a ppt describing the tools to monitor depth of anaesthesia

1. Monitoring depth of anaesthesia
Dr Neha Devaraj
2nd year anesthesia resident
Mumbai

2. Need……
• To ensure adequate depth without inadvertently loading the
patients with potent drugs
Increased risk of awareness
• Multiple trauma
• CS
• Cardiac surgery
• Hemodynamic instability

3. History
• The word "anesthesia" was first used by the Greek
philosopher Dioscorides to describe the narcotic effect of the
plant mandragora.
• The word reappeared in the 1771 Encyclopaedia Britannica,
where it was defined as a "privation of the senses.
• Plomley, in 1847, was the first to define depth of anesthesia.
He described three stages:
intoxication, excitement (both conscious and unconscious),
and the deeper levels of narcosis.

4. History
• 1847- John Snow
Five degrees of narcotism for ether anaesthesia
Refined by Guedel into 4 stages on the basis of
– Somatic muscle tone
– Resp parameters
– Ocular signs
• 1954-Artusio
Divided Guedel stage I into 3 planes
• 1957-Woodbridge
Defined anaesthesia as having 4 components
– Sensory blockade
– Motor blockade
– Blockade of autonomic reflexes
– Loss of consciousness

7. Anatomy of pain pathway
• Afferent fibres from nociceptors
Dorsal Root Ganglion of Spinal Cord
Terminates in grey matter of dorsal horn
Fibres cross to form antero lateral pathways
Reticular formation
Somatosensory cortex , sec somatosensory cortex & postero
parietal cortex
• This pathway is blocked by GA , m.c at ascending reticular
system

8. NOXIOUS STIMULI
SOMATIC
• Sensory
– Gives rise to pain and thus
memory , blocked by opioids
• Motor
– Causes movement , blocked
by muscle relaxants
AUTONOMIC
• Breathing
– Blocked by MR
• Hemodynamic
– Rise in BP,HR blocked
• Pseudomotor by
– Sweating spinal/
• Hormonal epidural
– Stress response

9. • Increasing concentration of GA
–Decrease in ability of brain to carry out
tasks (congnition) & to remember it later
(amnesia)
–This occurs before noticeable autonomic
effects

10. COGNITION
LONG TERM MEMORY
A) procedural memory
B) declarative memory
I. Somatic memory
II. Episodic memory
• Procedural & somatic memory
form the implicit memory
– Effortless retrival
• Episodic forms the explicit
memory
– Recall needs effort
– More sensitive to GA
SHORT TERM MEMORY
A) learning
B) decision making
C) information retrivation

11. Stages of awareness ( GRIFFITH)
1. Conscious awareness with explicit recall
2. Conscious awareness with no explicit recall
3. Subconscious awareness with implicit recall
4. No awareness or recall
White suggested that
• Anaesthesia is a continum
• No absolute unit of anesthesia depth to mark progress
along the continum

12. • So,
We need: Safe
: Non invasive
: Reliable
anesthesia depth indicator that should qualify
- Be readily interpreted
- Independent of anaesthesia technique
- Independent of surgical stimulus

13. Pharmacological principles of measuring
depth
1. Depth of anaesthesia is a pharmacodynamic measurement
2. Factors
a. Equilibrium of – drug conc in plasma
- drug conc at site of action
- measured drug effect
b. Relationship between drug concn & drug effect
c. Influence of noxious stimuli

14. Specific drugs and clinical situations
1. INHALATIONAL AGENTS
• Eger et all defined MAC (minimum alveolar concentration) of
inhaled anesthetics as the conc. required to prevent 50% of
subjects from responding (movement) to painful stimuli
• Types of MAC:
– MAC - intubation (movement and coughing)
– MAC - incision (initial Sx incision)
– MAC – BAR (prevent adrenergic response to skin incision
measured by venous concentration of catecholamines)
– MAC – awake (allows opening of eyes on verbal command
during emergence)
• MAC awake < MAC incision < MAC intubation < MAC BAR

15. • Nitrous oxide - 104
• Desflurane - 6.6
• Sevoflurane - 1.8
• Isoflurane - 1.17
• highest… Nitrous oxide - 104
• Lowest…. Methoxyflurane - 0.16
• MAC-BAR (1.7-2.0 MAC)MAC-awake (0.3-0.5 )

16. • In an unstimulated patient :
– [end tidal] = MAC awake, then absence of both implicit
and explicit memory
– As compared to somatic reflexes,hemodynamic responses
do not correlate well to end tidal drug conc.
– So, the relation between somatic and autonomic reponses
is poor during inhalational anaesthesia
• Skin incision :
– HR,RR,VT, pupil size…..increase for 12 min for halothane
anaesthesia
– BP may not change
• Factors affecting requirement of inhaled anaesthetics:
– Ageing Alcoholism Pregnancy
– NO Hypoxemia Hypo or hyperthermia
– Anaemia Concurrent drugs

