Anesthesia Awareness PACU presentation 5/14/08 -S. Zaghi MD
Definition • Definition of Anesthesia: • Anesthesia is a state in which the patient feels no pain. This may range from blocking the sensation of one small part of the body to total unconsciousness. • Definition of Anesthesia Awareness: • "unintended intra-operative awareness" occurs during general anesthesia, when a patient has not had enough general anesthetic or analgesic to prevent consciousness.
Definition • Components of Anesthesia Unconsciousness Amnesia Anesthesia +/- Muscle Relaxation Analgesia
Definition • How is Anesthesia “measured” • Classically done by measuring concentrations of inhaled anesthetic agents like: sevoflorane, desflorane.
Definition • Basic Concept of MAC • Defn: MAC = Miniumum Alveolar Concentration of inhaled agent where at 1MAC, 50% of targets do not move to surgical stimulus. • MAC for Iso: 1.1 • MAC for Sevo: 2.2 • MAC for Des: 6.6
Definition • Advanced understanding of MAC • MAC values are additive • Addition of N2O oxide • Use of narcotics, benzodiazepines, etc.. • MAC values are patient specific • Age • ETOH acute • Hypothermia • Pregnancy • Drugs • Hyponatremia 1 1.3
MAC Aware • .3 MAC – concentration at which 50% of patients lose consciousness • Bell Curve – 50% is obviously not enough! Awareness .3 .7
Importance • Patient Experience
Importance • Clip of AWAKE
Importance • Increase in patient advocacy
Importance • No reason to use monitors that are not effective… • Cost $$ 360 million in BIS probe cost/yr
Incidence • .1 -.2% of all patients undergoing general anesthesia. • 21 million patients have GETA, • Calculated 20-40K/yr experience • Even be higher in children • Incidence calculated for patients who received general anesthesia. • regional anesthesia (ie epidural) does not count for awareness
Risk Factors • Risk Factors • Routine use of paralytics (double incidence from .1% to .18%) • TIVA • Light anesthesia for sake of turnover • Hemodynamic instability • Procedures: Obstetric / Cardiac / Trauma • Patient Age • H.o of difficult airway • Limited cardiac reserve • H.o of substance abuse (chronic ETOH, anxiolytics, cocaine) .
Experiences • What is experienced: • Audio (48%) • Not being able to breath (48%) • Pain (30%) from the ET tube to severe pain from incision
Etiologies • Class 1: pt specific altered increase in expression of anesthetic receptors • Class 2: patient can’t tolerate anesthetic • Class 3: pt. hemodynamics are masked: • Class 4: anesthetic delivery failure
Detection • Hemodynamic • Typical indicators of physiologic and motor response, such as high blood pressure, heart rate, or movement, lacrimation, • Movement masked by the use of paralytic agents to achieve necessary muscle relaxation.
Detection • BIS monitor • Processed EEG from a single frontal electrode, into a numerical unit less value. • Values range from 0 – 100, and represent absence of brain activity to awake state • Usually aim for numbers 40- 60 for surgical anesthesia and aim in decreasing awareness. BIS <40 represent a deep hypnotic state.
Outcomes • PTSD (30%) • “worst experience of my life” vs uncomfortable • Unable to ascertain why some people the experience leads to PTSD others less so. • May remember these events in the PACU, on the floor or even once they are discharged.
Treatment • Reassurance • Honest discussion with the patient about the risk factors of awareness • Why awareness occurred, and likelihood of reoccurrence. • Pt should inform future anesthesiologist that has had awareness • Some patients should be referred for psychological evaluation and treatment/counseling • ASADatabase of awareness AwarenessDB.org (http://depts.washington.edu/awaredb/)
Prevention • Premedication with versed (amenstic) if anticipate light anesthesia • Ensure patient is asleep prior to intubation (appropriate doses) • Ensure frequent machine checks and fxn • Watch discussions in the OR • When giving beta blockers or antiHTN – worry about masking awareness • Avoid paralysis unless needed.
B-UnAware • study design • 2000 “high risk” patients, randomized to ETAG vs BIS 40-60; both ETAG and BIS values computer recorded at 1sec intervals for later comparison. Avidan MS, et al. Anesthesia awareness and the Bispectral Index. The New England Journal of Medicine 358, (11), 1097-1108: 2008.
B-UnAware • “High Risk” • Major Criteria: • long term use of Narcotics/ETOH/cocaine/ • EF <40% • h.o Anesthesia awareness, h.o difficult intubation • ASA 4 or 5 • Aortic Stenosis/ open heart surgery / ESLung Disease / • Minor Criteria • perioperative use of BBlockers • COPD, BMI >30 • tobacco 2packs/day
B-UnAware • Questionnaires where reviewed by blinded reviewers for specificity for intraoperative events. If two of three where in agreement but another was not, then a fourth expert was brought into to evaluate. • Then based on events, expert asked to identify when in normal intraoperative course the awareness could have occurred( to hence identify BIS or ETAG concentration]
B-Unaware Results • 90% (1754)of enrolled patients completed entire protocol. • 4 patients had definite awareness, 2 in the BIS group and 2 in the ETAG group. Overall incidence of .2% • 5 patients had possible awareness – often times recalled in only one interview point, often the 3rd most distal interview date. 4 BIS and 1ETAG overall incidence of .6%, • Of the 1754 patients who did not have awareness • 55% (964) of them had BIS values that where sustained over 60 • 75% (1315) of them had ETAG that where over .7
B-UnAware Experience Definite Definite Possible Possible Notice how the Turquoise line (BIS) consistently in range, but awareness still occurred