Best Muscle Relaxers
Simply put we use muscle relaxants to keep people from moving. The reason you'll see it used in anesthesia is that it's extremely helpful for endotracheal intubation at the beginning of a surgical case. Or, for example, in a large abdominal case where it's easier to pull open the belly when a patient is relaxed.
There are two types of muscle relaxants: depolarizing and nondepolarizing relaxes. As you might gather from the name, the difference between the two is that depolarizing muscle relaxants bind to both subunits of the ACH receptor and cause depolarization of the axon.
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During the initial depolarization you will see that the patient has fasciculations as an action potential is initially created. However, muscular blockade eventually results from the desensitization of the end-plate as the depolarizing agents continue to bind to the ACH receptor.
The nondepolarizing is, on the other hand, are competitive antagonists to the ACH receptor and prevent depolarization of the axon. These relaxants need to bind to only one instead of two alpha subunits on the end plate to prevent opening.
Depolarizers - Succinylcholine
Now let's talk about some examples of each. The depolarizers are pretty easy because there's only one that we use, which is succinylcholine.
Succinylcholine is not broken down by acetylcholinesterase, but by pseudocholinesterase.
This is an important fact to remember as there are patients who lack pseudocholinesterase. So, giving them succinylcholine may result in longer than intended muscle relaxation.
You may see succinylcholine used when you want rapid onset and termination of muscle relaxation. For example, if a trauma patient is coming in and you want to intubate ASAP, alternatively, it's great when a patient is at high aspiration risk because you want to minimize the amount of time that you'd otherwise need to mask when too late the patient before the muscle relaxant would work.
To that extent, it's worth knowing that succinylcholine has a short onset 60 to 90 seconds. And a short duration of 5-10 minutes.
This sounds like a great drug, right? Well, of course, there are downsides, which are also very important to remember. And,unfortunately, most of these things need to be memorized.
The first, you want to avoid giving succinylcholine to patients with hyperkalemia, which could lead to ventricular arrhythmias or patients with ventricular arrhythmias.
You'll see hyperkalemia in patients that have degenerative motor diseases such as spinal cord generation injuries or other neuromuscular diseases.
In addition, patients who are immobile are at high risk for hyperkalemia.
If you want to think about the pathophysiology as to why this is the case. First you have to remember that sodium enters the ion channels as potassium leaves when depolarization occurs.
Extra junctional receptors are normally suppressed by motor activity. Decreased motor activity leads to proliferation of extra junctional receptors, which are more likely to depolarize and open longer than your typical ACH receptor. Thus, releasing more potassium.
The depolarization by Succinylcholine administration on its own will lead to the efflux of potassium into plasma and acute hyperkalemia on top of a pre-existing hyperkalemic condition.
Burns is also another cause of hyperkalemia because of the elevated release of potassium due to damaged cells.
Other things to note about Succinylcholine is that it can cause bradycardia. Succinylcholine can act like ACH nicotinic receptors at the neuromuscular junction as well as muscarinic receptors, which mediate parasympathetic responses, ie bradycardia after stimulating muscarinic receptors in the sinus node.
Succinylcholine can also be a trigger for malignant hyperthermia.
Let's move on to the non-depolarizers.
These are the agents that end on “onium”. Most of these are hepaticly or really cleared. They can be classified according to their length of action by short, medium or long.
You may have heard or read that Rocuronium can have a quick onset if given at higher doses to match the onset of succinylcholine.
While this is true, the difference is that Rocuronium has a longer duration than Succinylcholine. So, if you have a patient that's difficult to ventilate or intubate. And you might need them to be able to breathe on his own after a failed intubation, you'd want something like Succinylcholine, which would wear off much quicker.
Rocuronium by nature is really an intermediate-acting drug but can be used as a short-acting drug.
Another intermediate acting drug that you might see used is Vecuronium. The two other intermediate-acting drugs that I've listed here, Cisatracurium and Atracurium, are worth knowing a little bit about, since these are the ones given for patients with renal failure or hepatic dysfunction.
The main thing to know about these drugs is they undergo Hofmann elimination, which means they're spontaneously broken down at physiologic temperatures.
They have a slow onset and intermediate length of action.
Finally, Pancuronium is the longest-acting with the slowest onset.
There are two main categories of muscle relaxants:
- non-depolarizing (everything else)
Succinylcholine is a rapid and short-acting drug used to rapid sequence intubation. You want to avoid succinylcholine in patients with burns, crush injuries, spinal cord injuries, muscular dystrophy or disuse syndromes.
Other side effects of succinylcholine include increased ICP, muscle fasciculations, post-op muscle aches and malignant hyperthermia.
Finally, the nondepolarizing agents differ in the metabolism and onset duration.