2019B01 Discuss the potential adverse effects of suxamethonium.

Unsafe paralysis

·   Can’t intubate, can’t oxygenate -> desaturation -> death

·   Can’t protect airway -> aspiration

Histamine release

·   Direct effect on mast cells. Not immune mediated.

·   Degranulation of mast cells

·   H1 (G_q):

o Vasodilatation, capillary leak, ↓mAP

o ↓AV node conduction, coronary vasoconstriction

o Bronchoconstriction

·   H2 (G_s):

o ↑contractility, coronary vasodilatation

o Bronchodilatation

Anaphylaxis

·   1 in 2000-2500

·   Similar to rocuronium

·   Depends upon population

·   1^st exposure: activation of specific T cell, IgE produced by specific B cell, fixes on mast cells and basophils

·   2^nd 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, young children, AV node pathology

·   Rx: atropine

Hyperkalaemia

·   Depolarisation -> open cation channel -> K⁺ efflux (also Na⁺ and Ca²⁺ influx)

·   Normal: ↑K⁺ ~0.5mmol/L

o May be significant if renal failure with existing ↑K⁺

·   Denervation: ↑↑K⁺ -> arrhythmia, cardiac arrest

o ↑ extrajunctional receptors with γ-ε substitution -> ↑channel opening time

o e.g. burns, critical illness myopathy, muscular dystrophy

o Highest risk 1 week – 3 months after onset

·   ECG changes (in order)

o Repolarization abnormalities (tall T waves)

o >Atrial paralysis (small or absent P wave)

o Conduction delay (AV block, wide QRS)

o 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

Masseter spasm

·   ? forme fruste malignant hyperthermia, ? due to under-dosing

·   Especially in children

·   May impede intubation

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 up to 10x then infuse 24 hours

·   General Rx: stop drug, sedate, intubate, ventilation, cool

·   Treat complications: e.g. hyperkalaemia, arrhythmia