Awareness during general anesthesia ; concepts and controversies

1
Awareness during general anesthesia concepts
and controversies

• Seminars in Anesthesia, Perioperative Medicine
  and Pain (2006) 25, 211-218
• Stuart A Formal M.D, Ph D
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2
Introduction

• In 1845, Horace Wells
• N2O anesthesia
• Pt moved and cried out
• No recall of his operation
• In 1846, W.T.G. Morton
• Ether anesthesia
• Surgeons considered it a success
• Pt. had been aware, no pain.
• From a pts perspective, Wells anesthetic may be
  considered more successful than Mortons.

3
Introduction

• Aims
• Explain how and why awareness during GA
• Summarizing research on the multiple actions of
  GA which are mediated by different parts of CNS.
• Review clinical research on awareness during GA
• Emphasis on recognizing high-risk pts and
  strategies for preventing this problem.

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Anesthetic actions and anatomic substrates

• GA affect a wide variety of neural functions,
  suppressing different neural circuits at
  different concentrations.
• Most important therapeutic actions of GA
• Suppression of memory, awareness, and movement.
• Supression of these three CNS functions occurs at
  different anesthetic concetrations.
• Each of these anesthetic-sensitive actions is
  controlled by different neural circuits in the
  CNS.

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Anesthetic actions and anatomic substrates

• Movement in response to pain
• Supressed by anesthetic effects that are almost
  entirely within spinal cord
• Inhibition of awareness and memory
• due to effects on different brain circuits
• Different GA act via distinct mechanisms at the
  molecular level.
• Many intravenous and volatile anesthetics affect
  the CNS
• Gamma-aminobutyric a. type A(GABAA) Receptor
• Dominant inhibitory NT-activated ion ch. in the
  brain.
• N2O and ketamine
• Inhibit Glutamate and acetylcholine R.

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Assessment of anesthetic depth how do we know
how much is enough?

• Surgical anesthesia
• Suppression of movement in response to surgical
  pain.
• Movement after an incision is suppressed in half
  of subjects at MAC
• MAC BAR(MAC- Blockade of Autonomic Reflexes)
• Deeper planes of GA
• Suppress autonomic responses to extremely painful
  stimuli
• MAC-awake
• Suppress perceptive awareness(Perceive
  non-painful stimuli, interpret them, and respond
  appropriately)
• About 1/3 of MAC for volatile agents
• about 2/3 of MAC for N2O

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Assessment of anesthetic depth how do we know
how much is enough?

• Ensuring adequate anesthesia
• Large patient-to patient variability
• Some factors
• Old ages require less anesthetic
• Core temperature influence sensitivity to
  anesthetics
• Chronic exposure to neuro-depressants
• Induce resistance to GA
• Acute exposure to neuro-modulators
• Can shift anesthetic requirements upward or
  downward.
• Anxiety and pain
• Increased surgical pain inc. MAC, MAC-awake
• Neuroaxial blockade
• Reducing MAC and dec. BZD and volatile anesthetic
  requirements
• Pharmacologic interventions

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Assessment of anesthetic depth how do we know
how much is enough?

• Introduction and widespread use of m. relaxants
• Dramatically changed clinical practice and
  created new unexpected problems
• M. relaxant
• Immobile pt. and optimal surgical conditions
• Enabling anesthetists to use lighter anesthesia
  and avoiding the cardiovascualr depression.
• NMB makes assessment of anesthetic depth much
  more difficult.

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Assessment of anesthetic depth how do we know
how much is enough?

• When muscle relaxants are used,
• Autonomic responses ( HR, BP, pupil size, and
  sweating) are observed. to assess pt responses
  to various stimuli.
• Modified by pt position, surgical events,
  neuroaxial blockade, and a variety of medications
  that do not affect consciousness.
• Monitors that use EEG signals to estimate depth
  of anesthesia
• Accurate assessment of explicit vs implicit
  memory
• Recall of validated word lists and performance on
  exclusion vs inclusion tests using post-exposure
  word lists.

