2019B Question 01
Discuss the potential adverse effects of suxamethonium.
Examiner Report
45.5 % of candidates achieved a pass in this question.
The major domains assessed in this question were cardiovascular effects, musculoskeletal effects, pressure effects (eg intra-ocular), electrolyte changes and other idiosyncratic effects such as anaphylaxis and malignant hyperthermia. Credit was given for other relevant correct material. Mechanisms and clinical context were expected.
Common problems encountered included presenting a long preamble on suxamethonium and a list of side effects with limited discussion on either mechanism or significance; focusing largely on a single domain to the detriment of others; failing to communicate the potential magnitude and severity of hyperkalaemia in at risk patient groups.
The question could be well answered with information in the standard textbooks, with Stoelting perhaps providing the most comprehensive coverage.
Model Answer
Structure:
- List of adverse events
- Genetics
| Effect | Description |
|---|---|
| Unsafe Paralysis | - Can’t intubate, can’t oxygenate → Desaturation → Death - Can’t protect airway → Aspiration |
| Masseter spasm | - ?From malignant hyperthermia, ?Due to under-dosing - Especially in children - May impede intubation |
| Histamine Release | - Direct effect on mast cells. Not immune mediated. - Degranulation of mast cells - H1 (Gq): - Vasodilatation, capillary leak, ↓ MAP - ↓ AV node conduction, coronary vasoconstriction - Bronchoconstriction - H2 (Gs): - ↑ Contractility, coronary vasodilatation - Bronchodilatation |
| Anaphylaxis | - 1 in 2000-2500 - Similar to rocuronium - Depends upon population - 1st exposure: Activation of specific T cell, IgE produced by specific B cell, fixes on mast cells and basophils - 2nd exposure: Systemic degranulation of mast cells, IgE mediated |
| Myalgia | - ?Due to depolarisation → Fasciculation - Risk factors: Young, muscular - Prevention: 5% ED95 non-depolarising relaxant prior (not very effective) - Treatment: Analgesia, NSAID (not very effective) |
| ↑ Intra-ocular Pressure | - Depolarisation → Contraction of extraocular muscle - ↑ 10cmH2O - Avoid in open globe injury - Coughing during laryngoscopy causes greater ↑ IOP |
| ↑ Intra-gastric pressure | - Depolarisation → Contraction of abdo wall - ↑ 10cmH2O - But also ↑ lower oesophageal sphincter tone - Risk of GOR +/- aspiration if barrier pressure ≤13cmH2O |
| Arrhythmia | - Sinus bradycardia, AV block, asystole - Due to agonism at cardiac mAChR - Risk factors: Big dose >2mg.kg-1, young children, AV node pathology - Rx: Atropine |
| Hyperkalaemia | - Depolarisation → Open cation channel → K+ efflux (also Na+ and Ca2+ influx) - Normal: ↑ K+ ~0.5mmol.L-1 - May be significant if renal failure with existing ↑ K+ - Denervation: ↑↑ K+ → Arrhythmia, cardiac arrest - ↑ Extrajunctional receptors with γ-ε substitution → ↑ Channel opening time - e.g. Burns, critical illness myopathy, muscular dystrophy - Highest risk 1 week – 3 months after onset - ECG changes (in order) - Repolarization abnormalities (tall T waves) - >Atrial paralysis (small or absent P wave) - Conduction delay (AV block, wide QRS) - Cardiac arrest ~8-9mM (sine wave, asystole) |
| Suxamethonium Apnoea | - Two stage metabolism by plasma cholinesterase (PChE) to inactive products - Two alleles for PChE. Variations: Normal, dibucaine-resistant (DR), fluoride-resistant, silent - Treatment: Sedate and ventilate in ICU; consider FFP or dialysis - Follow up: Testing of patient and family |
| Malignant hyperthermia | - Hypermetabolic reaction to volatile anaesthetics and suxamethonium - Mutation in RYR1 gene encoding skeletal muscle ryanodine receptor/channel on sarcoplasmic reticulum - Continuous Ca2+ release, tetany, ↑ temp, rhabdo - 80% mortality if untreated - Specific Rx: Dantrolene 1mg.kg-1 up to 10x then infuse 24 hours - General Rx: Stop drug, sedate, intubate, ventilation, cool - Treat complications: e.g. Hyperkalaemia, arrhythmia |
Genetics
| Alleles | Frequency | Dibucaine number | Offset time |
|---|---|---|---|
| 2 x normal | 96% | 80 | 5min |
1 x normal 1 x DR |
≤4% | 60 | 20-30 mins |
| 2 x DR | 0.03% 1 in 30,000 |
20 | 8 hours = suxamethonium apnoea |