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How Does the Neuromuscular Blocker Succinylcholine Work?

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I am a board-certified anesthesiologist in Lake Tahoe, California. I write from the perspective of both a doctor and a patient.

Succinylcholine can be a dangerous but useful medication for surgery and emergency airway management.

Succinylcholine can be a dangerous but useful medication for surgery and emergency airway management.

What Is Succinylcholine?

During surgery and a few other select situations (e.g., during resuscitations where a breathing tube must be placed), it is helpful to be able to completely relax (paralyze) the patient's muscles. Obviously, this endeavor should only be undertaken by trained professionals in appropriate situations. Medications called neuromuscular blocking agents are used for this purpose.

Succinylcholine, a neuromuscular blocker used in hospitals and surgery centers, paralyzes the muscles quickly and profoundly. This drug, sometimes shortened to "sux," is given after unconsciousness has been induced by anesthetic agents. Most often, it is given as one dose prior to placing the breathing tube. Helping an unconscious patient breathe is much easier if the chest muscles are relaxed. In addition, since the breathing tube needs to be placed between the vocal cords, it helps to relax the muscles around the cords so that placement is as easy and atraumatic as possible.

(Note: As an anesthesiologist, I personally don't care to use the phrase "paralytic drug" or any similar term. But, if you hear your anesthesiologist use this term, don't panic. They are just talking about routine medications that we use to make surgery safer by reversibly causing profound muscle relaxation while you are under anesthesia).

Other commonly used muscle relaxants or neuromuscular blockers are called "non-depolarizers" because they bind and block the receptor without activating it, and thus there are no depolarizations or contractions.

How Does Succinylcholine Work?

Before understanding how succinylcholine makes muscles relaxed, it is important to have a basic understanding of how a muscle contraction is initiated.

When a muscle needs to move, an electrical signal is sent down a nerve fiber. the axon (end) of the nerve fiber contacts a muscle fiber. This point of contact is known as the neuromuscular junction.

At the neuromuscular junction, the nerve releases a chemical called acetylcholine (Ach for short). Ach crosses from the nerve to the muscle side of the connection and triggers further electrochemical changes that culminate in muscle contraction.

Succinylcholine is a "Depolarizing Neuromuscular Blocker"

Succinylcholine is usually given intravenously and binds to the same receptors as acetylcholine. In doing so, the end effect of succinylcholine is to block acetylcholine from being able to bind or act. The effect of this binding is that sux causes the same subsequent reactions that acetylcholine would. This reaction is called "depolarization," which leads to other reactions that in the end, cause the muscles to contract. After this contraction, however, the acetylcholine can still not bind and no further contractions can occur, causing muscle relaxation or "paralysis".

Because succinylcholine first causes depolarization and contraction, it is classified as a "depolarizing neuromuscular blocker." In fact, it is the only depolarizing blocker in use today. The contractions of the muscles of the body are seen as "fasciculations."

The diagram below shows the neuromuscular junction, the place where the nerve (1) and muscle (2) fibers meet. Acetylcholine is released from its storage vesicles (3) and binds to its receptors on the muscle (4) to initiate muscle contraction.

When succinylcholine is present, it binds to the receptors (4) and blocks them so that acetylcholine cannot bind there.

Succinylcholine binds at the sites meant for acetylcholine. It causes a muscle contraction, and makes it impossible for the muscles to contract again for a period of time, causing profound muscle relaxation, or muscle paralysis.

Succinylcholine binds at the sites meant for acetylcholine. It causes a muscle contraction, and makes it impossible for the muscles to contract again for a period of time, causing profound muscle relaxation, or muscle paralysis.

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Characteristics of Succinylcholine

Onset of Action: approximately 30 to 60 seconds after a full dose, given intravenously.

Duration of Action: the block with succinylcholine starts to wear off in 3 minutes and is completely gone by 15 minutes.

Mechanism of Action: agonist at acetylcholine receptors on motor endplate.

Metabolism: by an enzyme called "pseudocholinesterase," found in the blood.

Succinylcholine Use During Anesthesia

The routine use of succinylcholine may be declining in anesthesia practice as other similar, but potentially safer, medications are getting better. Succinylcholine is still the gold standard by which other muscle relaxants are measured because it works so quickly, profoundly, and reliably.

At the beginning of general anesthesia, a combination of drugs is used to cause unconsciousness. This is necessary to ensure that a patient cannot sense (see, hear, feel) anything during their surgery. Many surgeries will require relaxation of the muscles or will require a breathing tube. In order to place the breathing tube, as mentioned, the muscles need to be relaxed quickly and reliably.

