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MD Anaesthesiology Exam Questions on Neuromuscular Blocking Drugs

Discussed from Miller's Anesthesia, 10e

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SECTION: NEUROMUSCULAR BLOCKING AGENTS

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Q13 — Outline the clinical presentation & treatment of overdosing with neuromuscular blocking drugs [NOVEMBER 2008]

Clinical Presentation of NMB Overdose

• Prolonged neuromuscular block — skeletal muscle paralysis beyond the intended surgical period
• Respiratory paralysis / apnea — the most dangerous consequence, requiring mechanical ventilation
• Upper airway compromise — loss of pharyngeal and laryngeal muscle tone
• Residual paralysis in PACU — approximately 30–50% of patients show TOF ratios <0.90 at extubation when quantitative monitoring is not used
• Symptoms of muscle weakness — patients may complain of diplopia, ptosis, difficulty swallowing, or inability to sustain head lift
• Hypoxic events — impaired ventilatory response to hypoxia due to residual block of respiratory muscles
• Cardiovascular effects — some agents (especially benzylisoquinoliniums) release histamine; succinylcholine causes bradycardia and dysrhythmias at high doses

Treatment

1. Maintain ventilation — positive-pressure ventilation with O₂ until neuromuscular function recovers
2. Quantitative monitoring — confirm TOF ratio using acceleromyography (AMG) or electromyography (EMG); safe extubation requires TOF ≥0.90 (or ≥1.0 by AMG)
3. Pharmacologic reversal:
   • Anticholinesterases (neostigmine 30–50 mcg/kg) — inhibit acetylcholinesterase, increasing ACh at the NMJ; effective only for shallow-to-moderate block; have a ceiling effect
   • Sugammadex (2–4 mg/kg for moderate block; 16 mg/kg for immediate/profound reversal of rocuronium or vecuronium) — encapsulates steroidal NMBDs directly; no ceiling effect; reverses even deep/profound block
4. Supportive care — adequate analgesia/sedation if muscle paralysis is prolonged, DVT prophylaxis, prevent decubitus ulcers

— Miller's Anesthesia, 10e, pp. 3221–3222 & p. 3417

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Q14 — Outline the side effects and clinical considerations of depolarizing muscle relaxants [JUNE 2008]

Mechanism

Side Effects

Clinical Considerations

• Black Box Warning: Routine use in children is CONTRAINDICATED due to risk of cardiac arrest with hyperkalemia in undiagnosed Duchenne muscular dystrophy
• Pretreatment with small nondepolarizing NMBD (defasciculating dose) reduces fasciculations but antagonizes subsequent succinylcholine — so the dose of succinylcholine should be increased after defasciculation

— Miller's Anesthesia, 10e, pp. 3218, 3226, 3232–3247

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Q15 — Discuss the factors that modify reversal of neuromuscular blocking agents [NOVEMBER 2007]

Q24 — Describe the biotransformation of intravenous muscle relaxants [NOVEMBER 2007]

Factors That Modify Reversal

• Type of NMBD: Intermediate-acting agents (rocuronium, vecuronium, cisatracurium) are easier to reverse than long-acting ones (pancuronium)
• Dose used: Larger doses = deeper block = harder to reverse; neostigmine has a ceiling effect at 30–50 mcg/kg

Desflurane > Sevoflurane > Isoflurane > Halothane > Nitrous oxide/TIVA

• Respiratory acidosis and metabolic alkalosis increase residual block risk (postoperative factor)
• Hypothermia slows drug metabolism and delays recovery

• Aminoglycosides (gentamicin, neomycin) and polymyxins potentiate NMBDs by inhibiting presynaptic ACh release and reducing postjunctional sensitivity
• Clindamycin and tetracyclines also augment blockade

• Furosemide: Enhances neuromuscular block by inhibiting cAMP production and reducing ACh output
• Dantrolene: Depresses mechanical response to stimulation (via Ca²⁺ release block), potentiating nondepolarizing block
• Steroids: Antagonize nondepolarizing NMBDs (facilitate ACh release); however, prolonged combined use in ICU causes critical illness myopathy
• Azathioprine: Minor antagonism of NMBD-induced block

