Questions – Skeletal Muscle Relaxants

by Pravin Shukle, MD

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    00:01 Let's try a question. Mitch don't twitch. Mitch is a patient who was given a muscle relaxant.

    00:07 During administration, the patient had a slight drop in blood pressure and a slight increase in heart rate.

    00:13 The patient did not have any twitching during induction.

    00:16 Question. Which of the agents was likely used as a muscle relaxant? Would it be succinylcholine? Would it be pancuronium? Would it be tubocurarine? Would it be baclofen? Or would it be dantrolene? The best answer here is pancuronium.

    00:35 Technically, you could say C, D and E are okay as well. Let's take a look at the explanation.

    00:41 So, succinylcholine is a depolarizing agent. The patient will often twitch during that first phase of injection.

    00:49 Once the first phase is finished, they don't twitch anymore. They are not reversed by acetylcholinesterase inhibitors.

    00:56 Tubocurarine is an old drug. It was used back in the 1900. It's not really used much anymore.

    01:02 It has a similar action to pancuronium though. Baclofen is a great drug but it's an antispasmodic.

    01:10 It has an action at the spinal cord. Now dantrolene, once again, used in malignant hyperthermia, it is not commonly used outside of that use.

    01:20 Now, remember pancuronium can block muscurinic cholinergic receptors in the heart, causing tachycardia.

    01:27 And it may drop the blood pressure initially. But really, these are minimal effects.

    01:32 I've used pancuronium many times and I've never had worrisome drops in my experience, and I've never had worrisome in rise in heart rate. There is small changes but they are not big.

    01:43 So, the answer here is going to be pancuronium.

    01:48 Let's go on to a different question. Mitch won't twitch now. So, Mitch is a patient who was given a muscle relaxant.

    01:55 During administration, the patient had a slight drop in blood pressure and a slight increase in heart rate.

    02:00 After surgery, he was given glycopyrrolate which is an atropine-like compound, and pyridostigmine.

    02:08 Which of these agents could be potentiated by the pyridostigmine during the first phase of blockade? Is it succinylcholine? Is it pancuronium? Is it tubocurarine? Is it baclofen? Or is it dantrolene? Good. So, this is not the kind of question that you'll get on a exam. But I'm putting this question to you now so that when you do questions like this on the exam, it will be much easier.

    02:38 I made this question harder. And let's go through the answers so that you really understand the signs.

    02:44 Succinylcholine is a depolarizing agent. These patients often twitch during the first phase of injection.

    02:52 They are not reversed by acetylcholinesterase inhibitors during the first phase of blockade.

    02:58 Now, during phase 1, pyridostigmine may actually increase blockade because succinylcholine is depolarizing the muscle alongside the increased acetylcholine in the cleft. So in other words, they may have more twitching because there's more depolarization because there's more acetylcholine.

    03:20 Now, one more piece of information I want to give you is that acetylcholinesterase does not hydrolyze succinylcholine.

    03:27 So, when succinylcholine is getting broken down, it's getting broken down outside of the acetylcholinesterase pathway.

    03:33 And therefore, that's why these agents don't increase succinylcholine clearance.

    03:42 Next question, which of the following drugs can cause hyperkalemia and cardiac arrest in patients with a spinal cord injury? Succinylcholine, pancuronium, tubocurarine, baclofen or dantrolene? Right, succinylcholine. Succinylcholine is a depolarizing agent. Muscle contraction may result in potassium leakage from muscle to serum. Patients with spinal cord injury have no resting tone. And depolarizing these muscles will therefore be more disruptive to the muscles, and you may see more potassium being released.

    04:24 Spinal cord injury patients also tend to have more potassium release through another unknown mechanism on top of what I just described.

    04:36 Let's take a look at a patient who has epilepsy and burns. A 15-year-old female with a history of epilepsy was burned during a house fire. She is being prepped for surgery.

    04:47 Which of the following medications could be used as a muscle relaxant while reducing seizure threshold? Would it be A, diazepam? B, baclofen? C, rucoronium? D, tubocurarine? Or E, succinylcholine? Here's the answer, diazepam is a benzodiazepine, and once again, it acts through the GABA A receptor to cause less membrane depolarization in the brain, the midbrain, and in the spinal cord.

    05:21 What this does is it results in increased seizure threshold and fewer seizures.

    05:27 So, this is a great drug to use in this case. The other agents that you probably have seen are more for muscle relaxants, it's not really relevant here, so this is why diazepam was the answer.

    05:40 Alright, there you have it. I'm sure that you'll do very well on your exams, just keep in mind that succinylcholine isn't the only depolarizing agent, and it is not reversed by the acetylcholinesterase. And the other issue that I want to make sure that you remember is malignant hyperthermia, dantrolene, less than 1 minute. Good luck.

    About the Lecture

    The lecture Questions – Skeletal Muscle Relaxants by Pravin Shukle, MD is from the course CNS - Pharmacology.

    Included Quiz Questions

    1. It can block the muscarinic cholinergic receptors in the heart.
    2. It has no antidote.
    3. Fasciculations are expected in the first phase of the blockade.
    4. Malignant hyperthermia is one of the life-threatening complications of pancuronium.
    5. Its main site of action is the presynaptic terminal.
    1. It is not reversed by acetylcholine esterase inhibitors.
    2. It has a slow onset of action.
    3. It causes tachycardia and hypotension in the first phase of the injection.
    4. It stimulates the cholinergic receptors in the central nervous system.
    5. It is only active on receptors in the heart.
    1. Diazepam
    2. Pancuronium
    3. Succinylcholine
    4. Tubocurarine
    5. Rocuronium

    Author of lecture Questions – Skeletal Muscle Relaxants

     Pravin Shukle, MD

    Pravin Shukle, MD

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    no time to relax crunch time..
    By Rufaro M. on 20. November 2020 for Questions – Skeletal Muscle Relaxants

    Concepts explained in a logical manner great for revision and exam preparations.