A patient receives vecuronium for neuromuscular blockade. At end of surgery, TOF (train-of-four) ratio is 0.6. The anaesthetist administers neostigmine 2.5 mg + glycopyrrolate 0.5 mg. Why is neostigmine NOT fully reliable for reversal when TOF <0.9, and what is the safer alternative?
- A Neostigmine works at all degrees of blockade; no alternative is needed if full dose is given
- B Physostigmine should replace neostigmine as it crosses the blood-brain barrier
- C Neostigmine cannot adequately reverse deep blockade (TOF <0.4); sugammadex 2 mg/kg is the safe alternative for moderate blockade (TOF ≥2 twitches) ✓
- D Edrophonium is preferred as it acts faster and has fewer cholinergic side effects
Explanation
Anticholinesterases (neostigmine) have a ceiling effect — they are unreliable at deep neuromuscular blockade (TOF count 0–1) and may cause cholinergic overstimulation without achieving complete reversal. The target before administering neostigmine is TOF count ≥4 (ideally 4 twitches with fade <25%). Sugammadex (a modified gamma-cyclodextrin) encapsulates and inactivates aminosteroidal relaxants (rocuronium, vecuronium) with a dose-response: 2 mg/kg for moderate block (TOF ≥2), 4 mg/kg for deep block (1–2 twitches), 16 mg/kg for immediate reversal at 3 minutes post-rocuronium. Sugammadex reliably achieves TOF ratio >0.9 irrespective of block depth.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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