Drugs Used During Anesthesia: Neuromuscular Blocking Agents
During the course of a general anesthetic, an anesthesiologist may use over ten different medications to induce, maintain and help you emerge from the anesthesia. As an anesthesiologist, I know the value of having such a wide variety of medications available to give the best anesthetic for each patient.
The basic goals of general anesthesia are the same no matter what type of procedure you are having — to keep you unconscious and pain free until it's time to wake up.
Some surgeries require that your muscles be completely relaxed because even when you are unconscious, your muscles may maintain some tone. Also, with general anesthesia, you may be able to breathe on your own, even with a breathing tube in place. But, if a surgeon is operating on a lung, or in your abdomen or even on some joints, it is much easier, and often mandatory, that your muscles be completely relaxed — as in, reversibly paralyzed.
The medicines that we use to paralyze the muscles are called neuromuscular blockers. They can also be referred to as paralytics, NMBs, or neuromuscular blocking agents.
What are Neuromuscular Blockers?
Neuromuscular blockers are medications that act at the neuromuscular junction to stop the muscle from being able to contract.
The microscopic space where muscle fibers and nerve endings come together is called the "neuromuscular junction". The axon terminal of the nerve fiber and the motor endplate of the muscle fiber form this junction.
When the nerve (1) is stimulated (something has 'told' your brain that your muscle needs to move), the vesicles (3) open up to release acetylcholine into the synapse. Acetylcholine is the chemical 'messenger' that activates the muscle to move.
Once across the synapse (space between nerve and muscle in this case), the acetylcholine binds to the acetylcholine receptors (4) on the motor end plate (2). This triggers a chain of electrochemical reactions that causes the muscle fiber to contract and your movement can be completed. This happens at many motor end-plates simultaneously, so that whole muscles can contract with the desired force.
How Do Neuromuscular Blockers Work?
Neuromuscular blockers bind to the acetylcholine receptor (ACh receptor) at the motor end plate. This blocks acetylcholine from being able to bind to it and no muscle contraction can take place.
There are two types of neuromuscular blockers — depolarizing and non-depolarizing.
Depolarizing Neuromuscular Blockers
There is only one depolarizing blocker in practice today. Succinylcholine is not as widely used as it once was, but is still a useful drug in many situations.
Succinylcholine binds to the end plate and stimulates it, causing a depolarization and therefore, muscle contraction. After this muscle contraction is over, though, no further depolarizations can take place until the muscles have recovered.
So, succinylcholine works because it binds to the acetylcholine receptor, causing stimulation of it, and then muscle contraction. It is paralytic because after this depolarization and muscle contraction happens, acetylcholine cannot bind to its receptor for a period of time and no further contractions can occur.
Succinylcholine is used most commonly during the induction of anesthesia. It works very, very quickly and wears off quickly as well. But, it provides reliable 'paralysis'. This is necessary and useful for intubation- placement of the breathing tube after the patient is unconscious.
Non-depolarizing neuromuscular blockers also bind the acetylcholine receptor on the motor end plate. They just block the receptor though, without causing a depolarization. So, there is no muscle contraction, just a blockade of the ability to have one.
Examples of non depolarizing blocking agents in use today are vecuronium and rocuronium.
Each available NMB has its own set of characteristics. The choice of which one to use in a surgery depends on these characteristics (how long does it last, how is it metabolized, what are the side effects, etc).
The Neuromuscular Junction- Neuromuscular Blockers Block the Acetylcholine Receptors Preventing this Reaction from Occurring.
How are NMBs Used during General Anesthesia?
During the induction of anesthesia, as mentioned, the vocal cords and the muscles of the throat may need to be paralyzed if a breathing tube is going to be used. Traditionally, succinylcholine has been used for this purpose since it provides reliable, complete paralysis that wears off quickly and so doesn't last much beyond the time needed for the intubation. But, succinylcholine has some side effects and risks associated with it.
As non-depolarizing agents have been developed, they have gotten safer, more reliable and shorter acting. One agent in particular, rocuronium, has a fast enough onset that it may be used for this purpose. Drugs used for intubation must act quickly because patients are not breathing during the time that the drug is taking effect (they are, however, already unconscious). Older agents either weren't fast enough, or had a less satisfactory safety profile.
Once anesthesia is underway, muscle relaxation must be maintained for surgeries that require it. The nondepolarizers are used for this purpose. A special nerve stimulator can be used to test and see if muscle function has returned. When the muscle twitches after the current is applied, the NMB is wearing off. If it is not close to the end of surgery, more muscle relaxant may need to be given.
At the end of surgery, obviously, the paralytic agent should be allowed to wear off before the patient returns to consciousness. Being awake and paralyzed or profoundly weak is not an enjoyable experience, so anesthesiologists should monitor the level of muscle relaxation carefully as the end of surgery approaches. Without muscle function, the patient will not be able to breathe, so it is essential for weakness to be reversed. Anesthesiologists are trained to administer, monitor and reverse muscle relaxation.
A Word about Anesthesia Awareness
The neuromuscular blockers are not used during every surgery, nor are they the only drug used. Before a patient is 'paralyzed', unconsciousness should be assured.
One of the biggest fears people have about anesthesia is that they will be awake and paralyzed. While true anesthesia awareness under these conditions does happen, it really is quite rare. I try to reassure my patients that our monitors and techniques help us prevent this from happening and that most people who do experience this have risk factors for it. The paralytic makes it harder to detect, but its use does not mean you will have a problem.
(I've had several general anesthetics myself with muscle relaxants and have had no issues, nor do I fear this complication knowing what I know about these or other anesthesia drugs).