Neuromuscular Blocking Agents (NMBAs) - Complete Guide

1. Historical Background

2. Basic Pharmacology and Classification

3. Neuromuscular Junction Physiology (Basis for Drug Action)

4. Depolarizing NMBAs

Succinylcholine (Suxamethonium)

• Binds NMJ nicotinic receptors non-competitively
• Causes sustained depolarization of the motor end plate
• Initial depolarization causes fasciculations (brief muscle contractions)
• Peri-junctional Na+ channels close and cannot reopen until the end plate repolarizes
• ACh cannot generate further EPPs on an already-depolarized membrane
• Result: flaccid paralysis (Phase I block)

• Onset: 45-60 seconds (fastest of all NMBAs)
• Duration: 6-11 minutes
• Elimination: Rapid hydrolysis by plasma pseudocholinesterase (butyrylcholinesterase) to succinylmonocholine, then to succinic acid and choline
• Pseudocholinesterase is not present at the motor end plate - it acts systemically before succinylcholine reaches the NMJ; only a small fraction survives to reach the end plate

• RSI: 1.5 mg/kg IV (based on total body weight, even in obese patients, as pseudocholinesterase activity scales with body habitus)
• ED95 is ~0.3 mg/kg, but this dose produces unacceptably slow onset for emergency intubation

• RSI - gold standard drug for rapid onset + short duration
• Electroconvulsive therapy (with methohexital)
• Short procedures requiring brief muscle relaxation

Adverse Effects of Succinylcholine

1. Hyperkalemia (most dangerous)

2. Fasciculations and Myalgia

• Occurs in >90% of patients receiving succinylcholine
• Muscle pain (myalgia) in ~50% post-procedure
• "Defasciculating" pretreatment with a small dose of nondepolarizing NMBA has inconsistent evidence
• Using 1.5 mg/kg causes less fasciculation than 1.0 mg/kg

3. Bradycardia and Cardiac Arrhythmias

• Succinylcholine is an ACh analogue and can stimulate muscarinic receptors in the heart
• Sinus bradycardia is more common in children; can occur in adults with repeat dosing
• Atropine treats it if needed, but hypoxia must first be excluded
• Rare: VF, asystole (hard to distinguish from laryngoscopy-related vagal stimulation)

4. Increased Intraocular Pressure (IOP)

• Succinylcholine raises IOP transiently
• Use cautiously with open globe injuries; however, if faster intubation prevents hypoxia-related injury, the benefits may outweigh risks

5. Increased Intracranial Pressure (ICP)

• Can transiently raise ICP, making its use in severe TBI controversial
• Hypoxia and hypercapnia from delayed intubation may be far more damaging than the transient ICP rise

6. Malignant Hyperthermia (MH)

• Rare, genetically predisposed individuals (ryanodine receptor mutations)
• Triggered by succinylcholine and volatile anesthetics
• Causes: rapid temperature rise, muscular rigidity, rhabdomyolysis, metabolic acidosis
• Treatment: stop the trigger, dantrolene 1-2.5 mg/kg IV every 5 min (max 10 mg/kg), active cooling
• MH hotline: 1-800-644-9737

7. Masseter Spasm

• Particularly in children and young adults
• Persistent severe spasm warrants suspicion of MH

8. Pseudocholinesterase Deficiency

• Allelic variants (e.g., Asp70Gly polymorphism) reduce enzyme activity
• Heterozygous: recovery prolonged 3-8x (tens of minutes)
• Homozygous: recovery up to 60x longer (prolonged apnea, hours)
• Also occurs with liver disease, renal disease, malnutrition, pregnancy, organophosphate exposure
• Management: ventilatory support until spontaneous recovery; fresh frozen plasma can be used in extreme cases

5. Nondepolarizing (Competitive) NMBAs

Chemical Classes

• Pancuronium, vecuronium, rocuronium, pipecuronium
• More likely to have vagolytic/cardiovascular effects
• Eliminated hepatically and/or renally
• Reversed by sugammadex

• Atracurium, cisatracurium, mivacurium, tubocurarine (historical)
• More prone to histamine release
• Often undergo non-organ-dependent elimination (Hofmann elimination, ester hydrolysis)
• NOT reversed by sugammadex

