FRCA Notes

Suxamethonium Apnoea

The core curriculum asks us to be able to describe 'the complications of anaesthetic drugs, including suxamethonium apnoea'.

Resources

  • Suxamethonium apnoea is a condition characterised by abnormal plasma cholinesterase function that leads to prolonged duration of neuromuscular blockade from suxamethonium
  • It is an autosomal recessive genetic deficiency of plasma cholinesterase function
  • A single nucleotide polymorphism at a locus on chromosome 3 causes altered enzyme activity

  • Four alleles exist:
    1. Eu (usual) - normal activity
    2. Ea (atypical) - dibucaine resistant
    3. Es (silent) - absent activity
    4. Ef (fluoride resistant)

  • >97% are homozygous for the normal alleles (Eu) and therefore have normal activity
  • 2-4% are heterozygous or homozygous for abnormal alleles, which prolongs suxamethonium activity to varying degrees

  • Multiple acquired factors can reduce plasma cholinesterase activity

Non-drug causes

  • Pregnancy
  • Failure states: cardiac, renal, liver
  • Thyrotoxic crisis
  • Malignancy
  • Massive blood transfusion

Drug causes

  • Multiple drugs can cause acquired plasma cholinesterase deficiency, either by direct interaction or by acting as a competitive inhibitors of cholinesterases
  • Examples include:
    • Metoclopramide
    • Ketamine
    • Lithium
    • Ester local anaesthetics
    • Oral contraceptive pill
    • Cytotoxic drugs
    • Acetylcholinesterase inhibitors e.g. neostigmine, edrophonium
    • Trimetaphan (itself a non-depolarising NMBA)

  • Formal diagnosis is by ascertaining the patient's dibucaine number
  • Dibucaine, a.k.a cinchocaine, is an amide local anaesthetic, which inhibits plasma cholinesterase

  • When given at a concentration of 10-5mol/L and using benzylcholine as a substrate, it will inhibit:
    • Normal Eu:Eu plasma cholinesterase by 80%
    • Variant forms of plasma cholinesterase to a lesser degree e.g. Ea:Ea plasma cholinesterase by only 20%

  • The percentage inhibition of a patient's plasma cholinesterase is called the 'dibucaine number'
  • It indicates the genetic variation of the individual, but does not make an assessment of the amount of enzyme in the plasma

Genotype Incidence Duration of block Dibucaine number
Eu:Eu 96% Normal 80
Eu:Ea 1 in 25 + 60
Eu:Es 1 in 90 + 80
Eu:Ef 1 in 200 + 75
Ea:Ea 1 in 2,800 ++++ 20
Ea:Ef 1 in 20,000 ++ 50
Es:Ea 1 in 29,000 ++++ 20
Es:Es 1 in 100,000 ++++ -
Ef:Es 1 in 150,000 ++ 60
Ef:Ef 1 in 154,000 ++ 70


Suxamethonium apnoea is an anaesthetic emergency and I would seek senior anaesthetic support as well as making a rapid but thorough assessment of the patient

Initial management

  1. Maintain and if necessary secure airway with intubation

  2. Apply high flow oxygen and exclude other causes of failure to breathe

  3. Ensure haemodynamic stability
    • Consider administration of FFP, which contains plasma cholinesterase
    • May be unhelpful if the donor also has suxamethonium apnoea!

  4. If attempted wake-up of patient who remains paralysed, re-anaesthetise immediately
    • Apply nerve stimulator and keep anaesthetised until ToF 4 without fade
    • Keep sedated and ventilated until effects wear off - may require transfer to ICU

Subsequent management