## Depolarising vs Non-depolarising Muscle Relaxants **Key Point:** Succinylcholine is the ONLY depolarising muscle relaxant in clinical use. It mimics acetylcholine and causes sustained depolarisation of the muscle membrane. ### Mechanism of Succinylcholine 1. **Depolarisation phase**: Succinylcholine binds to nicotinic acetylcholine receptors and causes sustained depolarisation 2. **Fasciculation**: Visible muscle contractions occur as motor units fire uncontrollably (lasts 5–30 seconds) 3. **Flaccid paralysis**: Prolonged depolarisation renders the membrane inexcitable, causing paralysis ### Clinical Consequences of Fasciculation | Complication | Mechanism | Clinical Significance | |---|---|---| | Hyperkalaemia | Potassium release from muscle | Can cause cardiac arrhythmias; contraindicated in burns, crush injuries, denervation | | Myoglobinuria | Muscle breakdown products | Risk of acute kidney injury | | Increased intracranial pressure | Muscle contraction increases cerebral metabolism | Avoid in head injury | | Increased intraocular pressure | Direct mechanical effect | Avoid in open-eye injury | | Postoperative myalgia | Uncoordinated muscle contraction | Prevented by pre-treatment with non-depolariser | **High-Yield:** Succinylcholine is the only depolarising agent; all others (vecuronium, atracurium, cisatracurium, rocuronium, pancuronium) are non-depolarising competitive antagonists that do NOT cause fasciculation. **Clinical Pearl:** Pre-treatment with a small dose of non-depolarising agent (e.g. 0.01 mg/kg vecuronium) prevents fasciculations and their complications — a standard practice in Indian anaesthesia protocols. ### Duration - **Succinylcholine**: Ultra-short acting (5–10 minutes) — metabolised by plasma pseudocholinesterase - **Non-depolarisers**: Intermediate (30–40 min) or long-acting (60–90 min)
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.