Phase II block (dual block) following suxamethonium is characterised by which pattern on peripheral nerve stimulator?

• A No fade on TOF, no post-tetanic facilitation
• B TOF count of zero with sustained tetanus
• C Fade on TOF with post-tetanic facilitation, resembling non-depolarising block ✓
• D TOF ratio >0.9 with preserved tetanus at 50 Hz

Explanation

Classical Phase I (depolarising) block shows no fade on TOF and no post-tetanic facilitation; tetanus is sustained and all four TOF twitches are equal. With prolonged or repeated dosing of suxamethonium, Phase II (desensitisation) block develops: the NMJ becomes desensitised, and the block changes character to resemble non-depolarising block — fade appears on TOF, post-tetanic facilitation occurs, and reversal with neostigmine becomes partially effective (unlike Phase I which is worsened by anticholinesterases). This is clinically important in patients with pseudocholinesterase deficiency.

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

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