• Qualitative monitoring (tactile/visual TOF) → higher risk of residual block; clinicians cannot detect fade when TOF ratio >0.30–0.40
• Quantitative monitoring (AMG, EMG, MMG) → accurate and mandatory for safe extubation

• Moderate block (TOF count 1–4) responds well to neostigmine
• Deep/profound block (post-tetanic count only) requires sugammadex

— Miller's Anesthesia, 10e, pp. 3168–3177 & pp. 2698–2740

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Q16 — Discuss the factors that modify neuromuscular blocking action of muscle relaxants [SEP/OCT 2004]

Q17 — Describe how you will differentiate different types of neuromuscular blockade. Discuss the factors which modify neuromuscular blocking action of muscle relaxants [SEP/OCT 2003]

Types of Neuromuscular Blockade

Factors Modifying NMB Action

• Renal failure: Prolongs block of renally cleared drugs (pancuronium, dTc, gallamine); atracurium/cisatracurium are safe
• Liver disease: Increased volume of distribution → delayed onset, apparent resistance after single dose; repeated doses prolong block for hepatically eliminated drugs
• Age: Neonates have immature NMJ; elderly have reduced plasma clearance
• Hypothermia: Slows Hofmann elimination and enzymatic hydrolysis

• Inhaled anesthetics: Potentiate nondepolarizing block (desflurane > sevoflurane > isoflurane)
• Antibiotics: Aminoglycosides, polymyxins, clindamycin potentiate block
• Electrolytes: Hypokalemia and hypocalcemia augment nondepolarizing block; hypermagnesemia potentiates block by reducing ACh release
• pH: Acidosis potentiates; alkalosis reduces nondepolarizing block
• Myasthenia gravis: Extreme sensitivity to nondepolarizing agents; resistance to succinylcholine

— Miller's Anesthesia, 10e, pp. 3168–3177 & pp. 2066–2090

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Q18 — Train of Four Stimuli [MARCH 2003]

Definition

Interpretation

Key Principle: Fade

• Nondepolarizing NMBDs block presynaptic α3β2 nAChRs → impaired ACh mobilization → progressive reduction in twitch height T1→T4 = FADE
• Depolarizing block (succinylcholine Phase I) → no fade; all four twitches equally reduced

Clinical Limitation

Comparison with Other Stimulation Patterns

— Miller's Anesthesia, 10e, pp. 3348–3349

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Q19 — Recurarisation [MARCH 2003]

Mechanism

• Residual drug redistribution: The NMJ has a lower blood supply than plasma; after reversal, when plasma ACh levels fall (as neostigmine effect wanes), residual NMBD at the NMJ re-exerts its effect
• Duration mismatch: Reversal agents (neostigmine half-life ~77 min) may wear off before the NMBD has been eliminated
• Insufficient reversal dose: Particularly when reversal was attempted during deep block
• Concurrent factors: Hypothermia, respiratory acidosis, antibiotics in PACU can worsen residual block

Risk Factors (Miller's Table)

• Use of long-acting NMBDs (pancuronium > intermediate agents)
• Large doses of NMBD
• Qualitative (subjective) monitoring only
• Deep neuromuscular block at time of reversal
• Inhalational anesthesia
• Hypothermia
• Postoperative opioid/antibiotic use

Clinical Features

• Hypoxemia, desaturation
• Airway obstruction
• Inability to sustain head lift >5 seconds (TOF ratio ~0.60 — an unreliable clinical sign)
• Difficulty swallowing → aspiration risk

Prevention

• Use quantitative monitoring (confirm TOF ratio ≥0.90 before extubation)
• Use sugammadex preferentially for steroidal NMBDs
• Avoid premature extubation

— Miller's Anesthesia, 10e, pp. 3165, 3358–3360

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Q20 — Newer Neuromuscular Blocking Drugs [MAY 2000]

