Muscle Relaxants: Comprehensive Guide to Types, Indications, and Contraindications
Classification of Muscle Relaxants
Muscle relaxants used in anesthesia are divided into two main categories based on mechanism of action: depolarizing agents (succinylcholine) and non-depolarizing agents, with the latter further subdivided into steroidal compounds (rocuronium) and benzylisoquinoline compounds (atracurium, cisatracurium, mivacurium) 1, 2.
Depolarizing Muscle Relaxants
Succinylcholine (Suxamethonium)
Indications:
- Rapid-sequence intubation - remains the gold standard with onset time of approximately 1 minute and fastest recovery 3
- Emergency airway management requiring immediate intubation 1
- Laryngospasm - highly effective in all cases when administered at 1.0 mg/kg IV (or 4.0 mg/kg IM/sublingual) 1
- Electroconvulsive therapy 1
Contraindications and Precautions:
- Modified Summary of Product Characteristics by ANSM due to safety concerns 1
- Malignant hyperthermia susceptibility 4
- Hyperkalemia risk in patients with burns, crush injuries, denervation, or neuromuscular diseases 1
- Bradycardia/cardiac arrest risk - particularly in children under 3 years (requires atropine 0.02 mg/kg co-administration) 1
- Increased intracranial pressure - use with extreme caution in neurosurgical patients 2
- Prolonged paralysis in patients with pseudocholinesterase deficiency 1
Non-Depolarizing Muscle Relaxants: Benzylisoquinoline Compounds
Atracurium
Indications:
- Abdominal laparotomy or laparoscopy surgery (GRADE 1+ recommendation) 1, 5
- ENT laser surgery (GRADE 2+ recommendation) 1, 5
- Airway obstruction related to supraglottic devices - dose of 0.1-0.2 mg/kg if adequate anesthesia depth 1, 5
- Patients with renal or hepatic failure - PREFERRED agent due to organ-independent elimination via Hofmann elimination and ester hydrolysis (GRADE 2+ recommendation) 5, 6, 7
Contraindications and Precautions:
- Histamine release at higher doses causing potential cardiovascular effects 6, 7
- Laudanosine accumulation (metabolite) may cause CNS excitation at extremely high doses, though rarely clinically significant 6
- No dose adjustment required in renal or hepatic failure 5
Pharmacokinetics:
- ED95: 0.23 mg/kg (range 0.11-0.26 mg/kg) 7
- Onset at 0.4-0.5 mg/kg: 3-5 minutes to maximum block 7
- Duration: Recovery begins at 20-35 minutes, 95% recovery at 60-70 minutes 7
- No cumulative effect with repeated dosing 7
Cisatracurium
Indications:
- General surgical procedures requiring neuromuscular blockade 8
- ICU ventilation - recovery time approximately 50-55 minutes after infusion termination 8
- Patients with renal or hepatic dysfunction - similar organ-independent elimination as atracurium 8
Contraindications and Precautions:
- Pregnancy Category B - use only if clearly needed 8
- Magnesium salts enhance neuromuscular blockade (important in toxemia of pregnancy management) 8
- Volatile anesthetics (isoflurane/enflurane at 1.25 MAC) may reduce infusion requirements by 30-40% 8
Drug Interactions:
- Succinylcholine - can be used safely after varying degrees of recovery; prior succinylcholine administration may slightly increase cisatracurium infusion requirements 8
- Antibiotics (aminoglycosides, tetracyclines, polymyxins, lincomycin, clindamycin) enhance blockade 8
- Phenytoin/carbamazepine chronic therapy may cause resistance and require higher infusion rates 8
Mivacurium
Indications:
- Short surgical procedures requiring brief neuromuscular blockade 1
- Tracheal intubation when intermediate duration is acceptable 1
Contraindications:
- Similar to other benzylisoquinoline compounds
- Pseudocholinesterase deficiency - significantly prolonged duration 1
Non-Depolarizing Muscle Relaxants: Steroidal Compounds
Rocuronium
Indications:
- Rapid-sequence intubation - alternative to succinylcholine at doses ≥1.0 mg/kg under relatively light anesthesia 3
- Standard tracheal intubation at 0.6 mg/kg with appropriate induction technique including opioid 3
- Maintenance of neuromuscular blockade during surgery 4
- Pediatric anesthesia 4
Contraindications and Precautions:
- Anaphylaxis risk - documented in guidelines on reducing anaphylaxis during anesthesia 1, 4
- Residual paralysis - requires reversal monitoring 4
- QT interval prolongation 4
- Malignant hyperthermia - not a trigger but use with caution 4
- Myasthenia gravis - extreme sensitivity, use reduced doses 4
- Extravasation - can cause tissue injury 4
Special Populations:
- Hepatic impairment - prolonged duration, reduce maintenance doses 4
- Renal impairment - prolonged duration, reduce maintenance doses 4
- Geriatric patients - may have prolonged duration 4
- Pediatric patients - approved for use with appropriate dosing adjustments 4
Drug Interactions:
- Volatile anesthetics potentiate effect by approximately 35% (isoflurane/enflurane) or 20% (halothane) 1, 4
- Antibiotics, magnesium, lithium, local anesthetics, procainamide, quinidine enhance blockade 4
- Propofol - no significant effect on duration 8
Withdrawn Agents
Pancuronium and Vecuronium - withdrawn from market in France 1
Rapacuronium - associated with dose-dependent respiratory side effects limiting usefulness above 1.5 mg/kg for rapid-sequence induction 3
Critical Monitoring Requirements
Neuromuscular monitoring is MANDATORY throughout anesthesia (GRADE 1+ recommendation) 1:
- Corrugator supercilii muscle is the recommended monitoring site due to sensitivity to muscle relaxants and kinetics comparable to laryngeal muscle 1, 5
- Train-of-four (TOF) ratio ≥0.9 at adductor pollicis required to eliminate residual neuromuscular blockade 9
- No clinical test is sensitive enough to detect residual blockade - only quantitative instrumental monitoring is reliable 9
- Failure to monitor quantitatively leads to undetected residual blockade and increased morbidity/mortality 9
Reversal Agents
Pharmacological reversal is strongly recommended following muscle relaxant use to prevent residual neuromuscular blockade 9:
Neostigmine (Anticholinesterase)
- Wait for four detectable TOF responses at adductor pollicis before administering 9
- Administering too early (before four TOF responses) results in longer time to full reversal and is ineffective 9
- Must be given with anticholinergic agent (atropine or glycopyrrolate) 7
Sugammadex (Selective Reversal for Steroidal Agents)
- Selective for steroidal muscle relaxants (rocuronium, vecuronium) 1
- Faster reversal than neostigmine with better results than succinylcholine recovery (mean 4.7 minutes) 1
- May be ineffective in rare cases, potentially requiring emergency tracheal access 1
Common Pitfalls to Avoid
- Never rely on clinical tests alone for assessing recovery - quantitative monitoring is essential 9
- Do not administer neostigmine before four TOF responses are visible - this is ineffective 9
- Avoid testing mask ventilation before muscle relaxant in routine cases - it is probably not recommended and lengthens induction time (GRADE 2-) 1
- Always have suxamethonium and atropine immediately available in pediatric settings - reduces cardiac arrest and severe airway obstruction by nearly half 1
- Adjust volatile anesthetic doses - they potentiate muscle relaxants and may require 30-40% reduction in infusion rates 1, 8
- Monitor for residual blockade - associated with higher 24-hour morbidity/mortality, critical respiratory events, postoperative pneumonia, and delayed recovery discharge 9