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How do you evaluate greatness:
based on success or technique?
.
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MAC
CP50
How do you evaluate the depth:
based on brain or spinal effects?

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Hey MAC, do you really belong here?
• GA produces its effect by acting on the brain
• MAC doesn’t evaluate the effect on brain
• MACBAR --> the concentration which suppress
autonomic responses
• MACawake --> the concentration needed to
suppress a voluntary response to verbal
command; but no noxious stimulus
• Aren’t we in need of a potency measure that
rely only on brain effects…?
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Makeup change a lot externally; don’t
be deceived
• Processed EEGs are not physiological ‘live’ measures of
cortical activity
• They are probability measures of anaesthetic depth based on
a proprietary database
• Different agents attain the same endpoint like LOC at
different BIS --> so you can’t compare
• Arbitrary nature & intervariability dilutes your mission
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Sticking with the original….
• Raw EEG has its own problems as a measure of DOA
• EEG directly assess the functional changes in the brain
following GA
• But one common endpoint: EEG Burst Suppression (BS)
• BS MAC (volatile agent) / BS CP50 (iv agent)
• To determine the anaesthetic concentration that can induce
EEG burst suppression in 50% of patients
• A true measure of the brain-effect of GA
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Fresh ideas may pave the path for
innovation..
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Burst Suppression as an end point:
Conventional parameters
• For lighter planes, can use clinical endpoint like LOC
• For deeper planes, choices are less
• An endpoint with suppression duration for >1 sec like BSR or
burst pattern or suppression duration are agent-specific and
influenced by pharmacokinetic differences in distribution and
metabolism
• So such an index will not be focusing only on the potency of
the drug
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Burst Suppression as an end point:
The ‘Silent second’
• ‘Silent second’ = burst suppression lasting at least 1 s.
• Well defined endpoint for testing of anaesthetic potency in
animal models
• In the intact animal, it depends only on the potency of the
drug and the dose administration rate of the infusion.
• The onset of burst suppression is achieved without agent
specific EEG characteristics.
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The younger parameter:
MAC BS AND CP50 BS Pilge et al.
• MACaw and MACBAR are too presented as a fraction of MAC
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AGENT MAC BS CP50 BS
SEVOFLURANE MAC x 1.4
ISOFLURANE MAC x 1.3
PROPOFOL CP50 x 1/3

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The veteran with the juniors
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AGENT MAC MACAW MACBAR MAC BS
SEVOFLURANE 2.0 0.6 MAC 3.5 MAC MAC x 1.4
ISOFLURANE 1.15 0.5 MAC 1.3 MAC MAC x 1.3
DESFLURANE 6.0 2.5 MAC 1.3 MAC ____

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The hurdles in the journey
• Ketamine
• Age
• Baseline delta EEG
• Correlation with conventional DOA monitors:
– MAC--> Too deep!
– Processed EEG-->
• The problems with too deep inhalational
anaesthesia
– Neuroanaesthesia
– Hemodynamics, POD
– Other drug dosing, premedication
– N2O
• For propofol, it may be a good parameter
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New wine in old pot !!!!
• BS MAC (volatile agent) / BS CP50 (iv agent): S. Pilge et al BJA
2014
• Power Spectral Density, Amplitude & slope of the burst in
first 2 sec Fleischmann et al. Frontiers in Human
Neuroscience 2018
• EEG Approximate entropy: Bruhn et al Anesthesiology 2000
• The spindle waves: Hajihira et al, BJA 2015
• Burst suppression: Hoffman et al, Anesth Analg, 1995
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A new window of research

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MAC: THE MR RELIABLE
3/26/2023
DESIRABLE ATTRIBUTES OF MAC
QUANTITATIVE METHOD
CAN BE APPLIED TO ALL AGENTS
SHOULD BE REPRODUCIBLE
EASE OF MEASUREMENT
SHOULD NOT INCREASE WITH STIMULUS INTENSITY

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