— Discuss the advantages of newer muscle relaxants in anaesthesia [OCT/NOV 2011]

Classification of Newer NMBDs

• Rocuronium — intermediate-acting; rapid onset (1–2 min with 0.6–1.2 mg/kg); good RSI alternative; reversed by sugammadex
• Vecuronium — intermediate-acting; minimal cardiovascular effects; primarily hepatic elimination
• Cisatracurium — the pure 1R-cis isomer of atracurium; intermediate-acting; Hofmann elimination only (no ester hydrolysis); minimal histamine release; 4–5× more potent than atracurium → 5× less laudanosine produced

• Mivacurium — short-acting; hydrolyzed by butyrylcholinesterase (clearance 50–100 mL/kg/min); three stereoisomers; clinical duration 15–20 min; avoid in atypical cholinesterase
• Atracurium — intermediate; undergoes both Hofmann elimination and ester hydrolysis; 10 optical isomers; organ-independent metabolism

Advantages of Newer Muscle Relaxants

— Miller's Anesthesia, 10e, pp. 3246–3295

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Q21 — Hofmann Elimination [JUNE 1999]

Definition

Drugs

• Atracurium: ~40% via Hofmann elimination + ester hydrolysis
• Cisatracurium: 77% via Hofmann elimination (remainder via organ-dependent routes; renal accounts for 16%)

Process

1. Laudanosine (tertiary amine) — crosses the blood-brain barrier; CNS-stimulating in high concentrations; however, plasma concentrations from clinical doses are negligible and adverse effects are unlikely
2. Monoquaternary acrylate — no neuromuscular or significant cardiovascular activity

Clinical Significance

• No organ dependency: Safe in renal failure, hepatic failure, or multi-organ dysfunction
• Cisatracurium produces 5× less laudanosine than atracurium (due to greater potency — smaller doses used)
• Rate of elimination: At pH 3.0/4°C (storage conditions), atracurium is stable; at physiological pH/37°C it degrades spontaneously
• Hypothermia slows Hofmann elimination → prolonged block in hypothermic patients

— Miller's Anesthesia, 10e, pp. 3290–3291

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Q22 — Indications for neuromuscular blocking agents in the intensive care unit [OCTOBER 2014 & MAY/JUNE 2006]

Indications for NMBDs in ICU

• Improvement of patient-ventilator synchrony in severe ARDS
• Prevention of breath stacking and barotrauma
• Reduction of peak airway pressures

• Reduces work of breathing
• Decreases total body O₂ consumption in septic/critically ill patients

• Prevent coughing/straining during suctioning in raised ICP patients

• Endotracheal intubation
• Bronchoscopy, tracheostomy placement

• Post-cardiac surgery, therapeutic hypothermia

• Stops motor activity (but does NOT stop cerebral seizure activity — EEG monitoring essential)

• To control severe muscle spasms

Choice of Agent in ICU

• Cisatracurium is preferred in ICU due to organ-independent Hofmann elimination — no accumulation in renal/hepatic failure
• Atracurium also acceptable but produces more laudanosine
• Avoid vecuronium in prolonged use due to accumulation of active metabolite 3-desacetylvecuronium (particularly in renal failure)
• Avoid pancuronium — long-acting, renally eliminated

Complications of Prolonged NMB in ICU (Miller's Box 24.2)

• Inadequate ventilation if ventilator disconnects
• Inadequate analgesia/sedation masked by paralysis

• Deep vein thrombosis, pulmonary embolism
• Peripheral nerve injuries (positioning)
• Decubitus ulcers
• Retention of secretions, atelectasis, pneumonia
• Critical illness myopathy (CIM) — especially with concurrent corticosteroids; loss of myosin in myocytes
• Critical illness polyneuropathy (CIP) — affects 50–70% of multiorgan failure patients
• Prolonged paralysis after stopping relaxant

— Miller's Anesthesia, 10e, pp. 2401–2454

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Q23 — Discuss about recent advances in delayed recovery and reversal of neuromuscular block [APRIL 2012]

