravascular tracer
compound
-Transcranial doppler
-Jugular venous oxygen
saturation
-Cerebral oximetry
-Tissue-Level Blood Flow Monitoring
Techniques
-Tissue Partial Pressure of Oxygen
Monitoring
EVOKED POTENTIALS :
A) Sensory evoked
potentials:
1) SSEP
2) BAEP
3) VEP
B) Motor evoked potentials:
C) EEG
D) BIS

5. NON INVASIVE NEUROPHYSIOLOGIC MONITORING:
- INTRAVASCULAR TRACER COMPOUND
- TRANSCRANIAL DOPPLER
- JUGULAR VENOUS OXYGEN SATURATION
- CEREBRAL OXIMETRY
5

6. INTRAVASCULAR
TRACER COMPOUND:
Passage of
agents during
computer tomographic
or magnetic resonance
imaging for determining
6

7. TRANSCRANIAL DOPPLER
USG:
Real time non invasive measurement of
.
Uses:
-Carotid endarterectomy: detection of
emboli, postoperative hypoperfusion,
postoperative occlusion.
-Cardiac surgery
-Subarachnoid hemorrhage
-Head injury: autoregulation, vasospasm,
brain death.
7

8. 8

9. JUGULAR BULB OXIMETRY:
Sjvo2 (jugular bulb venous oxygen saturation) measures the
and to represent the balance between cerebral oxygen
supply and demand.
A fibreoptic catheter is placed in a retrograde fashion into the jugular bulb
through IJV under fluoroscopic guidance.
Uses:
-Interpretation of jbo saturation
-Neurosurgical anesthesia
9

10. CEREBRAL OXIMETRY:
- Cerebral oximetry is a non invasive
technique which uses reflectance
oximetry to measure the
.
- Typically, two sensors are applied
to both sides of the forehead.
- As 66-80% of cerebral blood
volume is venous, determines –
10

11. 11

12. EVOKED POTENTIALS:
-Evoked potentials are the electrical activity generated in response to either a
.
-Evaluation of neurological disorders
-Extremely small amplitude (microvolts) electrical potentials generated by nervous tissue in
response to stimulation.
-Represented as waveforms.
12

13. • SOMATOSENSORY
EVOKED POTENTIAL:
-Electrical to
electrical stimulation of peripheral nerves
(median or tibial).
-Stimulus is a brief electric pulse delivered to the
distal portion of the nerve.
-The resulting evoked potential (nerve impulse)
follows the sensory pathways to brain where
EEG records its delivery.
-Measures
.
-Useful during surgeries of back, spinal cord.
13

14. - Negative predictive value – 99.9%
- Positive predictive value – 42%
- Premedication with atropine, morphine, diazepam
can attenuate SSEP.
- Increase in latency upto 10% from baseline and
reduction of amplitude upto 50% in noted.
- Factors affecting :
Technical
Physiological (temperature, hemoglobin, BP)
Anesthetic effect: propofol, narcotics,
benzodiazepines, ketamine, etomidate, nitrous
oxide.
14

15. • BRAINSTEM AUDITORY EVOKED POTENTIAL:
- Monitoring the
- Assessment of peripheral auditory function and the integrity of central auditory pathways.
- Indicated in posterior cranial fossa surgery.
15
Medial

16. • VISUAL EVOKED POTENTIAL:
-Retina is stimulated with light flashes and response is captured by electrodes
on the parietal- occipital lobe.
-Indicated for post operative visual outcome assessment.
-Measures the intactness of visual pathways.
16

17. • MOTOR EVOKED
PATHWAYS:
- Measures motor pathways, pyramidal
tracts by cortical stimulation and measured
motor responses.
- Indicated in spine surgeries.
- Influenced by ischemia, metabolic
changes, compression.
- 3-5 short duration but high voltage pulses.
17


18. 18

19. • EEG (ELECTROENCEPHALOGRAM):
- The EEG is a surface recording of the summation of excitatory and inhibitory postsynaptic
potentials spontaneously generated by the pyramidal cells in the cerebral cortex.
19

20. Indications for EEG:
-Depth of Anesthesia monitoring.
-Detection of seizures
-Temporal lobe epilepsy/laser thermal ablation
-Electrocortical stimulation mapping
-Shunt placement during carotid endarterectomy
-Cardiac surgery
-Acute monitoring of perioperative changes
-Progress and prognosis of coma
-Ischemia and cerebral protection by barbiturate therapy.
20

