Phase II block (dual block) can occur with succinylcholine when it is used as:
- A A single large bolus of 1 mg/kg
- B A dose of 0.5 mg/kg in a patient with normal plasma cholinesterase
- C An infusion delivering cumulative doses exceeding 4–6 mg/kg ✓
- D Combined with vecuronium at an induction dose
Correct answer: C. An infusion delivering cumulative doses exceeding 4–6 mg/kg
Explanation
Phase II block develops when the neuromuscular junction is exposed to succinylcholine repeatedly or in large cumulative amounts (typically exceeding 4–6 mg/kg), transforming the initial depolarizing block into a desensitization block that resembles non-depolarizing blockade (fade on TOF, post-tetanic facilitation). Unlike Phase I block, Phase II block can paradoxically be partially reversed by anticholinesterases. A single standard induction dose does not cause Phase II block in patients with normal plasma cholinesterase activity.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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