Delayed Recovery — Causes

1. Residual NMBD effect — most common; TOF <0.9 in 30–50% of PACU patients
2. Drug interactions — aminoglycosides, volatile agents, magnesium
3. Metabolic factors — hypothermia (slows Hofmann elimination, enzymatic hydrolysis), electrolyte imbalance
4. Atypical butyrylcholinesterase — prolonged succinylcholine or mivacurium effect
5. Organ dysfunction — renal failure accumulates vecuronium metabolite (3-desacetylvecuronium) and renally cleared NMBDs
6. Phase II block from succinylcholine overdose

Recent Advances in Reversal

• Modified γ-cyclodextrin; first agent based on encapsulation principle
• Selectively binds rocuronium and vecuronium (also pipecuronium) in a 1:1 tight inclusion complex (association:dissociation = 25,000,000:1)
• No need for anticholinergic co-administration (atropine or glycopyrrolate) — no muscarinic side effects
• Dosing:
  • Moderate block (TOF count ≥2): 2 mg/kg IV
  • Deep block (post-tetanic count 1–2): 4 mg/kg IV
  • Immediate reversal (RSI reversal): 16 mg/kg IV
• Reversal time to TOF ratio ≥0.9 is dramatically faster than neostigmine
• Pharmacokinetics: Volume of distribution 18 L, half-life 100 min, 80% excreted unchanged in urine

• AMG, EMG, mechanomyography now allow objective confirmation of TOF ratio ≥0.9 (or ≥1.0 for AMG) before extubation
• This is the only reliable method to rule out residual block

• A new cucurbit[n]uril-type encapsulating agent with potential to reverse all NMBDs (including benzylisoquinoliniums), though not yet in clinical use

• Recent guidelines recommend against routine "prophylactic" neostigmine at the end of every case (may paradoxically impair recovery if TOF already >0.9)
• Use only when quantitative monitoring confirms incomplete recovery

— Miller's Anesthesia, 10e, pp. 3282–3296 & pp. 3417–3446

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Q24 — Describe the biotransformation of intravenous muscle relaxants [NOVEMBER 2007]

Q25 — Describe the effect of clinically used muscle relaxants on the liver [NOV 2009]

Biotransformation of IV Muscle Relaxants (Miller's Table 24.8)

• Rapidly hydrolyzed by butyrylcholinesterase (pseudocholinesterase) in plasma → succinylmonocholine → succinic acid + choline
• Duration of action directly determined by BuChE activity and genotype (dibucaine number 20 vs 80)

Effect of Muscle Relaxants on the Liver (Q25 — NOV 2009)

• Elimination is mainly through bile → deacetylated to 3-desacetylvecuronium in liver
• Cirrhosis → increased volume of distribution and decreased clearance → prolonged elimination half-life
• Duration of 0.1 mg/kg is actually shorter (distribution-dependent), but 0.2 mg/kg gives duration increased from 65→91 min in cirrhotic patients
• Biliary obstruction: Duration prolonged by 50% due to reduced hepatic uptake

• Central compartment volume (+33%) and steady-state volume (+43%) increased in cirrhosis
• Clearance may be decreased; onset is slower; duration prolonged

• Cirrhosis: t½ increases 114→208 min; volume of distribution ↑50%; clearance ↓22%
• Cholestasis: Clearance reduced by 50%; t½ prolonged to 270 min

• Miller's states: "In contrast to all other NMBDs, the plasma clearances of atracurium and cisatracurium are slightly increased in patients with liver disease"
• Because a larger distribution volume is associated with greater clearance for these agents (both central and peripheral compartment metabolism occur via Hofmann elimination)
• These are the drugs of choice in patients with hepatic failure

— Miller's Anesthesia, 10e, pp. 1819–1864 & pp. 2301–2325

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SECTION 6: CHOLINESTERASE INHIBITORS AND OTHER PHARMACOLOGICAL ANTAGONISTS TO NMBA

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Q1 (Section 6) — Write a short note on Sugammadex [OCTOBER 2015 & APRIL 2014]

What is Sugammadex?

