What is the difference between non-depolarizing (NDMR) and depolarizing muscle relaxants?

Differences Between Non-Depolarizing and Depolarizing Muscle Relaxants

Non-depolarizing muscle relaxants (NDMRs) competitively block acetylcholine receptors at the neuromuscular junction, while depolarizing agents cause sustained depolarization of the motor end plate, with fundamentally different mechanisms of action, onset times, and side effect profiles. 1

Mechanism of Action

Depolarizing Muscle Relaxants

• Mechanism: Bind to acetylcholine receptors and cause prolonged depolarization of the motor end plate
• Example: Suxamethonium (succinylcholine)
• Characteristics:
  • Acts as an agonist at the nicotinic acetylcholine receptor
  • Causes initial muscle fasciculations before paralysis 2

Non-Depolarizing Muscle Relaxants

• Mechanism: Competitively block acetylcholine receptors without causing depolarization
• Examples: Rocuronium, atracurium, cisatracurium, vecuronium
• Characteristics:
  • Act as antagonists at the nicotinic acetylcholine receptor
  • No fasciculations 1

Clinical Characteristics

Onset and Duration

Characteristic	Depolarizing (Suxamethonium)	Non-Depolarizing
Onset time	Very rapid (30-60 seconds)	Slower (rocuronium fastest at 54.9 ± 10.9s) [3,4]
Duration	Ultra-short (5-10 minutes)	Intermediate to long (30-60+ minutes) [1]
Recovery	Spontaneous, rapid	Slower, may require reversal agents [1]

Clinical Applications

Depolarizing Agents (Suxamethonium)

• Preferred for:
  • Rapid sequence induction (RSI) due to fastest onset 1
  • Electroconvulsive therapy 1
  • Brief procedures requiring short duration paralysis
  • Emergency airway management when rapid onset is critical 5

Non-Depolarizing Agents

• Preferred for:
  • Longer surgical procedures requiring sustained relaxation
  • Patients with contraindications to suxamethonium
  • Rocuronium (≥0.9 mg/kg) can be an alternative to suxamethonium for RSI 4, 5
  • Cisatracurium is preferred in patients with renal or hepatic impairment 6

Side Effects and Contraindications

Depolarizing Agents (Suxamethonium)

• Side effects:
  • Muscle fasciculations (71% may experience post-operative myalgia) 2
  • Hyperkalemia (potentially fatal in susceptible patients)
  • Malignant hyperthermia
  • Increased intraocular, intracranial, and intragastric pressure
  • Bradycardia (especially with repeated doses)
• Contraindications:
  • Primary muscle damage (myopathies)
  • Up-regulation of nicotinic acetylcholine receptors (chronic motor deficit)
  • Burns, spinal cord injuries, and prolonged immobilization 1

Non-Depolarizing Agents

• Side effects:
  • Prolonged neuromuscular blockade
  • Residual paralysis if inadequately reversed
  • Histamine release (more with older agents, minimal with newer agents)
• Specific considerations:
  • Benzylisoquinolines (atracurium/cisatracurium) preferred in renal/hepatic failure 1
  • Steroidal agents (rocuronium, vecuronium) may have prolonged effects in renal/hepatic impairment 6

Reversal

Depolarizing Agents

• No specific reversal agent
• Action terminated by plasma cholinesterase metabolism

Non-Depolarizing Agents

• Reversal options:
  • Neostigmine (acetylcholinesterase inhibitor) + anticholinergic
  • Sugammadex (specific for steroidal NDMRs like rocuronium)
• Sugammadex advantages:
  • Faster reversal than neostigmine
  • Can reverse deep blockade
  • Recommended for patients with neuromuscular disease 1, 6

Special Populations

Obesity

• Suxamethonium: Dose based on actual body weight (1.0 mg/kg)
• Non-depolarizing agents: Dose based on lean body weight 1

Neuromuscular Disease

• Avoid suxamethonium in myasthenia, myopathies
• Reduce doses of non-depolarizing agents in myasthenia
• Monitoring of neuromuscular blockade is essential 1, 6

Monitoring

• Intraoperative monitoring of neuromuscular blockade is strongly recommended for all muscle relaxants to prevent residual paralysis 1, 6
• Corrugator supercilii muscle monitoring provides the best correlation with laryngeal muscle relaxation 1

Clinical Decision Making

When choosing between depolarizing and non-depolarizing muscle relaxants, consider:

1. Required speed of onset
2. Desired duration of action
3. Patient-specific contraindications
4. Availability of reversal agents
5. Monitoring capabilities

For rapid sequence induction, either suxamethonium (1 mg/kg) or high-dose rocuronium (≥0.9 mg/kg) with sugammadex availability is appropriate, with the choice depending on patient-specific factors and the anticipated duration of the procedure.