Comprehensive Drug Table

Individual Nondepolarizing Drug Profiles

Rocuronium

• The preferred alternative to succinylcholine for RSI
• Onset at 1.2 mg/kg: ~60 seconds, providing intubating conditions equivalent to succinylcholine
• At 1.5 mg/kg IV: onset as fast as 30 seconds
• Duration at 1.2 mg/kg: ~45-60 minutes
• No cardiac muscarinic blockade, no histamine release
• Dosing in obese patients: based on total body weight (TBW) in the ED
• Fully reversible by sugammadex (16 mg/kg for immediate reversal)
• Hepatic elimination; use with caution in severe liver disease

Vecuronium

• Intermediate duration
• No histamine release, no cardiac muscarinic effects - ideal for hemodynamically unstable patients
• Used for post-intubation paralysis in ICU: 0.1 mg/kg IV
• Hepatic + renal elimination
• Preferred for maintenance paralysis when prolonged blockade is needed

Atracurium

• Unique elimination: Hofmann elimination (spontaneous non-enzymatic degradation at physiologic pH and temperature) + ester hydrolysis
• Independent of renal or hepatic function - ideal for multi-organ failure
• Releases histamine (bronchospasm risk)
• Metabolite laudanosine can cause CNS excitation/seizures (relevant in prolonged ICU infusion)

Cisatracurium

• Isomer of atracurium; same Hofmann elimination but minimal histamine release
• Preferred over atracurium in ICU due to less laudanosine production and no histamine release
• Intermediate-to-long duration
• Organ-independent elimination - ideal in renal/hepatic failure

Pancuronium

• Long-acting; still used in some long surgeries and ICU settings
• Vagolytic: blocks cardiac muscarinic receptors - causes tachycardia and mild hypertension
• 80% renal elimination - avoid/reduce dose in renal failure
• Historically associated with prolonged weakness in ICU (critical illness myopathy)

Mivacurium

• Short-acting nondepolarizing agent (like succinylcholine in concept)
• Metabolized by plasma cholinesterase (like succinylcholine) - prolonged action with pseudocholinesterase deficiency
• Histamine release at faster injection rates

6. Monitoring Neuromuscular Blockade

Train-of-Four (TOF) Stimulation

• TOF ratio = 1.0: no blockade
• TOF ratio < 0.9: clinically significant residual block (risk of aspiration, respiratory failure)
• All 4 twitches absent: deep block

Other Stimulation Patterns

• Tetanic stimulation: High-frequency continuous stimulation; fade indicates nondepolarizing block
• Post-tetanic count (PTC): Counts twitches after tetanic stimulation; used when TOF count = 0 (very deep block)
• Double-burst stimulation (DBS): Two brief tetanic bursts; better than TOF for detecting residual block manually

Clinical note

7. Reversal of Neuromuscular Blockade

Acetylcholinesterase Inhibitors (Traditional Reversal)

• AChE inhibitors do NOT reverse succinylcholine - they actually enhance depolarizing blockade (by preserving more ACh at the NMJ)
• They cannot fully reverse deep nondepolarizing blocks (TOF count 0-1); must wait for spontaneous recovery to TOF count ≥ 2
• Must always co-administer an antimuscarinic (atropine or glycopyrrolate) to prevent bradycardia, excessive secretions, and bronchospasm from the systemic increase in ACh

desflurane > sevoflurane > isoflurane > halothane > N2O/opioid/propofol-based anesthesia

Sugammadex - Selective Relaxant Binding Agent

• Can reverse deep (even immediate post-dose) aminosteroid blockade
• Does not require antimuscarinic co-administration
• Works regardless of depth of block
• More reliable than neostigmine

• Only works for aminosteroid NMBAs (rocuronium, vecuronium, pancuronium); NOT for benzylisoquinolines or succinylcholine
• Expensive
• Less available outside OR settings
• Should NOT be used as a "rescue" in cannot-intubate/cannot-oxygenate scenarios - sugammadex speed and completeness of reversal are variable and do not address co-administered sedatives

8. Drug Interactions

9. Clinical Applications

Rapid Sequence Intubation (RSI)

1. Pre-oxygenation
2. Induction agent (e.g., etomidate 0.3 mg/kg, ketamine 1-2 mg/kg, propofol)
3. NMBA (succinylcholine 1.5 mg/kg TBW OR rocuronium 1.2 mg/kg TBW)
4. Cricoid pressure (controversial)
5. Laryngoscopy and intubation at ~60 seconds

