Regarding the pharmacology of depolarising and non-depolarising muscle relaxants, all of the following statements are TRUE EXCEPT:

Explanation

## Depolarising vs Non-depolarising Muscle Relaxants: Key Distinctions ### Mechanism of Action **Key Point:** Depolarising agents (succinylcholine) mimic acetylcholine and cause sustained depolarisation, while non-depolarising agents competitively antagonise acetylcholine binding. ### Electromyographic Patterns | Feature | Depolarising (Succinylcholine) | Non-depolarising | |---------|--------------------------------|------------------| | **Initial response** | Fasciculations, then flaccid paralysis | Immediate flaccid paralysis | | **Train-of-four (TOF)** | No fade (all four twitches equal) | Progressive fade (4th < 3rd < 2nd < 1st) | | **Post-tetanic potentiation (PTP)** | Absent or minimal | Present (post-tetanic twitches > baseline) | | **Double-burst stimulation (DBS)** | No fade | Fade present | ### Why the Correct Answer is Wrong **High-Yield:** Non-depolarising agents produce **fade on train-of-four** and **post-tetanic potentiation**. The statement says they produce "sustained contraction on high-frequency stimulation," which is **FALSE** — they actually show fade and potentiation, not sustained contraction. Sustained contraction without fade is a hallmark of depolarising agents. ### Clinical Pearl **Post-tetanic potentiation (PTP)** is a distinguishing feature of non-depolarising blockade. After a 5-second tetanic stimulus at 50 Hz, the post-tetanic twitches are transiently potentiated (stronger than baseline single twitches). This occurs because the non-depolarising agent has a slower dissociation rate, allowing some receptor recovery during the tetanus, followed by temporary acetylcholine accumulation. ### Correct Statements Explained 1. **Succinylcholine depolarisation:** Causes visible fasciculations (muscle twitching) followed by flaccid paralysis — ✓ TRUE 2. **Non-depolarising competitive blockade:** Produces fade on TOF and DBS — ✓ TRUE 3. **Succinylcholine metabolism:** Hydrolysed by pseudocholinesterase (plasma cholinesterase); duration 5–10 minutes — ✓ TRUE 4. **Non-depolarising on high-frequency stimulation:** Shows fade and PTP, NOT sustained contraction — ✗ FALSE **Mnemonic:** **FADE = Non-depolarising; NO FADE = Depolarising** — Train-of-four fade is the gold standard test to distinguish the two classes intraoperatively.