Phase II block can occur after large or repeated doses of succinylcholine. Which of the following accurately describes a Phase II block compared to Phase I block?

• A Phase II: tetanic fade absent; reversible with neostigmine
• B Phase II: depolarised endplate with no response to any stimulus
• C Phase II: occurs only in pseudocholinesterase deficiency
• D Phase II: resembles non-depolarising block; tetanic fade, post-tetanic potentiation, TOF ratio <0.7; partially reversible with anticholinesterase ✓

Explanation

Phase I (depolarising) block from succinylcholine shows: no TOF fade, no post-tetanic facilitation, augmented by anticholinesterases, no reversal with neostigmine. Phase II (desensitisation) block occurs with prolonged receptor occupancy, typically after cumulative doses >4–6 mg/kg. It resembles non-depolarising block: TOF fade present, post-tetanic potentiation present, and it can be partially reversed with neostigmine (though reversal is unpredictable). The transition from Phase I to Phase II is marked by the appearance of fade on TOF monitoring. Tachyphylaxis occurs with repeated succinylcholine boluses.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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