Comprehensive Notes on Anesthetic Depth Monitoring and BIS

Overview: Anesthetic depth monitoring and BIS

• Definition of anesthesia: “a reversible state of drug-induced unconsciousness in which the patient neither perceives nor recalls noxious stimulation.”
  • Basic elements of anesthesia:
  • unconsciousness
  • amnesia (loss of memory of pain or distress)
  • analgesia
  • muscle relaxation
  • diminished motor response to noxious stimuli
  • reversibility
• Learning objective alignment: GA depth assessment, BIS alternatives, clinical trials, and translation to practice.

Basic elements and agent classes in anesthesia

• Hypnotic agents: produce drowsiness and unconsciousness.
  • Examples: volatile agents, Propofol, Ketamine.
• Sedatives/Anxiolytics: promote calm, reduce anxiety, facilitate relaxation.
• Amnestics: memory loss, either directly (e.g., Midazolam) or indirectly via unconsciousness.
• Analgesics: abolish sensation/pain and reflexes; include opioids (Fentanyl, Morphine, etc.) and local anesthetics.
• Muscle relaxants (NMBDs): produce immobility (e.g., Succinylcholine, Rocuronium, Vecuronium).

Inhaled anesthetics and MAC

• MAC = minimum alveolar concentration required to prevent 50% of subjects from gross purposeful movement in response to skin incision.
  • Concept: MAC is a unifying and additive concept for inhaled agents.
• Key MAC values and related concepts:
  • MACawake ≈ ext{MAC}{awake} = rac{1}{3} ext{ to } rac{1}{4} ext{ MAC}
  • MAC95 ≈ 1.2–1.3 MAC $ext{MAC}</em>{95} o 1.2 ext{ to } 1.3 imes ext{MAC}$
  • MACBAR (blockade of adrenergic response) > 1.5 MAC ext{MAC}{BAR} > 1.5 imes ext{MAC}
• Mediation of endpoints:
  • Hypnosis/unconsciousness: mediated in the cortex
  • Immobility: mediated in the spinal cord
• Inhaled anesthetics are highly synergistic with opioids, benzodiazepines, and nitrous oxide (N2O).
• There are no uniform clinical signs to assess depth; signs include tachycardia, hypertension, sweating, etc., but none are definitive.

Intravenous hypnotics and TIVA

• Propofol, Barbiturates (e.g., thiopental/STP), Etomidate, Ketamine are used for induction and hypnosis.
• Induction dynamics:
  • Often a large bolus followed by redistribution.
• TIVA (total intravenous anesthesia):
  • Usually propofol ± analgesics (e.g., remifentanil)
  • Pharmacologic analgesia may be minimal unless ketamine is used.
• Depth assessment challenges with TIVA (no end-tidal agent to cue depth): reliance on clinical signs and EEG-based monitors.
• Verbal responsiveness, eyelash reflex loss, and corneal reflex loss as indicators of depth during IV techniques.

Opioids and analgesics in anesthesia

• Opioids (Fentanyl, Morphine, Hydromorphone, Remifentanil) provide potent analgesia but are not true anesthetics; they are weak hypnotics.
• Opioids are highly synergistic with hypnotics:
  • Decrease MAC by up to ~60–70%
• Considerations:
  • Opioids alone at high doses can lead to inadequate hypnosis (awareness risk) if not properly supplemented with hypnotics.
  • Opioid-only anesthesia poses risk in patients with limited circulatory reserve (cardiac disease, trauma).

Goals of a satisfactory anesthetic

• Ensure patient safety: adequate perfusion of all organ systems; stable CV and respiratory status.
• Avoid under- or over-dosing of anesthetic agents.
• Ensure unresponsiveness to noxious stimuli and minimal movement.
• Prevent awareness or recall of events during the procedure.
• Facilitate optimal operating conditions for the surgeon.

How deep is my patient? Clinical question

• Core concern: are they adequately anesthetized, paralyzed, and narcotized for the procedure?
• Traditionally, depth of anesthesia is judged by clinical signs and guided by experience (the clinical “art of anesthesia”).

Depth of anesthesia: assessment methods

• Physical assessment of hypnosis/analgesia:
  • Heart rate (HR) and blood pressure (BP)
  • Movement in response to stimuli
  • MAC for inhaled agents (Et) — unavailable during TIVA
  • EEG and processed EEG (e.g., BIS)
  • EMG activity
  • Neurophysiologic monitoring: Brainstem Auditory Evoked Potentials (BAEP), lower esophageal sphincter (LES) contractility and tone, spontaneous and evoked potentials