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Types of awareness during general anesthesia
events

• Intraop. awareness
• Duration
• Experience of pain and/or anxiety
• Explicit recall is present.
• Awake paralysis
• Aware, experiencing pain and anxiety, and able to
  remember these experiences.
• Most case
• Explicit recall are brief, and usually no pain
• Awareness without explicit recall
• Vague memories
• Dreams or dream-like experiences
• Associated with intraoperative events

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Types of awareness during general anesthesia
events

• Awareness can also be classified as preventable
  or not.
• Most cases have been deemed preventable
• Drug administration errors
• Mis-labeled drug syringes
• Empty vaporizers
• Leaky gas delivery circuits
• Dysfunctional or misused drug infusion pumps
• Intravenous lines that stopping running
• Diffcult airway cases

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Incidence of awareness during general anesthesia

• Estimate the incidence of awareness
• Multiple post-anesthetic interviews, usually
  using a modified Brice interview
• Definite awareness
• Recall conversations or music that they hear in
  the OR during the period of awareness
• Probable awareness
• Hearing voices or feeling discomfort asso with
  intubation or surgery
• Possible cases
• More vague and dream-like
• Psychological sx. without explicit recall
• They have implicit recall of intraop. Events or
  that the trauma or the experience results in
  memory suppression.

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Incidence of awareness during general anesthesia

• One large study in Sweden
• definite awareness 0.06
• A 2004 study in US academic centers
• awareness with recall 0.13
• Uncertainties
• One possible explanation
• Awareness experiences reported days after surgery
  may represent pt memories formed in the PACU
  rather than OR.
• Pt may be reticent to report negative experiences
  such as awareness while still dependent on the
  care of hospital staff.
• Anesthetist may alter their anethetic technique
  subtly when they know that intraop. Awareness
  will be monitored.
• Pt may be more prone to give positive responses
  to questions about intraop. Awarenss, simply they
  are asked.

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Risk factors for awareness during general
anesthesia

• Anesthetic technique
• The type of surgery
• Use of neuromuscular blockade
• Light anesthesia techniques
• Cardiac anesthesia without hypnotic agents and
  N2O-narcotic-relaxant
• Up to 4 awareness
• Highest risk surgeries
• Prevent the hemodynamic consuquences of high
  conc. Of volatile or intravenous agents.
• Cardiac surgery 1-1.5 awareness
• Trauma surgery 11-43
• C/S under GA 0.4
• Ptrelated f.
• Chronic alcohol, antiepileptic, opiate, or other
  sedative drug use, history of awareness during
  GA., limited cardiovascular reserve, or ASA Class
  ?-?

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Psychological harm and awareness during general
anesthesia

• PTSD(post-traumatic stress disorder)
• Most harmful consequence
• Depression, anxiety attacks, sleep disorders,
  flashbacks to the experience, and nightmares.
• Pt who have no explicit recall of intraop events,
  but who develop symptoms suggestive of
  intraoperative awareness, such as recurrent
  dreams about being buried alive.
• A pts understanding of their experiences can
  affect the psychological impact of awareness.
• Pt may think their awareness is impossible
• Leading them to become confused or question their
  own sanity.

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Medicolegal consequences of awareness during
general anesthesia

• Legal action against their anesthesia providers
• Woman, complain of PTSD
• ASA closed claim database
• 1971 -2001 1 - 3 continue growing.
• Reported awards to pts for awareness with recall
• 1000 600, 000
• Several actions (Table 2)
• Reduce the psychological harm to the patient and
  probability of legal action.

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Strategies to prevent awareness during general
anesthesia
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To use electroencephalographic awareness monitors
or not

• Most visible controversy
• Use of EEG monitors for anesthetic depth
• A follow-up study to the Swedish awareness survey
  
• Routine use of BIS monitoring reduced
  intraoperative awareness by nearly 80
• Uncertainty about whether methodology in the two
  groups was identical make this result
  questionable.
• Randomization to BIS/No BIS was applied to high
  risk pts
• Incidence of explicit awareness
• 80 lower in pts with BIS guided anesthesia
• Incidence of probable and possible
  awareness was not reduced
• US multi-center study
• Higher rate of intraoperative awareness with
  recall among BIS monitored pts.
• Applied device incorrectly or they failed to
  appropriately use the information the monitor
  provides.

