Anesthesia Awareness
PACU presentation
5/14/08
-S. Zaghi MD
Definition
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Definition of Anesthesia:
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Anesthesia is a state in which the patient feels no
pain. This may range from blocking the sensation of
one small part of the body to total unconsciousness.
Definition of Anesthesia Awareness:
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"unintended intra-operative awareness" occurs
during general anesthesia, when a patient has not
had enough general anesthetic or analgesic to
prevent consciousness.
Definition
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Components of Anesthesia
Unconsciousness
Amnesia
Anesthesia
+/- Muscle
Relaxation
Analgesia
Definition
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How is Anesthesia “measured”
 Classically
done by measuring concentrations of
inhaled anesthetic agents like: sevoflorane,
desflorane.
Definition
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Basic Concept of MAC
 Defn:
MAC = Miniumum Alveolar Concentration of
inhaled agent where at 1MAC, 50% of targets do
not move to surgical stimulus.
 MAC for Iso: 1.1
 MAC for Sevo: 2.2
 MAC for Des: 6.6
Definition
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Advanced understanding of MAC
 MAC
values are additive
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 MAC
Addition of N2O oxide
Use of narcotics, benzodiazepines, etc..
values are patient specific
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Age
ETOH acute
Hypothermia
Pregnancy
Drugs
Hyponatremia
1
1.3
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MAC Aware
MAC – concentration at which 50% of patients
lose consciousness
 .3
 Bell
Curve – 50% is obviously not enough!
Awareness
.3
.7
Importance

Patient Experience
Importance
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Clip of AWAKE
Importance
No reason to use monitors that are not
effective…
 Cost:
$$ 360 million in BIS probe cost/year

Incidence
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.1 -.2% of all patients undergoing general
anesthesia.
21 million patients have GETA,
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Calculated 20-40K/yr experience
Even be higher in children
Incidence calculated for patients who received
general anesthesia.
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regional anesthesia (ie epidural) does not count for awareness
Risk Factors
Risk Factors
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.
Routine use of paralytics (double incidence from .1% to .18%)
TIVA
Light anesthesia for sake of turnover
Hemodynamic instability
Procedures: Obstetric / Cardiac / Trauma
Patient Age
H.o of difficult airway
Limited cardiac reserve
H.o of substance abuse (chronic ETOH, anxiolytics, cocaine)
Experiences
What is experienced:
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Audio (48%)
Not being able to breath (48%)
Pain (30%) from the ET tube to severe pain
from incision
Etiologies
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Class 1: pt specific altered increase in
expression of anesthetic
receptors
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Class 2: patient can’t tolerate anesthetic
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Class 3: pt. hemodynamics are masked:
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Class 4: anesthetic delivery failure
Detection
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Hemodynamic
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Typical indicators of physiologic and motor
response, such as high blood pressure, heart
rate, or movement, lacrimation,
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Movement masked by the use of paralytic agents
to achieve necessary muscle relaxation.
Detection
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BIS monitor
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Processed EEG from a single frontal electrode,
into a numerical unit less value.
Values range from 0 – 100, and represent
absence of brain activity to awake state
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Usually aim for numbers 40- 60 for surgical anesthesia
and aim in decreasing awareness. BIS <40 represent a
deep hypnotic state.
Outcomes
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PTSD (30%)
“worst experience of my life” vs
uncomfortable
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Unable to ascertain why some people the experience
leads to PTSD others less so.
May remember these events in the PACU,
on the floor or even once they are
discharged.
Treatment
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Reassurance
Honest discussion with the patient about the risk
factors of awareness
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Why awareness occurred, and likelihood of reoccurrence.
Pt should inform future anesthesiologist that has had
awareness
Some patients should be referred for psychological
evaluation and treatment/counseling
ASADatabase of awareness
AwarenessDB.org
(http://depts.washington.edu/awaredb/)
Prevention
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Premedication with versed (amenstic) if
anticipate light anesthesia
Ensure patient is asleep prior to intubation
(appropriate doses)
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Ensure frequent machine checks and fxn
Watch discussions in the OR
When giving beta blockers or antiHTN –
worry about masking awareness
Avoid paralysis unless needed.
B-UnAware
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study design
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2000 “high risk” patients, randomized to ETAG vs BIS
40-60; both ETAG and BIS values computer recorded
at 1sec intervals for later comparison.
Avidan MS, et al. Anesthesia awareness and the
Bispectral Index. The New England Journal of
Medicine 358, (11), 1097-1108: 2008.
B-UnAware
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“High Risk”
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Major Criteria:
 long term use of Narcotics/ETOH/cocaine/
 EF <40%
 h.o Anesthesia awareness, h.o difficult intubation
 ASA 4 or 5
 Aortic Stenosis/ open heart surgery / ESLung
Disease /
Minor Criteria
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perioperative use of BBlockers
COPD, BMI >30
tobacco 2packs/day
B-UnAware
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Questionnaires where reviewed by blinded reviewers
for specificity for intraoperative events. If two of three
where in agreement but another was not, then a fourth
expert was brought into to evaluate.
 Then based on events, expert asked to identify
when in normal intraoperative course the
awareness could have occurred( to hence identify
BIS or ETAG concentration]
B-Unaware
Results
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90% (1754)of enrolled patients completed entire
protocol.
4 patients had definite awareness, 2 in the BIS
group and 2 in the ETAG group. Overall incidence
of .2%
5 patients had possible awareness – often times
recalled in only one interview point, often the 3rd
most distal interview date. 4 BIS and 1ETAG
overall incidence of .6%,
Of the 1754 patients who did not have awareness
55% (964) of them had BIS values that where
sustained over 60
75% (1315) of them had ETAG that where over .7
B-UnAware Experiences
B-UnAware Experience
Definite
Definite
Possible
Possible
Notice how the Turquoise line (BIS) consistently in
range, but awareness still occurred