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Pharmacology - Muscle relaxants > Muscle relaxants > Flashcards

Muscle relaxants Flashcards

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14
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AChE has two sites at which ACh binds.

What are these sites and how do they differ?

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AChE has two sites at which ACh binds.

The anionic site to which choline is ionically bound to hold ACh close to the enzyme to allow activity to occur

The esteratic site where the actual breaking of the ester linkage takes place

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What do you think are the most likely target sites at the NMJ for these venoms and toxins?

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24
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Under normal conditions, how long is ACh present in the synaptic cleft?

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ACh is present in the synaptic cleft for only a very short time, less than 10 ms, because it is rapidly metabolized by acetylcholinesterase.

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25
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What happens when acetylcholinesterase is inhibited irreversibly by organophosphates?

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When acetylcholinesterase is inhibited irreversibly by organophosphates, ACh remains in the synaptic cleft and flaccid paralysis ensues. The paralysis resulting from ACh in the presence of organophosphate poisoning is exactly the same as that seen with SCh: agonists can cause paralysis.

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What percentage of nicotinic acetylcholine receptors need to be blocked before any clinical effect is observed?

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More than 75% of all receptors need to be blocked.

57
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What is the ED95 of a drug?

A

The effective dose of drug that depresses the measured twitch strength in response to a single stimulus of the motor nerve by 95% is called the ED95. After this dose, the twitch height is just 5% of the pre-drug value. Intubation of the trachea requires twice the ED95of the drug.

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Do you know what the unique feature of atracurium is?

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The unique feature of atracurium that is different from any previous neuromuscular blocking drug is that it undergoes spontaneous decomposition by a chemical reaction that is dependent only upon the pH and temperature.

This is Hofmann elimination and it is not dependent on liver or kidney function.

Consequently, atracurium can be used in patients with deranged organ function in the absolute certainty that neuromuscular blockade will wear off.

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Cisatracurium is more potent than atracurium and its onset time is correspondingly longer.
Why?

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The majority of the molecules of a non-depolarizing neuromuscular blocking drug in the neuromuscular junction are bound to the post-junctional nicotinic acetylcholine receptors.
Almost all of the acetylcholine receptors must be occupied before neuromuscular block occurs.
Therefore, the number of molecules of a non-depolarizing relaxant that need to enter the neuromuscular junction is roughly constant over a wide range of drugs.
Fewer molecules of a more potent drug are given, so the diffusion gradient is smaller and the onset is slower.

62
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How does being highly ionized affect the drug?

A

They have a relatively small volume of distribution that is equal to or slightly larger than the extracellular fluid volume, and they are not absorbed from the gastrointestinal tract and do not cross the blood-brain barrier. They cross the placenta only in small quantities.
The highly ionized nature means that these drugs and their metabolites are partially renally excreted.

63
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Do you know what the duration of action of the isomers of mivacurium is?

A

The most significant isomers (trans-trans and cis-trans) have a half-life of about 3 minutes.
The minority isomer (cis-cis) is metabolized much more slowly and half-life is over 30 minutes, but it is also less potent than the other two isomers.

64
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How is mivacurium metabolized?

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Like succinylcholine it is metabolized in the plasma by plasma cholinesterase. The enzyme metabolizes mivacurium at 88% of the rate of succinylcholine.

65
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Would you reverse mivacurium?

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Pharmacological antagonism with neostigmine is recommended, but when spontaneous recovery is underway it progresses rapidly, and pharmacological reversal of mivacurium gains relatively little.
Neostigmine tends to inhibit the activity of plasma cholinesterase.

66
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How is atracurium metabolized?

A

Hofmann elimination and ester hydrolysis.

Forty-five per cent of atracurium undergoes Hofmann elimination to laudanosine and a mono-quaternary acrylate.

The remainder of the drug is metabolized by non-specific ester hydrolysis to a mono-quaternary alcohol and a mono-quaternary acid.

Up to 10% of atracurium is excreted unchanged in the urine

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Is the elimination of atracurium affected by changes in pH?

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Variation of pH in the clinical range does not significantly affect atracurium elimination.

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Do you know how cisatracurium is metabolized?

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Hofmann elimination and ester hydrolysis.

Seventy-seven per cent of cisatracurium undergoes Hofmann elimination to laudanosine and a mono-quaternary acrylate.

The mono-quaternary acrylate is hydrolysed to a mono-quaternary alcohol that undergoes further Hofmann elimination to laudanosine.

Cisatracurium undergoes minimal ester hydrolysis and up to 15% of the drug is excreted unchanged via the kidneys.

In renal failure there is a very slight increase in half-life.

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Which drug produces more laudanosine: atracurium or cisatracurium?

A

Because cisatracurium is more potent than atracurium, fewer molecules are given and less laudanosine is produced. Laudanosine levels after a bolus dose are about 10% of those after an equipotent dose of atracurium.

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Pharmacology - Muscle relaxants (1 decks)

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