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To use electroencephalographic awareness monitors
or not

• A per pt. cost of 20 for BIS disposables and an
  assumed average malpractice payout of 50,000 if
  a malpractice payout is made for one in every
  2500 cases.
• High-risk patients
• The incidence of intraoperative awareness
• 0.5-2 or higher (ie, trauma)
• The patient should be informed of the risk of
  intraoperative awareness, and the use of
  awareness monitoring is justified.
• Currently available EEG monitoring is not a
  panacea for the problem of intraop. Awareness.
• Remember that BIS only reduces the incidence of
  awareness with recall, and not other more
  frequent types of pt awareness reports.
• In the end, decision regarding routine use of
  monitors depend on the cost of avoiding harm vs
  the value that the anesthesist and pt place on
  avoiding that harm.

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Anesthetic drugs, awareness, and
electroencephalographic monitoring

• Opioids
• Alone use
• Do not suppress awareness
• Large doses
• Unresponsive to pain
• Respond to loud noises and remain aware of their
  surroundings
• when added to N2O
• Do not alter the incidence of awareness
• Do not alter basal BIS measurements
• Opioids
• Reduce the amount of cortical arousal asso. with
  peripheral pain
• Reduce the possibility that surgical pain will
  cause pt to awaken.
• Psychological trauma asso. with awareness and
  pain is greater than that of awarenes without pain

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Anesthetic drugs, awareness, and
electroencephalographic monitoring

• Propofol, barbiturates, etomidate, and
  halogenated volatile agents
• Modulate GABAA R. activity
• Shift the cortical EEG to lower frequencies
• BIS and EEG based monitor
• Provide strong correlation with hypnosis for this
  group
• N2O and ketamine
• Do not modulate GABAA R., but they do produce
  hypnosis
• Unchanged or increased high frequency EEG signals
• High reported incidence of dreaming during
  anesthesia
• BIS and EEG monitors
• Do not accurately predict the depth of anesthesia
  
• New correlates of consciousness
• Lead to development of more universally
  applicable monitors for anesthetic depth.
• Potent analgesia- NMDA receptor inhibition in
  spinal cord.
• Suppress cortical arousal during painful
  stimulation reduce the prabability of awareness

22
Anesthetic drugs, awareness, and
electroencephalographic monitoring

• N2O-volatile mixtures
• MAC for N2O and voaltile agent
• Additive
• Eg, mixture of 0.5 MAC N2O 0.5 MAC volatile
  agent
• Supress movement in response to pain like 1 MAC
  volatile
• Hypnotic activities of N2O and volatile agent
• Sub-additive
• Eg, mixture of 0.5MAC awake N2O 0.5 MAC awake
  volatile agent
• Is not as hypnotic as 1 MAC awake volatile
• N2O
• Antagonizes the hypnosis induced by volatile
  agent, perhaps via direct cortical arousal.

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Summary and recommendations

• ASA Taskforce
• It is up to you to decide what is best for your
  patient.
• Anesthesist should educate themselves thoroughly
  about intraop. Awareness and pre-anesthetic
  evaluation(pts risk for this problem)
• Strategies to reduce the chance of awareness
  should be applied whenever possible.
• Postop. Patient should be asked questions
  designed to elicit reports of awareness
  experience
• Intraop awareness is suspected,
• Responsible anesthetist, their departmental
  administrators, and quality assurance team
  members
• Activate a series of interventions
• Defining the nature of the events and its causes,
  while minimizing its impact by providing
  supportive care to patients
• High risk for intraop awareness should be
  informed about their status
• Anesthetic plans should explicitly incorporate
  approaches to reduce this risk, including the use
  of EEG-based monitoring.