Mivacurium is the only non-depolarising muscle relaxant metabolised by plasma cholinesterase. Its duration of action in a patient with dibucaine number of 20 would be:

• A Normal (15–20 minutes) as dibucaine number only affects suxamethonium
• B Shortened due to compensatory enzyme upregulation
• C Normal because mivacurium is also metabolised by hepatic esterases
• D Prolonged (2–3 hours) due to markedly impaired plasma cholinesterase activity ✓

Explanation

Dibucaine number reflects pseudocholinesterase (plasma cholinesterase) activity: normal ≥70 (dibucaine inhibits >70% of enzyme), heterozygous atypical ~50–60, homozygous atypical ~20–30. Mivacurium, like suxamethonium, depends on plasma cholinesterase for rapid hydrolysis. A dibucaine number of 20 indicates homozygous atypical pseudocholinesterase — extremely low enzyme activity — leading to markedly prolonged duration for both suxamethonium (2–3 hours instead of 8 minutes) and mivacurium (2–3 hours instead of 15–20 minutes). Unlike suxamethonium, mivacurium block can be partially reversed by anticholinesterases.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.