---

MD Anaesthesiology Exam Questions on Neuromuscular Blocking Drugs

Discussed from Miller's Anesthesia, 10e

---

SECTION: NEUROMUSCULAR BLOCKING AGENTS

---

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

---

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

---

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

---

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

---

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

---

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

---

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

---

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

---

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

---

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

---

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

---

SECTION 6: CHOLINESTERASE INHIBITORS AND OTHER PHARMACOLOGICAL ANTAGONISTS TO NMBA

---

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

---

Summary Table: Questions Year-Wise

---