BIS, EEG, and Entropy Monitoring "Depth" of Anesthesia – Flashcards
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All anesthesia depth monitors are derived from
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EEG
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How do you define sedation?
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behavior, NOT vital signs (but tough because paralyzed)
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Wakefulness is characterized physiologically by
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EEG recording and electromyogram (EMG) recording
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Awake looks like what on EEG and EMG
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fast frequency, low-amplitude rhythm. Desynchronized - maximal motor activity for EMG
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Characteristics of REM sleep
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- EEG desynchronized - low amplitude - high frequency - looks same as awake only little to no EMG
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Characteristics of non-REM sleep (general anesthetics)
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- large amplitude - low frequencies - low motor tone
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Electromyogram mesures
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muscle activity
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Electrooculogram measures
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eye movements
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One of the main concerns for anesthesia
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how deep is my patient
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*Goals of a satisfactory anesthetic*
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- adequate perfusion of all organ systems - unresponsive to noxious stimuli - adequate cardiovascular and resp stability - ideally no or minimal patient movement - NO AWARENESS OR RECALL OF EVENTS DURING PROCEDURE - facilitate surgery
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Main goal of a satisfactory anesthetic
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no awareness or recall of events during the procedure
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Components of anesthesia
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- analgesia - amnesia - hypnosis - immobility - blunting of autonomic reflexes (maybe)
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Agent that relieves pain
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analgesic
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agent that results in memory loss
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amnestic
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agent that produces drowsiness and acts to induce sleep or sleep-like state
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hypnotic
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agent that facilitates a calm state
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sedative
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relaxes skeletal muscles
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muscle relaxant
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relieves apprehension, anxiety
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anxiolytic
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*What are some methods of monitoring the state of consciousness*
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- autonomic and motor signs - MAC level - EEG and processed EEG (BIS) - BAEP Brainstem auditory evoked potentials - lower esophageal contractility
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*How do you monitor the state of consciousness through autonomic and motor signs?*
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Heart rate (heart rate variability) and BP
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How do you monitor the state of consciousness through BAEP brainstem auditory evoked potentials?
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amplitude and latency
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How do you monitor the state of consciousness through lower esophageal contractility?
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esophageal pressure, spontaneous and evoked
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*Autonomic indicators of anesthetic depth*
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- HR - BP - pulse amplitude - sweating - tearing - mydriasis (pupil dilation) ALL VERY UNRELIABLE
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Lower 3rd of the esophagus is made of what and under what control
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Lower 3rd of esophagus is entirely smooth muscle and immune to NM blocking drugs, but under control of the ANS (parasympathetic division)
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How do you decrease number of contractions and diminish amplitude of contractions of the lower 3rd of the esophagus
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deepening levels of inhalation agents
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How do you measure lower esophageal contractility
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- Instrumentation- Water filled balloon placed in lower esophagus and connected to pressure transducer - Additional air filled balloon may be used to stimulate smooth muscle contractions - Monitor records lower esophageal pressure and determines max force of contraction and frequency of spontaneous contractions.
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Problems with measuring lower esophageal contractility?
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- does NOT help determine deepness/awakeness - NMB may not inhibit peristalsis of lower esophagus
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What can we give that inhibits the peristalsis of the lower esophagus?
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- opioids - anticholinergics (parasympathetic drugs)
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Risks of anesthesia awareness
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- Preop long-term use of anticonvulsant agents, opiates, benzodiazepines, cocaine - Cardiac ejection fraction less than 40% - History of anesthesia awareness* - Hx of dif intubation or anticip dif intubation - ASA physical status class 4 or class 5 - Aortic stenosis - End-stage lung disease - Marginal exercise tolerance not resulting from musculoskeletal dysfunction - Pulmonary hypertension - Planned open-heart surgery - Daily alcohol consumption
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Biggest risk of anesthesia awareness
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history of anesthesia awareness
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Risk of awareness simplified
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- ANY situation where depth of anesthesia may have to be weighed against the hemodynamic instability of the patient (trauma, open heart) - Patient history of awareness - Patients with potentially high tolerances to medications
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*When to use BIS*
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- ANY situation where depth of anesthesia may have to be weighed against the hemodynamic instability of the patient (trauma, open heart) - Patient history of awareness - Patients with potentially high tolerances to medications
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Explicit memory
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remember = "awareness" "There was country music on in the OR"
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*Implicit memory*
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changes in performance or behaviors that are produced by previous experience but WITHOUT CONSCIOUS RECOLLECTION of those experiences "I remember my central line because my neck hurts"
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Anesthesia awareness definition
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unintended intraoperative awareness defined as *EXPLICIT* recall of sensory perceptions during general anesthesia
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Incidence of anesthesia awareness
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0.1-0.2% but approaches 1% in high risk patients 20,000 - 40,000 cases yearly in USA
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High risk patients are defined as
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having at least one major criterion or two
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Overall awareness incidence
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0.1-0.19% (0.15%) About 3 patients in every 2000
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*Incidence of awareness in non obstetric and non cardiac surgery*
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0.2%
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Incidence of awareness in obstetric surgery?