Structure and Mechanism

• The γ-cyclodextrin structure has:
  • Hydrophobic inner cavity — traps the lipophilic steroid nucleus of rocuronium/vecuronium
  • Hydrophilic exterior — polar hydroxyl groups make the complex water-soluble
• Forms a 1:1 tight inclusion complex with rocuronium (association:dissociation ratio = 25,000,000:1)
• Affinity for vecuronium is 2.5× lower but still sufficient for effective reversal
• Some binding to pancuronium — insufficient for clinical reversal
• No interaction with benzylisoquinoliniums (atracurium, cisatracurium, mivacurium)

Mechanism of Action

Pharmacokinetics

• Volume of distribution: 18 L
• Elimination half-life: 100 minutes
• Plasma clearance: 120 mL/min
• ~80% excreted unchanged in urine over 24 hours

Dosing

Advantages Over Neostigmine

Side Effects / Concerns

• Anaphylaxis/hypersensitivity: Incidence ~0.039–0.7%; onset typically within 3 minutes; treat with epinephrine
• Transient prolongation of aPTT and PT (limited, <1 hour — not clinically significant for most surgeries)
• Coughing/movement (unmasking of inadequate anesthesia)
• Parosmia (abnormal smell sensation)
• Re-rocuronization: Avoid re-administration of rocuronium within 24 hours; if urgent re-intubation needed, use succinylcholine (1 mg/kg) or high-dose rocuronium (1.2 mg/kg with delayed recovery expected)

— Miller's Anesthesia, 10e, pp. 3282–3295 & pp. 3417–3446

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Summary Table: Questions Year-Wise

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Detailed Discussion Based on Miller's Anesthesia, 10th Edition

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1. RESIDUAL NEUROMUSCULAR BLOCKADE

Definition

"It is impossible to detect residual paralysis when using clinical tests or qualitative (subjective) assessment of TOF stimulation." — Miller's Anesthesia, 10e, p. 3349

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Incidence

• 31–50% of patients have TOF < 0.90 after surgery in current large-scale studies
• A recent multicenter investigation of 1,571 patients found 58% had TOF < 0.90 at extubation, even though 78% received neostigmine reversal
• Meta-analysis of 24 clinical trials: pooled incidence of residual block was 41% with intermediate-acting NMBDs (TOF < 0.90)
• Most recent prospective multicenter studies report an average incidence around 65%
• The range across studies is 5–93%, reflecting variability in measurement methods, drug choice, and monitoring practices

— Miller's Anesthesia, 10e, pp. 3352–3354

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Causes and Risk Factors

• No reversal agent given → higher risk
• Neostigmine ceiling effect: ineffective at deep block levels
• Short interval between reversal and extubation
• Reversal attempted at TOF count 0 or 1

• Respiratory acidosis and metabolic alkalosis
• Hypothermia (slows enzymatic drug metabolism)
• Drug administration in PACU (aminoglycoside antibiotics, opioids)

• AMG frequently overestimates recovery → lower apparent incidence unless normalized
• Time of measurement: incidence higher at extubation than at PACU admission

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Symptoms of RNMB (Clinical Features)

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Adverse Outcomes Associated with RNMB

1. Postoperative hypoxemia: TOF < 0.70 associated with hypoxemia in 60% vs. 10% in patients with TOF ≥ 0.70
2. Postoperative pneumonia: Bulka et al. (n=13,100) — risk of pneumonia 2.26× higher in patients not receiving neostigmine reversal
3. Respiratory complications (failure to wean, reintubation, pneumonia): OR 1.75 in patients not receiving neostigmine
4. Coma or death within 24 hours: Reversal of NMBDs was associated with OR 0.10 for this composite outcome (Netherlands study, n=869,483 patients)