21. Factors affecting EEG:
-Hypoxia
-Hypotension, ischemia
-Hypothermia
-Hypo/hyper-carbia
-Brain death
-Surgery: untoward events
21

22. 1. INTRAVENOUS AGENTS:
• Barbiturates, Propofol, Etomidate.
22

23. • Ketamine:
- High amplitude theta activity
with ketamine.
- EEG activity is variable and disorganised; hence BIS is not useful in knowing the
conscious level with ketamine use
- Increased epileptiform activity.
• BZD:
- Incapable of producing burst suppression or isoelectric EEG.
• Dexmedetomidine:
- Incapable of producing burst suppression or isoelectric EEG even at higher dosage.
23

24. 2. INHALATIONAL AGENT:
• Nitrous oxide: Used alone, causes decrease in amplitude.
• Isoflurane, Sevoflurane, Desflurane:
- Isoflurane at MAC 2-2.5 –> burst suppression on EEG.
24

25. • BIS(BISPECTRAL INDEX):
-Bispectral monitoring by Aspect medical systems is a non invasive monitoring.
-A BIS sensor is placed on the forehead and then connected through a cable to
a monitor.
-Together the sensor and the monitor and computes
the number between 0 and 100, which corresponds to the level of
consciousness.
-Differentiates the need for deeper hypnosis, more analgesia.
-Direct measurement of the functional effects of the drugs on the brain allows
individualized pharmacological treatment.
25

26. - BIS ANALYSIS: BIS guided anesthesia demonstrated superiority in
monitoring depth of anesthesia, minimize awareness under anesthesia,
reduction in anesthetic utilization, fast awakening.
26

27. Advantages:
- decreased risk of awareness
- decreased use of narcotics and sedative.
Disadvantages:
- falsely elevated in pacemakers
-poor correlation with sedation scoring in ICU (Ramsay scale).
27

28. 28

29. • ENTROPY:
- Described by Shannon 1948
- Entropy measures in a
system.
- Wave forms of entropy value of zero or near zero are predictable and those
with very high entropy value are totally unpredictable.
- 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 anesthesia, EEG will have more
regular pattern of wave forms which brings down entropy.
29

30. SE- STATE ENTROPY:
- Frequency ranges from 0.8hz and 32hz
-Corresponds to EEG dominated part of the spectrum.
RE- RESPONSE ENTROPY:
-Frequency ranges from 0.8-47hz.
-Includes higher FEMGf (frontal electromyography) activity dominated frequencies from fast
muscle activity of frontal muscle.
During light planes of anesthesia and sedation, FEMG activity is present.
Inadequate anesthesia or arousal at the end of anesthesia is associated with an abrupt
increase in FEMG activity.
target : 40-60
Low SE and high RE -- consider more analgesia and muscle relaxant.
30

31. 31

32. 32

33. • NARCOTREND:
- A new EEG monitor designed to measure the depth of anesthesia
- It uses power spectral analysis and automated pattern recognition algorithms to classify the EEG
into stages from A (awake) to F (GA with increasing to burst suppression) and generate an index
of depth of anesthesia.
- The newer version of the Narcotrend monitor includes Narcotrend Index, which is scaled
between 0 and 100.
33

34. DEPTH OF ANESTHESIA- GUEDEL CLASSIFICATION:
34

35. - Awareness is the postoperative recall of sensory perception during general anesthesia.
- The incidence is 1-2 per every 1000 patients.
- Awareness during anesthesia may occur despite apparently sound anesthetic management
and is usually not associated with pain. However, a few cases may experience excruciating pain
and have long term neuropsychiatric sequelae. This adverse event can also have serious
medicolegal implications.
- Anesthestic depth can not be measured directly in an unconscious patient, what can be
measured is response to stimulation.
35

36. Types of awareness:
- The formation of explicit and implicit memories during anesthesia and surgery
(awareness) is considered potentially damaging to the human psyche.
- Explicit memory may be
.
- Implicit memory
36

37. Consequences of Awareness:
- It may have psychological sequelae for the patient:
- The majority of patients who have suffered intraoperative awareness fears future surgery and
anesthesia.
- The occurrence of intraoperative awareness also has consequences for the anesthetist.
- Recent examination of the American Society of Anesthesiologists’ (ASA) Closed Claim Project
revealed that 2% of all claims were for awareness.
37

