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Depth of Anaesthesia -- Concepts

Presentation for Postgraduates in Anaesthesia

Ravi Shankar

on 1 November 2012

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Transcript of Depth of Anaesthesia -- Concepts

Depth of Anaesthesia -- Concepts This presentation comes to you from

Prof. M. Ravishankar
Dept. of Anaesthesiology & Critical Care
Mahatma Gandhi Medical College & Research Institute
Pondicherry, India. Define anaesthesia Guedel 1937 Triad
of Anaesthesia Controversy Anaesthesia
Reversible loss of consciousness,
neither perceives
nor recalls noxious stimuli No variable depth

All or None phenomenon Muscle relaxation Neither a requirement
nor a componant Stage 1 – Analgesia
Stage 2 – Delirium
Stage 3 – Surgical Anaesthesia
Stage 4 – Medullary depression Introduction of
Intravenous agents True only for
Ether anaesthesia Unconsciousness Analgesia Relaxation Rees & Gray 1950 Balanced Anaesthesia
Liverpool school
1970's Narcosis
Reflex Suppression Extensive use of Muscle Relaxants Define depth of anesthesia.

Define target drug % to reach these goals. Objective of this lecture is to Introduction of Muscle Relaxants Prys Roberts 1987 Then Show a relationship between a measurement (level of consciousness) and concentration of anaesthetic agent Show a relationship between a measurement ("adequacy" of anaesthesia) and concentration of the agent Diversity of studies to define endpoint MAC MAC
BAR Staged the level of anaesthesia Workable Definition of Anaesthesia demonstrate a relationship between a measurement (movement) and the dose or concentration of the anaesthetic agent Editorial: Depth of anaesthesia. Gilles Plourde MSC MD
CAN J ANAESTH 1991 / 38:3 / pp270-4 Different clinical end-points are mediated at different sites
receptor. MAC for isoflurane in goats?
Preferencially perfused brain 2.9 ± 0.7%
brain + spinal cord 1.2 ± 0.3% Akinesia Inhibition of movement after noxious stimulation
Mediated at Spinal Amnesia / Hypnosis Loss of response to verbal command
Controlled at Supraspinal level MAC MAC
Awake A Analgesia (Sensory)
A Akinesia (Motor)
A Autonomic reflex control
A Amnesia/hypnosis (Central) Anaesthesia defined
(gentlemen, this is no humbug). Eger EI 2nd, Sonner JM.
UCSF, San Francisco Best Pract Res Clin Anaesthesiol 2006, 20: 23-9 Abstract

All inhaled anaesthetics act on the central nervous system to produce two reversible conditions, immobility and amnesia, that define the anaesthetic state. No other reversible, clinically useful, conditions are essential to the definition. One other (analgesia)is unmeasureable and no other condition define anaesthesia; only immobility and amnesia supply such a definition.

50 Years of Research
> 500 research publications analysing site of action “Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” What is Pain First formulated by an IASP Subcommittee on Taxonomy. Bonica JJ, 1979. IF Pain = conscious perception
Anaesthesia = Unconsciousness Then

There could be no pain during
where person neither perceives nor recalls
noxious stimuli Conscious perception and reaction is not there during good General Anaesthesia
Autonomic Nervous System is independent of consciousness
It responds and needs to be suppressed Regional Block / Opioids can take care of Analgesia as well as autonomic reflex control Autonomic Reflex Control Analgesia What defines anesthesia: Amnesia – MAC awake
Akinesia: immobility - MAC Finally Inhaled Anaesthetics Both Amnesia and Akinesia provided by 1 molecule
% easily measured in exhaled gas
% directly relates to clinically relevant end-points:
Amnesia ~MACawake
Akinesia ~ MAC Poor correlation between the measured and the predicted propofol plasma concentrations during TCI (r = 0.14, P = 0.36) Scatter ++ between propofol % and BIS value “pre-eliminate” postop pain
decrease MAC = less agent needed
facilitate autonomic reflex control (MACBAR) Opioids Since inhaled anesthetics ensure both aspects of anesthesia, should we then not use opioids? In the non-paralyzed patient who does not move and is receiving a potent inhaled anesthetic, the risk of awareness is zero. Domino KB. Anesthesiology. 1999; 90:1053-61 ASA closed claims analysis 1 MAC sevoflurane prevents explicit awareness during surgical skin incision and tracheal intubation HR/BP (HTN/TK) do not correlate with hypnosis
If I give 2.0 % sevoflurane, hypnosis is ensured in the face of noxious stimulation. Molestad et al. Acta Anaesthesiol Scand 1998;42:1184-87 But then, do we need that much?
I can give opioids to reduce MAC,
or I can use muscle relaxants to ensure immobility! How much inhaled % is needed to ensure hypnosis?
EDhypnosis50 = MACawake = 0.35 MAC
EDhypnosis99.99 = ? 50 100 0.75% 1.5% Incidence awareness “vanishingly small” if MAC > 0.5 Eger EI II, Sonner JM. Anesth Analg 2005;100:1544 ETAG concentrations were less than 0.7 MAC in three cases of definite anesthesia awareness. Michael S. Avidan et al.
N Engl J Med 2008;358:1097-108. My sevoflurane targets Instead of giving that dose that might ensure hypnosis in most patients I prefer to use a dose that does ensure hypnosis in all patients. N2O MACawake for N2O is 0.52 – 0.59 MAC (63 – 66% N2O FA!) MACawake reduction is not linear: antagonistic Katoh T et al. Does N2O antagonize sevoflurane-induced hypnosis? BJA 1997;79:465-468 O2/air ( + opioid; dose if no NMB)
FA >1.1 - 1.4 % My Tagets O2/ N2O ( + opioid)
FA = 0.7 % 1.3MAC or ED95 for immobility (NMB only if really needed) My isoflurane targets O2/air ( + opioid; dose if no NMB)
FA >1.5 - 2 % O2/ N2O ( + opioid)
FA = 1.3 % 1.3MAC or ED95 for immobility (NMB only if really needed) My Sevoflurane targets THank YOu
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