Anesthesia Awareness

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Anesthesia Awareness

• Rita Katznelson, MD
• Department of Anesthesia
• Toronto General Hospital

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Intraoperative Awareness

• 0.1-0.2 of all adult patients undergoing GA
• 0.8-1.2 children
• 20 000 to 40 000 of the 20 million GA patients
• 33 of those patients develop serious
• psychological sequelae

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Media Coverage
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Media Coverage
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October 2004 The Joint Commission on
Accreditation of Healthcare Organizations
(JCAHO) Sentinel Event Alert April 2006 the
American Society of AnesthesiologistsPractice
Advisory for Intraoperative Awareness and Brain
Function Monitoring
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Objectives

• Definition of intraoperative awareness
• Definition of general anesthesia
• Monitoring of anesthesia depth
• Risk factors for intraoperative awareness
• Strategy to prevent awareness during the general
• anesthesia

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Intraoperative Awareness
Occurs when a patient becomes conscious during a
procedure performed under general anesthesia and
subsequently has recall of those
events. Dreaming is not considered
intraoperative awareness.
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Perceptions of Awareness

• Most common
• Sounds and conversation
• Sensation of paralysis
• Anxiety and panic
• Helplessness and powerlessness
• Pain
• Least common
• Visual perceptions
• Intubation or tube
• Feelings operation without pain

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Psychological Sequelae

• Sleep disturbances
• Nightmares
• Anxiety and panic attacks
• Flashbacks
• Avoidance of medical care
• Post-traumatic stress disorder

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Consciousness

• A state in which a patient is able to process
  information from surroundings.
• Assessed by purposeful responses
• Following voice commands
• Response to noxious stimuli

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Recall
The patients ability to retrieve stored
memories. Assessed by a patients report of
previous events. (events that occurred during
general anesthesia) Explicit memory is the
patients ability to recall specific events that
took place during general anesthesia.
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General Anesthesia
A drug-induced loss of consciousness during which
patients are not arousable, even by painful
stimulation.
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General Anesthesia
Main components Hypnosis ( unconsciousness)
Analgesia (decreasing pain) Amnesia (preventing
recall) Muscle relaxation (preventing movement)
Physiologic support (maintaining respiratory
and cardiovascular function, fluid management,
electrolyte control, and thermoregulation )
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Case Report
52 y.o. patient, previously healthy, no
medications Laparotomy complicated with severe
bleeding Hemodynamically unstable. Arm
withdrawal during attempt to insert arterial
line (intraoperative consciousness ???) No
recall of the intraoperative events
postoperatively
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Depth of anesthesia
memory
awareness
???
movement
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Depth of Anesthesia

• A continuum of progressive central nervous
• system depression and decreased responsiveness
• to stimulation
• Balancing act
• Anesthetic drugs
• Surgical stimulation

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Monitoring of anesthetic depth

• Anesthetic stages
• Minimum alveolar concentration (MAC)
• Brain Function Monitors

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Anesthetic Stages
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Anesthetic Stages
Stage I Analgesia Stage II Excitement
Stage III Surgical Anesthesia Stage IV
Overdose
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Minimum Alveolar Concentration (MAC)
Alveolar concentration of inhalation agent
requires to prevent movement in response to a
noxious stimulus in 50 of the population (Eger
et al. 1960s)
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Minimum Alveolar Concentration (MAC)
MAC-BAR ( MAC- Blockade of Autonomic Reflexes)
reflects deeper planes of general anesthesia
with suppression of autonomic responses to
extremely painful stimuli
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Minimum Alveolar Concentration (MAC)
MAC-Awake is the concentration suppressing appropr
iate response to command in 50 of patients 1/3
of MAC for volatile agents Memory is usually
lost at MAC-Awake
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MAC-BAR
Anesthetic concentration
No autonomic response
MAC
Immobility
MAC-Awake
Amnesia, analgesia
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Minimum Alveolar Concentration (MAC)
Agent MAC Desflurane
6.0 Enflurane
1.7 Halothane 0.77 Isoflurane
1.15 Sevoflurane
1.71
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Minimum Alveolar Concentration
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Minimum Alveolar Concentration (MAC)

• Main limitation is a large patient-to-patient
  variability
• Age
• Temperature
• Chronic and acute exposure to neuro-depressants
  / neuro-modulators
• Genetic factors

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Brain Function Monitors
Devices that record or process brain electrical
activity and convert these signals
mathematically into a continuous measure
typically scaled from 0 to 100.

