Muscle relaxants and reversal agents

Question 1

Doses for muscle relaxants

Answer 1

Suxamethonium: 1.5mg/kg
Rocuronium: 0.6 mg/kg, 1.2 mg/kg for RSI
Vecuronium: 0.1mg/kg
Atracurium: 0.6 mg/kg

Question 2

Onset for muscle relaxants

Answer 2

Suxamethonium: 30 seconds
Rocuronium: 2 minutes at 0.6mg/kg, 30-60 seconds at 1.2mg/kg
Vecuronium: 3 minutes
Atracurium: 3 minutes

Question 3

Duration for muscle relaxants

Answer 3

Suxamethonium: 5 minutes
Rocuronium: 40 minutes
Vecuronium: 40 minutes
Atracurium: 40 minutes

Note with repeat dosing, non-depolarising agents duration of action is prolonged especially with aminosteroid agents that are organ dependent (vec and roc). For infusions its more common to see atracurium (and cisatracurium used)

Question 4

Special features for the muscle relaxants

Answer 4

Suxamethonium: Fastest onset, fast offset, the only depolarising muscle relaxant in use so muscle fasciculations mark the onset of block

Rocuronium: Fastest onset at 1.2mg/kg, direct reversal agent suggamadex, no histamine release

Vecuronium: Intermediate onset, direct reversal agent suggamadex, no histamine release, very low anaphylaxis risk

Atracurium: Intermediate onset, non-organ dependent metabolism

Question 5

Unique side effects of muscle relaxants

Answer 5

Suxamethonium: hyperkalaemia - arrest, higher anaphylaxis risk, malignant hyperthermia, sux apnoea = prolonged action, myalgias, second dose bradycardia

Rocuronium: Higher anaphylaxis risk, hepatic and renal metabolism

Vecuronium: Hepatic and renal metabolism

Atracurium: Histamine release/anaphylactoid reactions

Question 6

Common side effects to all muscle relaxants

Answer 6

High anaphylaxis risk

Definitive muscle paralysis = apnoea (ventilation always required)

Needs reversal agent

Question 7

2 categories of muscle relaxants

Answer 7

Depolarising (sux) and non-depolarising (all others)

Non-depolarising are divided in to aminosteroids (roc, vec) and benzylisoquinoliniums (atracurium, cisatrac)

Question 8

Speed of onset

Answer 8

The less potent the drug, the faster the onset

The less potent, the more particles are needed to exert an effect. More particles mean a higher concentration gradient to drive the diffusion of drug molecule to the site of action.

Rocuronium is the least potent therefore with a dose of 1.2mg/kg dose it acts very quickly

= an application of Fick’s Law of Diffusion

Question 9

What is sux apnoea

Answer 9

Describes prolonged muscle paralysis after the administration of sux due to pseudocholinesterase deficiency (which can be acquired or genetic)

Question 10

Explain genetic pseudocholinesterase deficiency

Answer 10

96% of people have the normal gene with paralysis lasting about 5 minutes

4% may have a heterozygous mutation - paralysis lasting about 10 minutes

A very small number of people have a homozygous mutation whereby paralysis may last for hours

Question 11

How can you acquire a pseudocholinesterase deficiency?

Answer 11

Pregnancy
Kidney and liver disease
Heart failure
Thyroid disease
Drugs (lithium, neostigmine, cytotoxic agents)