17. 2. INTRAVENOUS AGENTS
A. HYPNOTICS
 Rapid rise & fall in peak plasma level d/t redistribution
 CNS depression lags behind plasma conc hysteresis on
curve
 Clinical end points :
a. Loss of verbal responsiveness
b. Loss of eyelash reflex
c. Loss of corneal reflex
d. Absence of movement in response to squeezing trapezius
 Only hypnotic doesn’t overcome the noxios stimuli of
laryngoscopy and intubation
So, to measure depth of anaesthesia other drugs have to be given
concurrently (opioids,NO,MR )

18. • MIR (Minimum Infusion Rate)
– This concept was introduced for TIVA by Sear et al
– ED50 & ED95 were calculated with respect to movement
response to skin incision, which were analogous to MAC
– This also gets affected by factors & pharmacokinetic
properties
B. OPIOIDS (NARCOTICS)
 Used to produce anaesthesia in patients with
• Severe valvular ds
• Cong heart ds
 But even high doses are not able to produce complete
anaesthesia in all pts
 Currently high dose opioid are supplemented with
amnestic (BZD) or low conc of potent inhalational
anaesthetic

19.  Cp50
a) That plasma conc. Of drugthat will prevent purposeful
movement to noxious stimuli in 50% population
b) For alfentanyl
Cp50 Intubation > Skin incision > Skin closure
c) End points for minimum infusion rate of opioids during
Sx:
» Increase in sys BP >15mm Hg of baseline
» HR > 90/min in absence of hypovolaemia
» Somatic responses
» Autonomic signs of inadequate anesthesia

20. Methods of monitoring depth of
anaesthesia
SUBJECTIVE
1. Autonomic response
a. Hemodynamic changes
b. Lacrimation
c. Sweating
d. Pupilary dilatation
2. Isolated forearm technique
OBJECTIVE
1. SEMG
2. LOC
3. HRV
4. EEG &derived indices
a. Spectral edge frequency
b. Median frequency
c. Bispectral index
5. Evoked Potentials
a. Auditory EP
b. Visual EP
c. Somatosensory EP
d. Auditory EP index

21. SUBJECTIVE- 1. Autonomic Response
• Can also be due to hypotension, hypoxia, hypo or
hyperthermia and suddden massive blood loss
• Greatly affected by built of the patient, baseline tone, cardiac
drugs
• PRST (patient response to surgical stimulus score)
– Poor indicator
– Hemodynamic response to noxious stimuli doesn’t
necessarily signify awareness, nor its lack guarantee
unconsciousness
– MR & opioids may blunt these responses & still do not
cause hypnosis

22. SUBJECTIVE – 2. Isolated Forearm Tech
• Purposeful movement to verbal command indicates lighter
planes
• Tech :
– Tourniquet put on pt’s arm & elevated above the sys
pressure before giving MR
– The arm is thus free to move during anaesthesia
– Tourniquet is periodically released to prevent ischaemia,
usually before giving top up
– Pt is then asked to move the fingers
• Limitations :
– Non specific startle response may be interpreted as consciousness
– Levels of anaesthesia to prevent movement using IFT are s;ightly
higher than regularly used
– Thiugh aware some are unable to move their arm

23. OBJECTIVE – 1. SEMG(Spontaneous surface
electromyogram)
• Can be recorded from various muscle group in pts not
completely paralysed
– Frontalis – facial N [FEMG] : falls during anaesthesia and
rises to preanaesthetic levels just before awakening
– Neck muscles
• No absolute scale
• FEMG + EEG gives better results
• ABM monitor system records both via same electrodes

24. OBJECTIVE – 2. LOC (Lower Esophageal Contractility)
Non straited muscles in lower half of oesophagus retain their potential
activity even after MR
First proposed by Evans and colleagues
SLOC (spontaneous LOC)
• Non propulsive
• Spontaneous
• Mediated via vagal nuclei &
RAS in brain stem
• Frequency of movement
increases as anaesthesia
depth decreases
• Absence 6 min before skin
incision correlates well with
no movement on incision
ELOC (evoked LOC)
• Small balloon is inflated in
the lower esophagus to
illicit
• Amplitude increase as
depth decreases

25. OBJECTIVE – 3. Heart Rate Variability
• Anaesthetic agent acts first on brain stem via ascending afferent
projections cerebral cortex
• Brainstem mediated autonomic tone is usually not affected by
factors other than anaesthesia
• HRV has 3 components (Kiode)
– Low frequency circadian fluctuations
– Medium frequency fluctuations d/t baro receptors relex
– High frequency fluctuations
• HRV : increase during inspiration resp sinus arrythmia
decreases during expiration [RSA]
– d/t parasym strech reflex in lungs & aorta vagal receptors in heart
– RSA is usually >10% variation in p wave interval over 5 min
– Level of RSA shows anaesthetic depth
– Disadv : Sx stimulus is more contributary than depth of anaesthesia

