awareness during anesthesia
TRANSCRIPT
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
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.
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.