How Does the Neuromuscular Blocker Succinylcholine Work?
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).
How Muscle Contraction is Initiated - The Neuromuscular Junction
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 the 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.
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 is 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
- Arryhthmias - 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 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 help of breathing until the drug does go away.