## Depolarising vs Non-depolarising Muscle Relaxants **Key Point:** Succinylcholine is the only clinically used depolarising (Phase I) muscle relaxant in India and worldwide. ### Mechanism of Succinylcholine 1. Binds to nicotinic acetylcholine receptors at the neuromuscular junction 2. Causes sustained depolarisation (unlike acetylcholine which causes brief depolarisation) 3. Prevents repolarisation and muscle contraction 4. Results in initial fasciculations followed by flaccid paralysis ### Clinical Features of Depolarising Agents - **Onset:** Very rapid (30–40 seconds) - **Duration:** Short (5–10 minutes) due to rapid plasma pseudocholinesterase metabolism - **Fasciculations:** Visible muscle twitching before paralysis - **Complications:** Hyperkalaemia, malignant hyperthermia, myoglobinuria, increased intracranial pressure **High-Yield:** Succinylcholine is the ONLY depolarising agent used clinically. All others (vecuronium, atracurium, cisatracurium, rocuronium, pancuronium) are non-depolarising (competitive antagonists). ### Comparison Table | Feature | Succinylcholine (Depolarising) | Non-depolarising (e.g., Vecuronium) | | --- | --- | --- | | Mechanism | Sustained depolarisation | Competitive blockade | | Fasciculations | Present | Absent | | Onset | 30–40 sec | 60–90 sec | | Duration | 5–10 min | 30–60 min | | Metabolism | Plasma pseudocholinesterase | Organ-dependent | | Hyperkalaemia | Yes (risk) | No | | Malignant hyperthermia | Yes (trigger) | No | | Reversal | None (wait for metabolism) | Neostigmine + glycopyrrolate | **Clinical Pearl:** Succinylcholine is preferred for rapid sequence intubation (RSI) in emergency settings due to its rapid onset and short duration, despite its risks.
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