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Prevention and Management

1. Quantitative monitoring throughout the perioperative period — mandatory standard
2. Appropriate NMBD selection — prefer intermediate-acting agents
3. Pharmacologic reversal based on depth of block:
   • TOF count ≥ 2 (moderate block): Neostigmine 30–50 mcg/kg OR Sugammadex 2 mg/kg
   • TOF count 0–1 (deep block): Sugammadex 4 mg/kg (neostigmine is inadequate)
   • Post-tetanic count 0 (profound block): Sugammadex 16 mg/kg
4. Do not extubate until TOF ratio ≥ 0.90 (EMG/MMG) or ≥ 1.0 (AMG) is confirmed
5. Avoid time-based decisions — "2 hours since last dose" does not reliably predict recovery; 37% of patients still had TOF < 0.90 at 2+ hours post-NMBD in one large study

— Miller's Anesthesia, 10e, pp. 3348–3370

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2. NEUROMUSCULAR MONITORING

Why Monitoring is Essential

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A. Sites of Monitoring

• Most studied, most validated
• TOF ratio at AP correlates with respiratory and pharyngeal muscle function
• Note: Laryngeal and diaphragmatic muscles recover faster than the AP — so full recovery at AP indicates full body recovery

• Flexor hallucis brevis (tibial nerve at ankle)
• Corrugator supercilii (facial nerve) — onset mirrors laryngeal muscles; NOT reliable for recovery assessment

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B. Stimulation Patterns

1. Single Twitch (0.1 Hz)

• One supramaximal stimulus every 10 seconds
• Measures degree of block as % of baseline twitch height
• Cannot detect residual block; needs baseline measurement
• ED₉₅ and ED₅₀ determined using this method

2. Train-of-Four (TOF) — 2 Hz × 4 stimuli

• Most clinically used method
• 4 stimuli at 2 Hz over 2 seconds; cycle can be repeated every 10–15 seconds
• TOF count: Number of visible/palpable twitches (0–4)
• TOF ratio (T4/T1): Key quantitative parameter; target ≥ 0.90

3. Tetanic Stimulation (50 Hz × 5 seconds)

• High-frequency burst; tests presynaptic ACh reserve
• Nondepolarizing block: Fade during tetanus → confirms incomplete recovery
• Depolarizing block (Phase I): No fade — uniform, sustained reduction
• Post-tetanic facilitation (PTF) occurs after tetanus — enhanced response for several minutes

4. Post-Tetanic Count (PTC)

• Used when TOF count = 0 (profound block)
• 50-Hz tetanus × 5 sec, followed by single twitches at 1 Hz
• Count the number of post-tetanic twitches (PTC 1–2 = deep block; PTC > 10 = shallow block ready for reversal)
• Helps predict time to spontaneous recovery and guide sugammadex dosing

5. Double-Burst Stimulation (DBS)

• Two short bursts of 50-Hz tetanic stimulation, 750 ms apart
• Better clinical (tactile) detection of fade compared to TOF at moderate block levels
• DBS ratio < 0.9 corresponds to TOF ratio < 0.9

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C. Types of Quantitative Monitors

• Gold standard; measures isometric force of muscle contraction
• Most accurate, but bulky and cumbersome — limited clinical use

• Measures compound muscle action potential (CMAP)
• Not affected by arm position or preloading
• Examples: TwitchView (Blink), TetraGraph (Senzime)
• Miller's recommends EMG as preferred modality for confirming TOF ≥ 0.90 before extubation

• Measures acceleration of the thumb (Newton's second law: F = ma)
• Most widely available clinically
• Examples: TOF-Watch, TOFscan, StimPod
• Limitation: frequently overestimates degree of recovery; TOF ratio must be normalized or threshold raised to ≥ 1.0 before extubation
• "AMG frequently overestimates the degree of neuromuscular recovery" — Miller's

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D. Current Practice and Guidelines

