2018B04 Compare and contrast the pharmacology of suxamethonium and rocuronium.

Suxamethonium

Rocuronium

Container

Plastic ampoule

Glass vial or ampoule

Contents: drug and additives

Aqueous solution pH 3.5

(Precipitate with thiopentone pH 11)

Suxamethonium chloride 50mg/mL

Aqueous solution pH 4

Precipitation rare

Rocuronium bromide 10mg/mL

Needs refrigeration

Y

Y

Life at room temp

(accept 10% loss of potency)

2/52

2/12

Needs reconstitution

No

No

Suxamethonium

Rocuronium

Admin:

-ED₉₅ mg/kg

0.3

0.3

-Intubating dose mg/kg

1-1.5 (4x)

0.6-1.2 (2-4x)

-IM

Y

2-3mg/kg

(Useful if larygospasm during paediatric gas induction without IVC)

N

-Intralingual

Y (as for IM)

N

-Phase 2 blockade mg/kg

3-5

N/A

Time course

-Onset IV 2xED₉₅ dose (mins)

0.5-1

(limited by cardiac output)

1-1.5 (<1 if 4x ED₉₅)

-Duration IV (mins)

5 (normal)

(ideal for very short case e.g. ECT)

30-40

(Longer if 4x ED₉₅)

Absorption:

N/A

N/A

Distribution:

-V_DSS

0.2L/kg

0.2L/kg

-%PPB

Unknown

10%

-Crosses placenta

Minimal

Minimal

Metabolism:

-Mode

PChE near 100%

Two stage

Products: succinic acid, choline

Hepatic <5%

-Active metabolites

Nil

17-desacetyl-rocuronium (much less active, hepatic elimination)

-t_1/2β (mins)

1-2

70

Excretion

(minimal)

(95%)

% in bile

-

60%

(liver failure -> prolonged)

% in urine

10% parent

100% metabolites

40%

(renal failure -> prolonged)

Suxamethonium

Rocuronium

Therapeutic effect:

-Action at junctional nAChR

Depolarising blockade

Non-classical receptor effect

?Non-competitive partial agonist

Non-depolarising blockade

Competitive antagonist

Use:

Rapid sequence induction

Y

Y (↑dose -> ↑speed of onset but ↑duration)

Surgical access e.g. laparotomy

Y (but usually too brief)

Y

Infusion

N (develop phase 2 block)

Y (but offset variable)

Monitoring:

-Single twitch

↓

↓

-TOF ratio

Phase 1 >0.7

Phase 2 <0.7

<0.3

-Response to tetany

Phase 1 sustained

Phase 2 fade

Fade

-Post-tetanic potentiation

Phase 1 yes

Phase 2 no

Yes

Side effects:

-Myalgia

Y

N

-Arrhythmia

Sinus bradycardia

AV block

Rare

-↑K⁺

Y

(contraindicated if ↑↑extra-junctional receptors e.g. burns, prolonged immobility, spinal cord injury)

N

-Histamine release

Y

N

-Anaphylaxis (arguments!!)

1 in 2000

1 in 2500

-↑Intragastric pressure

+10cmH₂O (but also ↑lower oesophageal sphincter tone)

-↑Intra-ocular pressure

+10cmH₂O? (avoid if open globe injury)

-Polymorphism -> prolonged paralysis

Rare

(avoid and test if FHx sux apnoea)

N

-Malignant hyperthermia

Y

N

Reversal:

-Spontaneous

Y and fast (5min after 4xED₉₅)

Y but slow (30-40mins after 2xED₉₅)

-AChEi reversal

Phase 1 augmented

Phase 2 antagonised

Antagonised

-Chelation reversal

No

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