Physical signs of anesthetic depth: what’s most reliable

• Most reliable indicators:
  • Gross purposeful movement in response to a stimulus
  • Reflexive movement to a stimulus
  • Immediate hemodynamic and respiratory responses to a stimulus
  • Response to soft stimulation (shaving, prepping, positioning)
• Reliable indicators:
  • Muscle tone (e.g., jaw tone)
  • Pupillary light reflex
  • Eyelash reflex: loss indicates moderate-to-deep anesthesia
  • Corneal reflex: persists until deep anesthesia
  • Lacrimation (tears) indicates light anesthesia
  • Position of the eyeball
• Less reliable indicators:
  • Heart rate
  • Respiratory rate
  • Blood pressure

Limitations of physical signs

• NMBDs suppress movement, masking some signs of light anesthesia.
• Drugs affecting autonomic responses can mask HR/BP changes (e.g., beta blockers, calcium channel blockers, anticholinergics, epinephrine).
• Scopolamine can cause mydriasis; narcotics cause miosis.
• Patient positioning and reflexes can influence signs.
• Overall: physical signs alone are not always reliable for depth assessment.

Awareness: memory and consciousness concepts

• Explicit (conscious) memory: conscious recollection of experiences.
• Implicit (unconscious) memory: changes in performance/behavior due to prior experience without conscious recollection.
• Intraoperative awareness: explicit postoperative recall of sensory perceptions during general anesthesia.

Intraoperative awareness: Michigan Awareness classification (NSU/Anesthesia & Analgesia)

• Class O: No awareness
• Class 1: Isolated auditory perceptions
• Class 2: Tactile perceptions (e.g., surgical manipulation or endotracheal tube)
• Class 3: Pain
• Class 4: Paralysis (feeling unable to move/speak/breathe)
• Class 5: Paralysis and pain
• Additional designation “D” for distress (fear, anxiety, sense of doom, etc.)
• Source: NSU Anesthesia & Analgesia, March 2010, vol. 110, issue 3, p. 813-815

Risk factors for awareness

• High demand/low supply of anesthetics; addiction, chronic pain, genetic factors (e.g., red hair), female gender.
• Low demand/very low supply scenarios: low perfusion states, EF < 40%, trauma, especially in younger patients, ASA 4-5, end-stage lung disease, poor exercise tolerance, pulmonary hypertension.
• When does awareness occur?
  • Any situation where depth of anesthesia must be weighed against hemodynamic stability (trauma, OB, cardiac surgery).
  • History of awareness; nitrous oxide/narcotic technique with little volatile agent; low provider experience; anticipated difficult intubation; equipment malfunction (vaporizers, circuits, end-tidal monitors, pumps, lines).
  • MAC < 0.7 age-adjusted is associated with higher awareness; NMBDs increase awareness risk if depth is insufficient.
  • Inadequately anesthetized, nonparalyzed patients usually move first.

Incidence and consequences of awareness

• Rare: approx. 0.1–0.2% in non-OB/non-cardiac surgery; estimated 20,000–40,000 cases/year in the USA.
• Obstetric: ~0.4%; Cardiac: ~1.1–1.5% (higher in major trauma: 11–43%).
• Consequences for patients: PTSD (explicit or implicit memory), anxiety, distress.
• Consequences for providers: psychological effects and litigation; average ~10 lawsuits/year in closed claims for anesthesia awareness.

Anesthetic depth monitors: EEG-based approaches

• All depth monitors are EEG-based or derived from EEG signals.
• EEG basics:
  • EEG is produced by neuronal activity (action potentials) and recorded as low-voltage current via scalp electrodes.
  • Measured in Hertz (Hz) or waves per second.
  • EEG can infer possibility of consciousness but does not directly measure consciousness.
• General pattern of EEG changes with increasing anesthesia depth: frequency slows from beta to alpha to theta to delta bands.
  • Beta (13–30 Hz) → Alpha (8–13 Hz) → Theta (3.5–8 Hz) → Delta (0–3.5 Hz)

BIS: Bispectral Index monitoring overview

• Introduced by Aspect Medical Systems in 1994; uses EEG analysis with a proprietary algorithm to estimate depth of anesthesia.
• BIS index scale: 0–100
  • 0: no EEG activity; 100: fully awake
  • BIS ≥ 80: likely to follow commands
  • BIS 70–79: Grey zone; ~50% of patients fail to follow commands
  • BIS < 70: memory function impaired; decreased explicit recall probability
  • BIS < 60: high probability of unconsciousness; better hypnotic depth
  • BIS 40–60: balanced general anesthesia; improved recovery; reduced intraoperative awareness
  • BIS < 40: deeper anesthesia associated with negative outcomes; possible cerebral ischemia if sustained
  • BIS < 20: cerebral ischemia risk
• Single-channel EEG data from 4 frontal sensors; differential amplification between two electrodes; ground and noise removal.
• Data are averaged over ~15–30 seconds to produce a single BIS value; there is an inherent lag in reflecting rapid changes.