1. Quantitative monitoring must be used to guide dosing of NMBDs and their antagonists throughout the perioperative period
2. Neostigmine is most effective when administered for antagonism of minimal block (TOF ≥ 0.2 spontaneously)
3. Neostigmine should NOT be administered at TOF count 0–1 without a concurrent plan for sugammadex
4. Time from last NMBD dose is an unreliable predictor of recovery — quantitative monitoring is mandatory
5. In a study, 37% of patients still had TOF < 0.90 at 2+ hours post-single NMBD dose — confirming time-based decisions are inadequate

— Miller's Anesthesia, 10e, pp. 3349–3409

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3. DEPTH OF ANAESTHESIA MONITORING

Overview

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A. Processed Electroencephalography (pEEG)

Principle

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B. Bispectral Index (BIS)

How BIS is Calculated

1. EEG bispectrum (phase relationships between EEG frequency components)
2. Burst suppression ratio (% of isoelectric EEG within a time epoch)
3. Beta ratio (ratio of high-frequency to low-frequency EEG power)
4. SyncFastSlow (synchronisation of low and high frequencies)

BIS Values and Clinical Interpretation

Clinical Applications of BIS

1. Titration of anaesthesia: BIS-guided titration reduces total anaesthetic drug consumption, with:
   • 2–4 minute reduction in awakening times
   • 4–8 minute reduction in PACU stay
   • Modest reduction in PONV (32% vs. 38%)
   • But no proven improvement in early home discharge
2. Prevention of awareness: The B-Aware trial demonstrated BIS monitoring reduced the rate of intraoperative awareness in high-risk patients
   • However, awareness can occur even at BIS 40–50 in some patients
   • Concerns raised: "titrating anesthesia to BIS values of 40–50 to reduce costs... may unintentionally increase the risk for awareness"
3. Postoperative delirium: Two RCTs showed BIS-guided titration (target 40–60) resulted in significantly lower delirium rates
   • But the ENGAGES trial found conflicting results
   • Cochrane review (~3,000 patients): processed EEG indices or evoked potentials decreased acute delirium and POCD at 3 months
4. Depth vs. mortality: The landmark trial by Short et al. (6,644 patients, 73 centres, 7 countries):
   • Patients randomised to BIS target 35 vs. 50 (deep vs. light anaesthesia)
   • No difference in all-cause mortality at 1 year
   • Confirmed that deeper anaesthesia does not independently increase perioperative mortality
5. Paediatric use: BIS has been successfully used in older children to decrease drug delivery and enhance recovery; utility in infants < 6 months is questionable
6. Sedation: BIS changes with increasing sedation but is too variable to be routinely useful for procedural sedation (e.g., not effective for midazolam sedation endpoint); did not improve quality of sedation during ERCP

Limitations of BIS

• Does not measure analgesia
• EMG artifact can falsely elevate BIS (muscle activity mimics high-frequency EEG)
• Nitrous oxide, ketamine, and neuromuscular blockers cause artefactual changes
• Interpatient variability is high — some patients have awareness at BIS 40
• Does not prevent adverse events unless the displayed value is acted upon

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C. Other Processed EEG Monitors

• Measures two components:
  • State Entropy (SE): 0.8–32 Hz range; reflects cortical hypnotic state (0–91)
  • Response Entropy (RE): 0.8–47 Hz; includes EMG; responds to nociceptive stimuli (0–100)
  • RE - SE gap suggests frontal EMG activity (nociception/arousal)
• Comparable performance to BIS; small reductions in drug use with entropy monitoring (quality of evidence poor per Miller's)

• Displays raw EEG, bilateral Patient State Index (PSI), density spectral array (DSA)
• PSI target for general anaesthesia: 25–50

• Based on automated stage classification of EEG into A–F stages

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D. Analgesic / Nociception Monitors

• Surgical Pleth Index (SPI)
• Nociception Level (NOL)
• Analgesia Nociception Index (ANI)

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E. End-Tidal Target Anesthesia

• MAC awake (~0.3 MAC): return of consciousness
• MAC = 1.0: 50% of patients do not move to skin incision
• MAC-BAR (~1.7 MAC): blocks autonomic response

