Phase II block (dual block) occurs with succinylcholine under which circumstance, and how does it differ from Phase I block on TOF stimulation?

• A Phase II block occurs after a single large dose of succinylcholine; it shows no fade on TOF, identical to non-depolarising block
• B Phase II block is specific to pseudocholinesterase deficiency patients and represents enzyme saturation
• C Phase II block is reversed by further succinylcholine administration and worsened by anticholinesterases
• D Phase II block develops with large cumulative doses (>3–4 mg/kg) or prolonged infusion; unlike Phase I (no fade), Phase II shows TOF fade and post-tetanic potentiation — similar to non-depolarising block ✓

Explanation

Phase I block (depolarising) from succinylcholine: sustained depolarisation of end plate, NO fade on TOF, NO post-tetanic potentiation, NOT reversed by anticholinesterases (worsened). Phase II block (dual block) occurs after large cumulative succinylcholine doses (typically >3–4 mg/kg or prolonged infusion >30 min): the block transitions to a state resembling non-depolarising blockade with TOF fade, post-tetanic potentiation, and partial reversal by neostigmine. Mechanism involves receptor desensitisation and intracellular changes. Clinically, this manifests as unexpectedly prolonged paralysis after a succinylcholine infusion.

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

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