How BIS works and what it tells us about the brain

• The BIS algorithm analyzes frontal lobe signals across frequencies (Bispectral analysis).
  • High-frequency beta activity (14–30 Hz) relates to light anesthesia.
  • Low-frequency delta activity (0.5–4 Hz) relates to deep anesthesia.
• BIS provides an index that correlates with reduced likelihood of explicit memory formation, but does not guarantee lack of consciousness.
• BIS value interpretation helps guide anesthetic dosing, but should not be the sole determinant of depth.

BIS parameters beyond the index

• BIS index: 0–100
• SQI (Signal Quality Index): 0–100%
• SR (Suppression Ratio): % of time EEG activity falls below a preset limit
• EMG (electromyography): 70–110 Hz; reflects muscle activity
• EMG can artifactually elevate BIS; NMBDs reduce EMG and may decrease BIS, though stable anesthesia with minimal EMG artifact minimizes this effect
• Various artifacts and interfering factors exist from devices and physiological states

Factors that influence BIS values

• EMG artifacts and NMBD effects:
  • Forehead EMG activity can increase BIS; NMBDs reduce EMG and may lower BIS
  • In stable anesthesia without EMG artifact, NMBDs have little effect on BIS
• External devices and equipment interactions:
  • Pacemakers, forced-air head warmers, electrocautery, ESU, endoscopic shavers can affect BIS readings
• Drugs and clinical conditions:
  • Ketamine and etomidate can transiently increase BIS (activation of EEG)
  • Opioids and benzodiazepines can decrease BIS due to synergistic effects
  • Cardiac arrest, hypovolemia/hypotension, cerebral ischemia/hypoperfusion, hypoglycemia, hypothermia, and certain neurological disorders can alter BIS readings
  • Post-ictal patterns, dementia, cerebral palsy, brain injury, brain death, and low-voltage EEG can modify BIS sensitivity
• Effects of NMBDs:
  • NMBDs do not directly suppress EEG activity but their use masks signs of movement, potentially affecting depth assessment by traditional signs
• Other considerations:
  • Ketamine may paradoxically raise BIS even when anesthesia depth is adequate
  • Electrical interference and artifact can lead to spurious BIS values

BIS clinical application: interpreting signals in real time

• Clinical states and BIS ranges:
  • “Light” anesthesia: BIS high; signs include hypertension, tachycardia, movement; consider increasing hypnotic/analgesic dosing
  • “Adequate” anesthesia: BIS in the 45–60 range; stable hemodynamics; minimal movement; continued monitoring
  • “Deep” anesthesia: BIS low; consider reducing hypnotic/analgesic dosing if hemodynamics are stable; assess for causes of deepening depth
• Practical strategies:
  • If BIS is high but patient is stable: assess surgical stimulation level and ensure adequate hypnotics/analgesics
  • If BIS is low with hypertension/movement: reevaluate dosing and possible other etiologies
  • Use BIS as part of a multimodal approach (not as a sole determinant)

Alternatives to BIS: other EEG-based/awareness monitors

• SNAP II (Stryker)
• S/5 Entropy Module (GE)
• Narcotrend (Monitor Technik)
• Cerebral State Monitor (Danmeter)
• SEDLine / Patient State Analyzer (Masimo)
• AEP Monitor (Danmeter)
• Masimo SedLine — Patient State Analyzer specifics:
  • 4 frontal EEG channels; bilateral frontal assessment
  • Patient State Index (PSi): 0 (no cortical activity) to 100 (awake); 25–50 optimal hypnotic state
  • Optional Root platform adds cerebral oximetry information
• Overall: different monitors have distinct algorithms and may be differentially affected by artifacts and surgical equipment

Masimo SedLine: Patient State Index (PSi)

• 4 frontal EEG channels; awake baseline establishment pre-induction
• PSi: 0–100; 25–50 indicates optimal hypnotic state for surgery
• Less interference from ESU and similar devices relative to BIS
• Masimo Root platform offers additional data (e.g., cerebral oximetry)

Clinical trials evaluating BIS monitoring

• Utility Trial (1997): propofol as primary hypnotic (no volatile agents)
  • BIS-guided anesthesia reduced propofol use by ~13–23%
  • Faster time to wake up and extubation by ~35–40%
  • ~16% faster eligibility for PACU discharge
  • BIS patients had better nursing assessments; no significant difference in intraoperative adverse events
• B-Aware Trial (2004): high-risk patients (airway, cardiac, trauma, C-section)
  • Awareness incidence: BIS-guided group 0.17% vs non-BIS 0.91%; p = 0.022
• Cochrane Review (2007): meta-analysis of 20 randomized trials (N ≈ 4056)
  • BIS-guided anesthesia reduced propofol use and MAC requirements
  • Faster recovery times to eye opening, command following, and PACU discharge
  • Reduced incidence of intraoperative recall in high-risk patients
• Avidan et al. / subsequent trials (2008–2011):
  • 2011 trial: recall occurred in 7 BIS-guided patients vs 2 recall in ETAG 0.7 MAC or better group (interpretation and design specifics vary by study)