— Miller's Anesthesia, 10e, pp. 3925–3951 (block 27) and block 10, pp. 3720–3721

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4. THROMBOELASTOGRAPHY (TEG) AND ROTATIONAL THROMBOELASTOMETRY (ROTEM)

Overview

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A. Principle of Operation

• A small blood sample (~0.36 mL for TEG; ~0.34 mL for ROTEM) is placed in a heated cylindrical cuvette (37°C)
• A pin (TEG) or cup (ROTEM) is suspended in the blood
• Rotational oscillation is applied:
  • In TEG: the cuvette oscillates ±4.75° at 9 rpm; the pin detects clot resistance
  • In ROTEM: the pin oscillates; the cuvette is stationary
• As clot forms, fibrin strands transmit torque from cuvette to pin → increasing signal amplitude
• As clot lyses, fibrin dissolves → amplitude decreases
• Results displayed as a characteristic waveform (the "thromboelastogram")

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B. TEG Parameters and Their Clinical Meaning

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C. ROTEM Assays (Specific Tests)

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D. TEG Assays

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E. Viscoelastic Tests vs. Conventional Coagulation Tests

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F. Clinical Applications

1. Cardiac Surgery and CPB

• Primary indication in Miller's — most extensive evidence base
• Cardiopulmonary bypass (CPB) causes multifactorial coagulopathy: platelet dysfunction, hyperfibrinolysis, dilutional coagulopathy, heparin effect, protamine excess
• TEG/ROTEM-guided transfusion algorithms have been shown to:
  • Reduce allogeneic blood product use
  • Be both efficacious and cost-effective
  • Class I recommendation with Level B-NR evidence from STS/SCA/AmSECT/SABM Patient Blood Management Guidelines

2. Massive Haemorrhage / Trauma

• Detects coagulopathy of trauma in real time
• Guides directed therapy: FFP, cryoprecipitate, platelets, fibrinogen concentrate, PCC
• Identifies hyperfibrinolysis → targeted antifibrinolytic therapy (TXA)

3. Liver Transplantation

• Liver failure creates global coagulopathy: factor deficiency, thrombocytopenia, hyperfibrinolysis
• TEG guides transfusion during hepatectomy and reperfusion phases

4. Obstetrics

• TEG in normal term pregnancy reflects a hypercoagulable state:
  • ↓ R time (faster clot initiation)
  • ↓ K time
  • ↑ α angle
  • ↑ MA (increased clot strength)
• These changes begin in the first trimester
• Useful in managing obstetric haemorrhage (PPH) and coagulopathy of HELLP/DIC

5. Antithrombotic Drug Monitoring

• POC monitors can stratify bleeding risk in patients on clopidogrel, prasugrel, or GP IIb/IIIa inhibitors preoperatively
• TEG Platelet Mapping: measures residual platelet reactivity → guides timing of surgery after antiplatelet agents

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G. Limitations of TEG/ROTEM

• Does not measure: individual clotting factor levels, vWF function, endothelial function
• Operator dependent: sample preparation, timing, temperature critical
• No shear stress: conditions are not identical to in vivo haemostasis (no vascular flow)
• Standardization lacking: TEG and ROTEM are not interchangeable — different activators, different parameters, different reference ranges
• Evidence for outcome improvement beyond surrogate transfusion endpoints needs further large-scale confirmation

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H. Point-of-Care Algorithm Approach

1. Is there ongoing bleeding?
2. Is fibrinogen low? (FIBTEM MCF < 8 mm) → Fibrinogen concentrate / cryoprecipitate
3. Is platelet function impaired? (EXTEM MCF low with normal FIBTEM) → Platelets
4. Is clotting time prolonged? (EXTEM CT prolonged) → FFP or PCC
5. Is fibrinolysis present? (APTEM vs. EXTEM comparison or LY30 > 8%) → TXA or ε-aminocaproic acid

— Miller's Anesthesia, 10e, pp. 7529–7534 (block 20) and pp. 8818–8819 (block 24)

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Summary Reference Table