Surgical Muscle Relaxation

• Allows surgery at lighter planes of anesthesia
• Especially abdominal wall relaxation for laparotomy
• Intermediate agents (vecuronium, rocuronium, cisatracurium) most commonly used

ICU Paralysis

• Indications: severe ARDS (to improve chest wall compliance, reduce ventilator dyssynchrony, decrease O2 consumption), status asthmaticus, refractory ICP elevation, therapeutic hypothermia
• Preferred agents: cisatracurium (organ-independent elimination, low laudanosine), vecuronium
• Must ensure adequate sedation and analgesia before and during paralysis - paralysis without sedation is torture
• Continuous train-of-four monitoring recommended
• Risk of ICU-acquired weakness (critical illness myopathy/neuropathy) with prolonged use

Electroconvulsive Therapy (ECT)

• Succinylcholine + short-acting barbiturate (methohexital) to prevent fractures from convulsive motor activity

Ophthalmic Surgery

• Avoid succinylcholine in open globe injuries (raised IOP risk)

10. Contraindications and Precautions

Succinylcholine Absolute/Strong Contraindications:

• History of malignant hyperthermia or family history
• Known/suspected pseudocholinesterase deficiency (relative)
• Hyperkalemia manifest on ECG
• Denervation syndromes >5 days old (spinal cord injury, stroke, ALS, muscular dystrophies)
• Burns >5 days old
• Crush injuries/immobilization >5 days old
• Open globe injury with high IOP (relative - weigh against intubation urgency)

Rocuronium precautions:

• Hepatic failure (decreased clearance; prolong duration)
• No absolute contraindications
• Reassess dosing in myasthenia gravis (exquisitely sensitive - even tiny doses cause prolonged block)

Myasthenia Gravis

• Patients are extremely sensitive to nondepolarizing NMBAs (reduced ACh receptor number)
• Resistant to succinylcholine (requires higher doses due to receptor loss)
• Nondepolarizing NMBAs should be used at significantly reduced doses or avoided

Eaton-Lambert Myasthenic Syndrome

• Impaired presynaptic ACh release
• Sensitive to both depolarizing and nondepolarizing NMBAs
• Use with great caution; titrate carefully

11. Residual Neuromuscular Block and Post-operative Complications

• Impairs pharyngeal function and upper airway protective reflexes
• Increases risk of aspiration pneumonia
• Causes hypoxic ventilatory response impairment
• Contributes to postoperative pulmonary complications

12. Special Populations

Pediatric

• Succinylcholine: bradycardia more common; some advocate atropine pretreatment in infants < 1 year (though evidence is limited)
• Succinylcholine is not recommended for routine pediatric intubation due to risk of undiagnosed myopathies (e.g., Duchenne) causing hyperkalemic cardiac arrest
• Exception: emergency (RSI) where fast onset is needed

Renal Failure

• Avoid pancuronium, d-tubocurarine (renally eliminated)
• Atracurium, cisatracurium preferred (Hofmann elimination - organ independent)
• Mivacurium acceptable (plasma cholinesterase)
• Rocuronium: some prolongation (partial hepatic, some renal excretion of metabolites)

Hepatic Failure

• Decreased pseudocholinesterase - prolong succinylcholine and mivacurium
• Rocuronium, vecuronium: primarily hepatically eliminated - prolonged effects
• Atracurium, cisatracurium preferred

Obesity

• Succinylcholine: use TBW
• Rocuronium: use TBW in emergency; IBW in elective anesthesia acceptable (but TBW safer in ED)

Pregnancy

• NMBAs do not cross placenta readily (quaternary ammonium, polar)
• Plasma pseudocholinesterase slightly reduced in pregnancy - mildly prolonged succinylcholine

13. Summary: Choosing an Agent

• Goodman & Gilman's The Pharmacological Basis of Therapeutics, Chapter 13
• Miller's Anesthesia, 10th Edition
• Morgan and Mikhail's Clinical Anesthesiology, 7th Edition
• Rosen's Emergency Medicine: Concepts and Clinical Practice
• Barash, Cullen, and Stoelting's Clinical Anesthesia, 9th Edition