The Ideal Short-acting Muscle Relaxant

Question 1

Regarding the ideal short-acting muscle acting relaxant (true or false):

Suxamethonium is the ideal short-acting muscle relaxant

Answer 1

False. Whilst suxamethonium produces optimal intubation conditions within 45 seconds and in the majority of individuals has a short duration of block, it has considerable side-effects.

Question 2

Regarding the ideal short-acting muscle acting relaxant (true or false):

It should produce intubating conditions within 1 minute, produce total paralysis for 5-10 minutes, and spontaneous recovery to TOF 0.9 within 15-20 minutes

Answer 2

True

Question 3

Regarding the ideal short-acting muscle acting relaxant (true or false):

The agent should be stable in solution and kept at a temperature of 4°C

Answer 3

False. It should be stable at room temperature and should have a long shelf life.

Question 4

Regarding the ideal short-acting muscle acting relaxant (true or false):

The duration of action is unaffected by hepatic or renal impairment

Answer 4

True

Question 5

Regarding suxamethonium, the following is a recognised side effect (true or false):

Renal impairment

Answer 5

False

Question 6

Regarding suxamethonium, the following is a recognised side effect (true or false):

Epileptiform activity

Answer 6

False

Question 7

Regarding suxamethonium, the following is a recognised side effect (true or false):

MH trigger

Answer 7

True

Question 8

Regarding suxamethonium, the following is a recognised side effect (true or false):

Prolonged apnoea

Answer 8

True (in susceptible patients)

Question 9

Regarding suxamethonium, the following is a recognised side effect (true or false):

Prolonged QT interval

Answer 9

False

Question 10

Regarding suxamethonium, the following is a recognised side effect (true or false):

Anaphylaxis

Answer 10

True. It is the muscle relaxant with the highest propensity to cause anaphylaxis.

Question 11

Regarding suxamethonium, the following is a recognised side effect (true or false):

Pain on injection

Answer 11

False

Question 12

Regarding suxamethonium, the following is a recognised side effect (true or false):

Hyperkalaemia

Answer 12

True

Question 13

It acts as a non-depolarising muscle relaxant (true or false)

Answer 13

False. It acts as a depolarising muscle relaxant.

Question 14

Suxamethonium is metabolised by plasma cholinesterase at the NMJ (true or false)

Answer 14

False. It is metabolised by plasma cholinesterase, but not at the NMJ. Metabolism occurs within the plasma. 80% of an administered dose of suxamethonium is metabolised before it reaches its effect site at the NMJ. The action of suxamethonium is terminated by diffusion down a concentration gradient from NMJ to plasma. The concentration gradient is maintained by metabolism of drug within plasma.

Question 15

Suxamethonium may be associated with myalgia, myoglobinuria and elevated CK levels (true or false)

Answer 15

True. These are all recognised side-effects of suxamethonium and are thought to relate to the fasciculations caused by the drug. Several treatment strategies have tried to obtund the severity of the myalgia. These include precurarisation, dantrolene, magnesium and vitamin C.

Question 16

Suxamethonium is the muscle relaxant most commonly implicated in anaphylactic reactions (true or false)

Answer 16

True. Muscle relaxants as a group are the agents most commonly implicated in anaphylactic reactions occurring during anaesthesia. Within this group suxamethonium accounts for approximately 50% of reactions. Atracurium and rocuronium probably carry an equal incidence accounting for market share. Vecuronium is associated with the lowest incidence of allergic reactions.

Question 17

Suxamethonium is a known trigger of MH in susceptible individuals

Answer 17

True. This is one of several idiosyncratic reactions to suxamethonium. The others are prolonged apnoea and anaphylaxis.

Question 18

Regarding onset and duration of block (true ro false):

Atracurium has a predictable duration of block

Answer 18

True. As a result of metabolism by Hofmann elimination and ester hydrolysis atracurium is unaffected by hepatic or renal impairment.