26. OBJECTIVE :4. EEG & derived indices
• EEG: a complex deflection of many waves (1-50mv)
• Level of brain activity can be interpreted by electronic
flattening of EEG & integrated amplitude of EEG waveforms.
a) CFM(cerebral function monitor)
• single trace
•Increasing cerebral activity broadening of trace
b) CFAM(cerebral frequency analysing monitor)
• Five frequency bands + 1trace demonstrating periods of
burst suppression.
Both influenced by diathermy
periods of poor electrode contact

27. c) CSA (compressed spectral array)
• EEG analog signal are obtained & processed over a period of
time & information is displayed in form of CSA(histograms) or
numerical data
• Advantage - more compact than raw EEG
• Disadvantage- complex
takes time to comprehend
changes within it are difficult to quantify
Numeric indices derived from power spectral analysis
a) Spectral edge frequency
- below this 95% of EEG power is contained
b) Median frequency (5 Hz for propofol)
- below this 50% of EEG power is distributed
c) BIS

28. BIS( bispectral index)
• Empirically derived complex parameter
• Combination of - time domain
- frequency domain
- higher order spectral sub
parameters
• Numeric index ranging from
100(awake) to 0 (isoelectric EEG)
• Correlates well with levels of responsiveness
• Periods an excellent prediction of level of consciousness
with propofol , midazolam & isoflurane

29. • BIS < 40 : response rate of 12%
• BIS > 60 : response rate of 25%
• Useful monitor to adjust anaesthetic dose with
decreased incidence of hemodynamic response
disturbances & improved recovery
• Helps in dose titration in elderly , paediatric &
cardiac sx.
• Disadvantage – senile dementia can be
confounding factor

30. • Several factors that interact with the BIS
index:
• Hypothermia : decrease in BIS index by 1.12
units/degree Celsius decline in temperature.
• Infusion of esmolol
• Epidural anesthesia decreases the amount of
hypnotic anesthetic needed for sedation.
• Ketamine doses that create unresponsiveness (0.25
to 0.5 mg/kg) does not change the BIS index.

32. OBJECTIVE : 5.Eps (evoked potential)
 Show response of more localised area of brain :
brainstem midbrain & cerebral cortex to specific stimuli
 EEG epocs are recorded time referenced to
repeatedly applied sensory stimuli
 So, it’s a time Vs voltage relationship
 This can be qualified by measuring post stimulus latency
& interpreting amplitudes

33. • Intra op 3 types of EPs used
a) SEP (somatosensory EP)
-tibial
-peroneal stimulation - somatosensory cortex
-median
b) VEP (visual EP)
Photo stimulation (flash) retina Lateral geniculate nucleus
primary visual cortex
c) AEP(auditory EP)
audible clicks Superior olivary nucleus inferior colliculus
medial geniculate nucleus primary auditory cortex

34. Auditory EP
1) Brain stem response - 10 miliseconds after stimulation
2) Early cortical response- 15 to 80 m sec
3) Lateral cortical response- 80 to 100 m sec
SEP INHALATIONAL NO
VEP LATENCY increases No effect
AEP AMPLITUDE decreases decreases

35. • Iv barbiturates…. increase brain stem EP latency
Etomidate & propofol….increase cortical EP and
decrease amplitude
• EPs are technically, clinically and practically difficult to
monitor
• Confounding artifacts:
– A) stimulus characteristics (intensity,duration,interval)
– B) electrode placement, technique
– C) age & gender
– D) anesthetic drug

36. • MLAER (Middle Layer Auditory Evoked Response
• 1998 : Thompson and Sharpe)
• Detects awareness by focusing on latency and amplitude
of Pa & Nb waves
Pa latency increase if < 12 ms awareness
Nb frequency = 47ms 100% sensitive & specific for
explicit memory of words (isoflurane)
Disadvantages : intermittent
: waveforms difficult to use clinically

37. Auditory evoked potential index
• Derived from AEP
• Result is a single numeric to monitor depth
• Diff b/w amplitudes of successive segments of the curve
• Index is obtained every 3 sec
• Index = 35 ; 100% specific & 53 % sensitive for
unconsiousness.
PET
• Limited study
• Invasive method
SQUIDS
• Noninvasive
• Measures functional activity of brain
• Expensive
• Depth, awareness,anoxia,ischaemia& unusual pathology

38. • M-Entropy module.
• It is proposed that the EEG can be adequately described with
methods from non-linear dynamics.
• Entropy is a mathematical concept to quantify non-linear
dynamics based on concept of spectral entropy which
originates from a measure of information called Shannon
entropy.
• When applied to the power spectrum of EEG, spectral entropy
is obtained and measures the regularity of the frequency
distribution

39. THANK YOU