Intra-Operative
Brain Function Monitors
Thomas Jan
Goals of General Anesthesia
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Anxiolysis
Analgesia
Hypnosis
Muscle Relaxation
Suppression of
Somatic/Autonomic
Responses
Intra-Operative Awareness
• Patient is not given enough general anesthesia and can
recall events during surgery
• Incidence is between 2/1000 to 10/1000 per year
– Incidence is greater in OB, Trauma, and Cardiac cases
• IOA heavily depends on interpatient pharmokinetic and
pharmodynamic variability
Intra-Operative Awareness Causes
• Equipment failure
• Inadequate anesthesia
• Patient co-morbidities
Intra-Operative Awareness
Consequences
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Anxiety, anger, depression, irritability, anguish
Post-op traumatic neurosis
Recurrent nightmares
Insomnia
Difficulty concentrating
Exaggerated startle response
Unnatural preoccupation with death
35% of patients had informed their anesthesia
care provider about their awareness and recall.
Monitoring Anesthetic Depth
• Indirect
– Clinical signs
– Respiratory Sinus Arrhythmia
• Direct
– Isolated Forearm Technique
– Auditory Evoked Potentials
– Facial EMG
– Electroencephalogram, Brain Function Monitors
Electroencephalogram
• Processed EEG signal
– BIS Monitor, State Entropy, Narcotrend, Patient
State Analyzer, SNAP, Danameter
• Goal: Targets brain activity and measures
depth of anesthesia
BIS Monitors decrease awareness
• Multi-center study in 2004;
2500 patients; Australia
• BIS decreases awareness in
high risk patients
– 2 cases of awareness in BIS
group
– 11 cases of awareness in control
group
• NNT 138 patients
• Cost of prevention $2200 per
one case of awareness
Can the BIS value go too low?
• Prospective Observational Study;
1064 patients; Florida teaching
hospital
– BIS < 45 was one of the predictors of
mortality after non-cardiac surgery
(RR 1.244)
– Mortality 5.5% all patients; 10.5% in
> 65 years of age
• Predictors:
– Patient co-morbidities
– Cumulative Deep Hypnotic Time
– Intraoperative SBP<80 mm
Brain Function Monitors
ADVANTAGES
• Monitor of sedation and hypnosis
• Prediction of brain activity
• Less anesthetic used
• Easier wakeups
DISADVANTAGES
• Does not measure movement
• Does not predict hemodynamic response
• Physiologic interference
• Interference from other equipment
Conclusion
• Studies have shown that brain function monitors can
decrease intra-operative awareness
• These devices are best used in high risk situations:
Trauma, Obstetrics, Cardiac Surgery, previous history of
awareness and explicit recall
• Could assist in delivery of multi-modal anesthetics in
complex cases
• Remaining problems:
– Opioid adjuncts have no effect on BIS, but lower hypnotic agent
requirements
– False sense of security
• Understand all the monitors you use in the OR!!!
References
• S Muravchick, "Defining and measuring the anesthetic state," in The
Anesthetic Plan: From Physiologic Principles to Clinical Strategies (St
Louis: Mosby-Year Book, 1991) 31.
• W H Liu et al, "Incidence of awareness with recall during general
anaesthesia," Anaesthesia 46 (June 1991) 435-437
• N Moerman, B Bonke, J Oosting, "Awareness and recall during
general anesthesia. Facts and feelings," Anesthesiology 79
(September 1993) 454-464.
• Different Conditions that could result in the bispectral index
indicating an incorrect hypnotic state, Dashabe, Ashraf A, Anesth
Analg 101(3);Sept 2005:765-73
• Anesthetic management and one-year mortality after non-cardiac
surgery, Terri G. Monk et al, Anesth Analg 2005;100:4-10
• The Incidence of Awareness During Anesthesia: A multicenter
United States Study, P.S. Sebel et al, Anesth Analg 2004;99:833-9