Question 19

Regarding onset and duration of block (true ro false):

Rocuronium, at a dose of 0.6 mg/kg, produces intubating conditions within 1 minute

Answer 19

False. Rocuronium is capable of producing intubating conditions within this time frame but not at a dose of 0.6 mg/kg. A greater mass of rocuronium, i.e. 1.0-1.2 mg/kg, would be required to ‘flood’ the receptors and increase the onset time.

Question 20

Regarding onset and duration of block (true ro false):

The terminal half-life of rocuronium is significantly pronged in patients with renal impairment

Answer 20

True. 35% of an administered dose of rocuronium is excreted as unchanged drug in the urine.

Question 21

Regarding onset and duration of block (true ro false):

Atracurium can have its onset time reduced by increasing the dose

Answer 21

True. The onset time of any muscle relaxant can be reduced, i.e. more rapid time to achieve block, by increasing the mass of drug administered and thus providing a steeper concentration gradient and a more rapid diffusion of drug to effect site. In the case of atracurium, this would not be a good strategy as with bigger doses there would be increased cardiovascular instability.

Question 22

Regarding onset and duration of block (true ro false):

Rocuronium has a terminal half-life of 60 minutes

Answer 22

False. It is 85 minutes.

Question 23

Regarding side-effects (true or false):

Histamine release is rarely associated with mivacurium

Answer 23

False. Mivacurium is known to cause histamine release. Histamine release is caused by the quaternary ammonium group(s) of the muscle relaxant. Mivacurium is a bisquaternary structure, i.e. it has two quaternary ammonium groups.

Question 24

Regarding side-effects (true or false):

Rocuronium is the agent of choice for patients with a history of hypersensitivity reactions

Answer 24

Rocuronium is associated with an incidence of anaphylactic reactions approximately equal to atracurium. Vecuronium is associated with the lowest incidence of allergic reactions and should be the agent of choice where use of a muscle relaxant cannot be avoided.

Question 25

Regarding side-effects (true or false):

Vecuronium is the agent associated with most cardiovascular stability

Answer 25

True. It is also the muscle relaxant with the lower incidence of allergic reactions.

Question 26

Regarding side-effects (true or false):

Rocuronium can be used safely in patients with burns and spinal cord injuries

Answer 26

True. all non-depolarising muscle relaxants can be safely used in patients with burns and spinal cord injuries, in the absence of any other contraindications. It is suxamethonium, a depolarising muscle relaxant which may cause hyperkalaemia.

Question 27

Regarding side-effects (true or false):

Atracurium causes minimal histamine release

Answer 27

False. Histamine release is caused by the quaternary ammonium group(s) of the muscle relaxant. Atracurium, as with mivacurium, is a bisquaternary structure, i.e. it has two quaternary ammonium groups. Cisatracurium, one of the ten stereoisomers of atracurium, is associated with less histamine release.

Question 28

True or false:

Atracurium is stable in solution and at room temperature

Answer 28

False. It is stable in solution at pH 3.4, as in the ampoule, but must be kept refrigerated at 4°C.

Question 29

True or false:

Vecuronium is a suitable agent for patients with hepatic impairment

Answer 29

False. The pharmacokinetics may be modified due to its hepatic metabolism. Atracurium, or even rocuronium, would be suitable agents.

Question 30

True or false:

Mivacurium has a predictable metabolism

Answer 30

False. It is metabolised by plasma cholinesterase and thus is subject to delayed breakdown with prolonged neuromuscular block in individuals with genetically determined variations of plasma cholinesterase activity, just as occurs with suxamethonium.

Question 31

True or false:

Rocuronium has a terminal half-life unaffected by renal impairment

Answer 31

False. 35% of rocuronium is eliminated as unchanged drug in the urine. In individuals with impaired renal function this results in prolonged bloc, which is clinically detectable.

Question 32

True or false:

Vecuronium is stable in solution when maintained at 4°C

Answer 32

False. Vecuronium is not stable in solution for long periods. It is presented as a white powder, which may be kept at room temperature, with an ampoule of water for reconstitution prior to use.