It is important for a muscle relaxant to take effect quickly. Once a patient is unconscious, and certainly once a muscle relaxant starts to work, breathing and oxygen delivery are impaired. The anesthesiologist becomes responsible for assuring these vital functions for the patient. A breathing tube should be placed as quickly as possible after this, and succinylcholine makes that possible.

Sux is not used alone because it has no anesthetic or sedative properties. Other drugs must be used first to ensure unconsciousness to avoid an awake, but "paralyzed" patient.

Side Effects and Risks of Succinylcholine

  • Arrhythmias - Succinylcholine can cause a slowed heart rate. This is most notable in children. Other abnormal heart rhythms are also possible, but less common.
  • Muscle Aches - Because sux causes the muscles to contract (called fasciculation), achy muscles are a common after-effect. There are measures an anesthesiologist can take to help minimize this, but they aren't always completely effective. More muscular people report a higher incidence of muscle aches.
  • Increased Intracranial Pressure - An anesthesiologist must know when not to use succinylcholine. Sometimes, like if a patient is bleeding into their brain, the pressure in the brain is already high and succinylcholine can make it higher. Usually, though, the risk is worth the benefit of getting the breathing tube in quickly in these patients.
  • Increased Intraocular Pressure - Also, if a patient has an injury to the eye where the fluid in the eye could leak, the increase in pressure caused by succinylcholine may cause the fluid to be pushed out. Again, the risk: benefit ratio must be considered.
  • Increased Potassium Level - Potassium is released from muscle cells as a result of succinylcholine administration. For most people, this is an inconsequential event. Some patients will have dangerously high levels of potassium (those with certain neuromuscular diseases, burns, or kidney failure, for example). Succinylcholine is not recommended in these patients.

Allergies and Abnormal Reactions

  • Allergy - Approximately 1:4000 times that succinylcholine is given, a patient will experience a true allergic reaction.
  • Malignant Hyperthermia - MH deserves its own full page, but basically, it is an inherited disorder that causes some people to have a life-threatening reaction to anesthesia. It is very, very rare, but serious. The reaction results in high fever, muscle rigidity, organ failure, and changes in blood chemistry that can lead to death if not treated aggressively and quickly.
  • Pseudocholinesterase Deficiency - Pseudocholinesterase is the enzyme that breaks down succinylcholine and thereby stops its action. Some people inherit abnormal types of pseudocholinesterase. This leads to a prolonged duration of action of succinylcholine. Instead of wearing off in 15 minutes, it can take several or up to 6 hours. Once it is recognized as the problem, the treatment is simply to continue sedation and ventilator to help with breathing until the drug wears off.

This content is for informational purposes only and does not substitute for formal and individualized diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed medical professional. Do not stop or alter your current course of treatment. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.


Chad on April 24, 2019:

Just had a disk replacement between c6 and c7. The Anesthesiologist admittedly was out of sequence and gave the sux before anything else. I thought I was dying, it was my first surgery and I’m 29 years old. The only way I could let him know something was wrong was to shake my feet which I could still move for some reason. During this he put a mask on my face and asked for a couple deep breaths but I couldn’t breathe. About a minute later I was out. The amount of pain and trauma during this is far too much for someone to go through while awake.

Luma hameed on April 04, 2019:

Thank u very much ..good job

Till now Im understand ..

from Iraq

Alexandra Ioffe on February 27, 2019:

I have read in a Medical suspense novel by Robin Cook "Foreign Body" how Succinylcholine was used to murder 3 patients in private hospital settings in India (and all 3 patients were Americans who arrived to India to have their elective surgeries to be done there) and patients' deaths which later occurred were ascribed to death from heart /and heart and stroke causes. And everything from the hospital CEO and up in bureaucracy ladder was done was done to prevent autopsy, by all means, to be done on all those 3 deceased patients. I highly recommend this book to read

JudyA on September 02, 2015:

Found info educational . I was given sux by accident for an outpatient procedure instead of Versed. It was the most horrifying experience of my life. You are completely paralyzed and aware of all that is going on and you can hear but totally unable to do anything to communicate. Thank God the DR came in and saved me.

Mel Flagg COA OSC from Rural Central Florida on August 13, 2012:

Love your hubs Doc!! This one was really interesting and well written as always. Great pic at the beginning too!! :D

Mmargie1966 from Gainesville, GA on August 13, 2012:

Wow Doc, that one was a bit scary. I must admit there is a lot of information that I don't quite understand. You sure have to trust your anesthesiology!

I really like the way you put it all out there. I like that you are informing us lay people. Every time I read one of your hubs I learn a lot. Thank you for that.

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