38. - The risk of awareness correlates with depth of anesthesia.
- Light anesthetics, particularly where the patient is paralysed by a neuromuscular blocking
agent, are associated with the highest risk of awareness.
-
a. Errors include the omission or late commencement of a volatile agent
b. inadequate dosing
c. failure to recognize the signs of awareness
d. Under-dosing of anesthetic agent may occur during hypotensive episodes, when anesthetic
is withheld in an attempt to maintain arterial pressure
e. equipment failure
38

39. - A number of surgical scenarios are
associated with a higher risk of intraoperative
awareness. These include: cardiac surgery,
emergency surgery, surgery associated with
significant blood loss and Caesarean section.
- The selection of anesthetic dose is based
upon the patient’s expected requirement.
Patients vary significantly. Compared with
young adults, there is a 25% increase in
minimum alveolar concentration (MAC) for
volatile agents in young children, and a 25%
reduction in the elderly.
39

40. - Factors associated with a degree of resistance to anaesthetic agents:
pyrexia
hyperthyroidism
obesity
anxiety
young age
tobacco smoking
regular, heavy alcohol use
use of recreational drugs (e.g. opioids, amphetamines, cocaine)
chronic use of sedatives
previous and repeated exposure to anaesthetic agents.
40

41. - Breathing system malfunctions and disconnections have been associated
with awareness.
- Vaporizers may malfunction in a number of ways, each having the
potential to deliver an inadequate dose of anesthetic.
- These include: an empty vaporizer, miscalibration, impurities in the volatile
agent and disconnection from the anesthetic machine.
41

42. METHODS OF MEASUREMENT:
- Clinical signs: heart rate, Blood pressure, respiratory rate, pupillary response,
perspiration, response to command.
- Clinical techniques: Isolated forearm technique, surface electromyogram,
lower esophageal contractility.
- Brain electrical activity monitoring: EEG, BIS monitoring, entropy, narcotrend.
- Evoked potentials monitors: MEP, SSEP, AEP, VEP.
42

43. Pharmacological masking of signs of inadequate depth of
anesthesia:
- A nonparalyzed but inadequately anesthetized patient usually communicates by movement.
- The use of muscle relaxants render such a patient motionless and can give the anesthesiologist a
false sense of security.
- Use of drugs like beta blockers or vasodilator agents which have to be given preoperatively for
disorders like hypertension may affect intraoperative haemodynamics.
- Sometimes the anesthesiologist may use these drugs to tackle intraoperative tachycardia and
hypertension without addressing the underlying cause like inadequate depth of anesthesia.
-
43

44. Clinical signs of awareness:
- In the absence of specific depth of anesthesia monitoring, cardiovascular parameters are usually
relied upon to gauge adequacy of anesthesia.
- It is assumed that light anesthesia will manifest itself by causing hypertension and tachycardia as
well as other signs of sympathetic nervous system stimulation, such as lacrimation, papillary
dilatation and sweating.
- It is important to put these variables into the context of the specific patient and their anesthetic.
44

45. - There are many factors that may cause blunting of these responses and therefore delay or
prevent the suspicion of inadequate anesthesia.
- Many of these are pharmacological:
a. Anti-cholinergic medications may dry secretions such and reduce sweating or lacrimation as
well as causing mydriasis.
b. Opioids can cause meiosis.
c. Many anti-hypertensive and anti-anginal medications may reduce a patient’s ability to mount
a tachycardic and hypertensive response to stress (e.g. β-blockers, calcium channel
antagonists, ACE-inhibitors).
- The presence of an epidural can also lead to relative hypotension as can hypovolaemia from
any number of causes.
-
Few disease conditions like hypothyroidism, autonomic neuropathy and limited cardiac reserve
may have these cardiovascular responses masked.
45

46. 46

47. PREVENTION:
- Premedication with BZD
- adequate dosage of anesthetics
- checking equipments
- monitoring depth of anesthesia
- monitoring end tidal volatile anesthetics.
0.8 to 1 MAC of anesthetic agent exhaled greatly decreases the risk of awareness.
47

48. References :
• Miller’s anaesthesia 9th edition.
• Clinical anaesthesia 8th edition by Paul G Barash
• Morgan & Mikhail’s Clinical Anesthesiology, 7e
• Objective anesthesia, 5th edition
• Purden PL et al Anesthesiology 2015;123:937-60
• Hardman & Aitkenhead - Continuing Education in Anaesthesia Critical
Care & Pain - 2005
48