• EEG
• Auditory evoked potentials (AEPs)
• Electromyographic (EMG) activity

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Brain Function Monitors

• Spontaneous electrical activity (EEG/EMG)
• Bispectral Index (BIS)
• Entropy
• Narcotrend
• Cerebral State Index
• Patient state analyzer
• SNAP index
• Evoked electrical activity (AEP monitoring)
• AEP Monitor/2

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Bispectral Index (BIS)

• Proprietary algorithm converts a single channel
  of
• frontal EEG into an index of hypnotic level
• BIS values scaled from 0-100
• Specific range of 40-60 low probability of
• consciousness under GA

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Bispectral Index (BIS)
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Entropy

• Describes the irregularity, complexity or
  unpredictable characteristics of a signal
• Single sine wave represents a completely
  predictable signal (entropy 0)
• Noise represents entropy 1
• State entropy cortical state (hypnosis)
• Response entropy EMG activity from inadequate
  analgesia

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Entropy
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EEG monitoring limitations

• Insensitive to nitrous oxide, ketamine, xenon
• Sensitive to Beta- blockers, muscle relaxants
• Hypothermia, hypoglycemia can affect the reading
• 5-10 of normal population has congenitally
• low-voltage EEG
• Subject to artifact from other electrical
  equipment in the OR.

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Signs of Awareness

• No obvious signs
• Patient movement
• Tachycardia, Hypertension
• Reflex activity
• Coughing/straining
• Papillary dilation
• Sweating/tears

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High Risk Patient Characteristics

• Substance use or abuse
• Limited hemodynamic reserve
• ASA IV V
• Previous episode of intraoperative awareness
• Chronic pain patients
• Younger age
• Tobacco smoking

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High Risk Surgeries

• Caesarian section (0.4)
• Major trauma/Emergency (11-43)
• Cardiac surgery (1.1-1.5)

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High Risk Anesthetic Techniques
Reduced anesthetic doses in presence of
paralysis Total intravenous anesthesia
Nitrous oxide-opioid anesthesia Rapid sequence
induction
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High Risk Situations

• Equipment problems
• Drug errors
• Other problems
• Laryngospasm/airway obstruction
• Difficult/Prolonged intubation
• Delayed extubation

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Emergency Management

• Stop painful stimuli
• Verbally reassure patient
• Rapidly deepen anesthesia
• Consider amnestic drugs like benzodiazepines
• Plan follow-up

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Further Care

• Interview
• Reassure
• Explain
• Be honest and sympathetic
• Arrange for follow-up

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The Modified Brice Interview

• What is the last thing you remember before
  surgery?
• What is the first thing you remember after
  surgery?
• Do you remember anything happening during
  surgery?
• Did you have any dreams during surgery?
• What was the worst thing about your surgery?

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ASA Practice Advisory

• Identify high-risk patients preoperatively and
  inform them about possibility of intraoperative
  awareness when circumstances permit
• Check equipment to assure that the desired
  anesthetic drugs and doses will be delivered

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ASA Practice Advisory

• The decision to administer benzodiazepine
  prophylactically should be made on a case-by-case
  basis for selected patients
• Intraoperative monitoring of depth of anesthesia
  should rely on multiple modalities, including
  clinical techniques and conventional monitoring
  systems( ECG, BP, end-tidal gas analyzer)

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ASA Practice Advisory

• The decision to use a brain function monitor
  should be made on a case-by-case basis.
• Cautions!!!
• Maintaining low brain function monitor
  values in an attempt to prevent intraoperative
  awareness may conflict with other anesthesia
  goals (preservation vital functions)

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Summary

• Intraoperative awareness is a recognized risk
• No ideal system/monitor to measure adequate depth
  of anesthesia
• More research is needed

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Thank you