Awareness during Anesthesia

ByAbdelrahman Mahmoud Soliman

Explicit memory—Conscious recollection of previous experiences (“awareness” is evidence of explicit memory).

Implicit memory—Changes in performance or behavior that are produced by previous experiences but without any conscious recollection of those experiences (“unconscious memory formation” during general anesthesia)

Definitions

Awareness—Postoperative recall of events occurring during general anesthesia.

Dreaming—Any experience (excluding awareness) that patients are able to recall postoperatively that they think occurred during general anesthesia and that they believe is dreaming.

Cont.

Incidence of awareness ranges from 0.01- 2% in different studies according to type of surgery and anesthetics.

1 in 100 in cardiac surgery 1 in 20 in trauma surgery 1 in 250 in emergency C-Section under GA Awareness is doubled with use of muscle

relaxant.

Incidence of awareness

PTSD. Anxiety. Fear of future surgery. Legal responsibilities.

Why we do care?

Awareness results from an imbalance between anesthetic requirement and anesthetic delivery

1. Normal Requirement—Low Delivery Errors in knowledge. Anesthetic machine failure.

Risk factors

2. Low Requirement—Very Low Delivery

Trauma patients. C- section under GA. Hypovolemic patients.

Cont.

3. High Requirement—Normal Delivery

Normal variability in the population. Patients tolerant to sedative, hypnotic and

analgesic drugs. Alcohol use Hyperthyroidism and hyperthermia.

1. Anesthesia training and continuing education

Physiology and pharmacology Equipment and clinical measurement.

Prevention of awareness

2- Preoperative Phase

Identify patients at risk Conduct preoperative checklist-based

equipment check. Inform, consent, reassure as appropriate.

Cont.

3- Intraoperative phase scan equipment regularly during each

case. Take care to avoid wrong drug

administration Administer adequate hypnotic drug Minimize use of muscle relaxant Respond rapidly to suspected inadequate

anesthesia Consider using an EEG-based monitor

Cont.

4- postoperative phase

Conduct a postoperative interview. Provide counseling for aware patients.

Cont.

Use of benzodiazipines can decrease the postoperative recall in case of unanticipated awareness.

Respond rapidly to signs of inadequate anesthesia by deeping level of anesthesia.

Benzodiazipines although have no retrograde amnesic properties can decrease postoperative recall.

1. Clinical signs

Monitoring depth of anesthesia

2- End tidal agent monitoring:

The minimum alveolar concentration (MAC): the minimum concentration at ambient pressure to prevent movement in 50% of non premedicated subjects to standard painful stimulus (skin incision).

Increase MAC:Pyrexia, hyperthyroidism, obesity, young age,

tobacco, chronic alcohol, chronic sedative use.

Decrease MACPregnancy , hypotension, old age,

hypothyroidism, hypothermia, opioids,

Factors altering MAC

1. EEG:Can be used as a measure of the depth of

anesthesia for several reasons: Represent cortical activity which affected

by anesthetics drugs, CMR and CBF, both are affected by anesthetics drugs and surgical stimulation.

It fails to measure clinical depth of anesthesia.

EEG based monitors

Based on Fourier spectral analysis and bispectral analysis

Monitor provide number on a scale 0-100 85-100: awake, light sedation. 85-60: deep sedation, impairment of

memory processing, arousable on stimulation.

40-60: surgical anesthesia, decrease probability of postoperative recall

Bispectral index

0-40: burst suppression > cortical electrical silence.

It is recommended to be maintained 40- 55 BIS decrease the incidence but don’t

eliminate awareness risk

Cont.

BIS limitation

Interindividual variability in depth of anesthesia for a given BIS value.

Minimally affected by opioids, therefore may not reflect balanced anesthetic regimen.

Changes in consciousness from ketmine and nitrous oxide are not faithfully represented.

Based analysis of irregularities in EEG signals which decreased as level of anesthesia increase, incorporating Fourier analysis.

EMG from facial muscles.Two readings are displaced 1- State enotropy ( SE): 0-91 based on EEG2- Rsponse enotropy ( RE): 0-100 based on

EMG

Enotropy

RE is hypothesized to represent analgesic component.

Studies showed that the device produce results comparable to those of BIS.

Anesthetic range from 40-60.

Other monitors

Evoked potentials Patient state index narcoted

Thank you