Sun17October0936AM 10

ED95 of a muscle relaxant is the dose required to reduce twitch height by 95% for a graded dose-response, or the dose required to reduce twitch height by 95% in 50% of patients.

The typical intubating dose for non-depolarising muscle relaxants is 2-3 times the ED95

Mivacurium is routinely administered at less than 2 times the ED95 for procedures requiring a short period of muscle relaxation and rapid recovery.

Which one of the following options best explains why mivacurium is the exception to this rule?

(Please select 1 option)

Mivacurium has a high potency

It is metabolised more slowly than succinylcholine via plasma cholinesterase Incorrect answer selected

It is not licenced for endotracheal intubation

Dose related histamine release occurs which frequently leads to tachycardia and hypotension This is the correct answer

There is an increased risk of anaphylaxis

Explanation

Mivacurium is a benzylisoquinolium ester and is presented as an acidic (pH 5.3-5.0) aqueous solution containing 2 mg/ml.

The ED95 has been estimated in the range of 0.08-0.15 mg/kg. Like atracurium, mivacurium releases histamine in a dose related fashion. At doses greater than 0.2 mg/kg, features such as hypotension, tachycardia, and cutaneous signs, such as erythema and flushing occur. These effects are less profound in paediatric patients. Complications can also be reduced by giving the drug slowly (>30 seconds) or in divided doses.

Although mivacurium is metabolised more slowly than succinylcholine, this has no bearing on the dose required for intubation.

Mivacurium is licensed for endotracheal intubation but caution should be taken for the reasons mentioned above.

The high potency of mivacurium increases the onset time but this is not the correct answer.

Despite the effects of histamine release, anaphylaxis with mivacurium is rare. The effects of the histamine release lasts for a few minutes.

Reference:

Barash, PG, et al. Clinical Anaesthesia. 6th ed. Philadelphia; Lippincott Williams & Wilkins: 2012. p 511.

Answer Statistics

1

45%

2

24%

3

3%

4

25%

5

5%

Times answered: 241