Intra-Operative Brain Function Monitors

Thomas Jan

Goals of General Anesthesia

• 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

• 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 MonitorsADVANTAGES• 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