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0.4%
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Incidence of awareness in cardiac surgery
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1.1-1.5%
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*Incidence of awareness in major trauma*
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11-43%
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*BIS Bi-Spectral index method of action*
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- single channels EEG data from 4 electrode sensor placed over frontal cortex - differential amplifier measures potential difference between electrodes 2 and 3 - electrode 1 = ground - electrode 4 = removes noise - Analyzes the signals from the brain at 14 Hz and 30 Hz (Bispectral) and smoothed with a running average
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BIS 100 = BIS 0 =
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100 = aware 0 = no activity at all
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50% of subjects fail to respond to verbal commands with BIS of
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67-79
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Prevention of implicit memory at BIS of
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84-91
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No cases of frank awareness with BIS
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< 50
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What BIS is good enough for suturing
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70 - chance of awareness is less at 60 but you can't really say who is deeper
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BIS deep sedation
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70
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BIS general anesthesia
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60
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*BIS deep hypnotic state*
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40
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BIS value of __ has high sensitivity for identifying drug-induced consciousness
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60
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BIS values less than ___ signify increasing amount of EEG suppression
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30
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BIS ranges from ___ to ___
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0 to 100 0 = unconsciousness and 100 is fully aware
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*What is the SQI, what does it do*
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Signal quality index - ranges from 0 to 100% - percentage of good epochs in the last 60 seconds that could be used to calculation of BIS - big avg over time (bovie really messes with this)
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What Hz for EMG
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electrical power of 70-110 Hz (given in dB with trend and bar graph)
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BIS uses what techniques
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BIS SQI EMG
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What limits BIS
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- ECG/pacemaker - Hypothermia and cerebral ischemia - Nitrous and ketamine anesthesia - neurologic disease - high electrode impedance
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ECG/pacemaker effects on BIS
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60 Hz, ECG/pacemaker causes EMG signals to increase BIS number (false high)
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Hypothermia and cerebral ischemia effects on BIS
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appear identical to deep sedation and decrease the BIS number (false low)
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*Nitrous and ketamine effects on BIS*
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0.25 - 0.5 mg induced unconsciousness, BIS didn't change 0.75% nitrous responsiveness to voice commands is lost but BIS does not change
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Neurological disease effect on BIS
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No data - genetically determined low voltage EEG awake at BIS of 40
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High electrode impedance effect on BIS
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may elevate (false high)
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Highest risk for awareness
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REWARMING (cold gives false low)
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BIS clinical studies
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Utility trial B-aware trial Cochrane Review Aidan Study
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Utility trial info
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- 160 pts for abd surgery (half sevo, half des then half of each used BIS) - 35-40% faster time to wake up with propofol - no change with sevo - CAN USE 30% LESS PROPOFOL AND 40% LESS SEVO!!! - faster PACU discharge - BIS pts higher % of pts with better ICU assessments and had better nursing assessments
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B-aware trial info
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- studied high risk patients (airway, cardiac, trauma, C-section surgeries) - 0.17% awareness with BIS vs 0.91% without - 82% reduction in incidence of awareness with recall - 11 cases aware without BIS, 2 with
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(B-aware trial) cost to prevent 1 case of intraoperative awareness
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$2200
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Cochrane Review Info
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- BIS guided anesthesia reduced propofol by 1.3 mg/kg/hr and MAC by 0.17 - recovery times to eye open, response to command, and PACU discharge all reduced - NOT reduce time for discharge to home - Reduced intra-op recall with high risk pts
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Avidan Study info (big one)
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- compared BIS value to maintain minimum 0.7 MAC - both can be equally effective in preventing awareness - Second trial: 7 pts aware/recall with BIS vs 2 pts in ETAG 0.7 MAC or better - MAC better than BIS
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Main points of the BIS test trial studies
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BIS is not a measure of anesthetic depth!!!!!
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What is BIS used as
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awareness monitor!
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Using a BIS to guide your anesthetic can result in
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Less agent Less PONV (because less agent) Less recovery time
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ASA practice advisory summary for awareness
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- identify high risk pts in preop and inform them about possibility of intraop awareness - check equipment to assortment desired anesthetic drugs and doses will be delivered - administer benzo after possible awareness
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Expense PER prevented event of intraoperative awareness using BIS (3 incidences per 2000)
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10k-25k
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What times are the best to have a BIS on in an average ambulatory case?
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induction and emergence
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ASA practice advisory summary for monitoring depth of anesthesia
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should rely on multiple modalities, including clinical techniques and conventional monitoring systems (ECG, BP, end-tidal gas analyzer) - BIS IS NOT A STANDARD OF CARE
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*Alternatives to BIS*
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- PSA (patient state analyzer - front to back EEG) - Entropy - Narcotrend (sleep stages)
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What does PSA stand for and what is it
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Patient state analyzer and front to back EEG
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PSA technique
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- Placed on pt prior to induction for baseline - Headpiece and special electrodes - Indication of level of consciousness -- Anterior to posterior EEG power shift -- 0-100 absence of brain activity to full awake --- Decrease drug use --- Controlled emergence - Not effected by Electrosurgical Equipment - Equally "effective" as BIS with similar advantages
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Entropy technique
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describes the irregularity, complexity or unpredictable characteristics of a signal (the amount of disorder) increased entry = increased variance = increased sleep
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Downside of entropy
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too much variability from person to person
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Hz range for entropy
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Used 0.8 - 32 Hz range (mostly hypnotic changes) compared with 0.8 - 47 Hz (hypnotic + facial muscle) to achieve a value
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What study showed us (through BIS) that we use more volatile gas than we need to
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Avidan study
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*Less gas =*
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less vasodilation = more stability intraop = faster emergence and recovery