Cost, practicality, and guidelines

• Cost considerations:
  • BIS monitor pads cost approximately $20–$40 each
  • Economic arguments claim potential savings per prevented awareness event could be in the range of $10,000–$25,000, considering reduced wake times and improved recovery
• Regulatory and professional guidance:
  • FDA clearance (1996): BIS may be associated with reduced incidence of awareness with recall; but reliance on BIS alone for intraoperative management is not recommended
  • ASA/angel policies emphasize multimodal monitoring and cautious integration of BIS with clinical signs
• Practical recommendation (ASA/Center guidance):
  • Identify high-risk patients preoperatively and counsel about intraoperative awareness
  • Check equipment and ensure delivery of correct anesthetic doses
  • Consider amnestic measures if awareness occurs
  • Use depth-of-anesthesia monitoring as one of multiple modalities (ECG, BP, end-tidal gas analysis) rather than a standalone determinant

Should you use BIS? practical considerations

• BIS can help reduce anesthetic usage, promote faster emergence, and reduce PONV, with potential safety benefits in certain populations
• It is not an infallible guarantee of unconsciousness or absence of recall; no monitor has 100% sensitivity or specificity for awareness
• NMBDs can mask light anesthesia; BIS should be used in conjunction with traditional vital signs and end-tidal gas monitoring
• Decision to use BIS should consider patient risk profile, institutional preferences, and clinician experience

Summary and practical takeaways

• Depth of anesthesia involves both hypnosis and analgesia, with reflexes and hemodynamic responses as auxiliary indicators.
• Inhaled agents’ depth relates to MAC and related concepts (MACawake, MAC95, MAC_BAR); hypnosis is cortical, immobility is spinal.
• IV hypnotics and TIVA present depth assessment challenges due to lack of end-tidal cues; EEG-based monitoring is particularly helpful here.
• BIS provides a quantified index (0–100) correlated with hypnotic depth and memory formation risk but is not a standalone predictor of consciousness.
• Use BIS as part of a multimodal monitoring strategy; be mindful of artifacts, NMBD effects, and drug interactions (e.g., ketamine can raise BIS).
• The literature suggests BIS can reduce anesthetic consumption and improve recovery in some settings, but findings are heterogeneous; it is not mandatory as an ASA standard monitor.
• Always tailor anesthesia depth monitoring to the patient and procedure, combining clinical judgment with multiple monitoring modalities and vigilance.

Key numerical references and formulas (LaTeX)

• MAC concepts:
  • ext{MAC} = ext{minimum alveolar concentration that prevents 50% of subjects from gross purposeful movement in response to skin incision}
  • ext{MAC}_{awake} o rac{1}{3} ext{ to } rac{1}{4} ext{ MAC}
  • $ext{MAC}_{95} o 1.2 ext{ to } 1.3 ext{ MAC}$
  • ext{MAC}_{BAR} > 1.5 ext{ MAC}
• BIS scale and targets:
  • $ext{BIS} <br /> ightarrow [0, 100]$
  • $ext{BIS}_{ ext{awake}} <br /> ightarrow ext{high value (close to 100)}$
  • ext{BIS}_{ ext{light}}
    ightarrow ext{high value (e.g., >70)}
  • $ext{BIS}_{ ext{adequate}} <br /> ightarrow 45 ext{ to }60$
  • ext{BIS}_{ ext{deep}}
    ightarrow <40
• EEG bands and transitions with increasing anesthesia:
  • $ext{Beta} = 13 ext{--}30 ext{ Hz} o ext{Alpha } 8 ext{--}13 ext{ Hz} o ext{Theta } 3.5 ext{--}8 ext{ Hz} o ext{Delta } 0 ext{--}3.5 ext{ Hz}$
• EMG frequency range: $70 ext{--}110 ext{ Hz}$
• Example BIS clinical ranges and associations:
  • BIS ≥ 80: likely command following
  • BIS 70–79: grey zone
  • BIS < 70: memory impairment
  • BIS 40–60: balanced general anesthesia
  • BIS < 40: deeper anesthesia with associated risks
• Example trial results:
  • Utility Trial: BIS-guided reduced propofol usage by about 13 ext{–}23\oldsymbol{\%}; faster wake/extubation by $35 ext{–}40\%$; PACU discharge faster by $16\%$
  • B-Aware: awareness from $0.17\%$ (BIS) vs $0.91\%$ (no BIS) in high-risk patients; p = 0.022
  • Cochrane Review: BIS-guided anesthesia reduced propofol and MAC; faster recovery; reduced recall in high-risk patients