Neuromuscular Blockers: Vecuronium and Rocuronium
Neuromuscular Blockers are Used as Part of a Balanced General Anesthetic
Relaxing the Vocal Cords Allows Easier and Safer Passage of the Breathing Tube
What are Neuromuscular Blockers?
Neuromuscular blockers are a class of drug used on unconscious patients in hospitals and most often in operating rooms. They cause profound muscle relaxation or paralysis. This is advantageous during surgery since muscle paralysis can aid in placement of the breathing tube. They especially make surgery easier for the surgeon since tense muscles are harder to work through and around.
Muscle relaxants can be categorized in several ways. Vecuronium and rocuronium are commonly used "non-depolarizing" neuromuscular blockers (NMBs). Other types of NMBs, such as succinylcholine, are discussed elsewhere.
How Neuromuscular Blockers Work
Nerve endings and muscle fibers meet up at a structure called the neuromuscular junction. Neuromuscular blockers paralyze the muscles by acting at neuromuscular junctions. In order to understand how these medications work, a brief explanation of the anatomy of this structure is helpful.
A signal from the brain or spinal cord travels down a nerve to its ending called the nerve terminal of an axon. This axon contacts or is in very close proximity to a muscle fiber. The signal in the nerve causes a chemical messenger to be released. This chemical -- acetylcholine (Ach) -- crosses the space (junction) to the muscle fiber. On the muscle fiber are receptors that are specifically designed to allow the Ach to bind.
When Ach binds its receptors, it causes contraction of the muscle.
Neuromuscular blockers such as rocuronium and vecuronium get to the receptors and bind to them. They block, but do not activate the receptors. Because of this, the acetylcholine is not able to bind to the occupied receptors. Thus, no muscle contraction can take place.
Diagram of Neuromuscular Junction
Metabolism of Neuromuscular Blockers
How long does the paralysis caused by neuromuscular blockers last? And, why doesn't it last forever?
Each medication has its own pharmacologic profile. This means they each have a unique onset time, metabolism and duration of action. Eventually, the body breaks down and removes the drugs or their by-products from the receptor and from the body.
The drug can be partially reversed as well. There are medications that increase the availability of acetylcholine in the neuromuscular junction. They do this by preventing the breakdown of Ach. When more Ach is available and stays around longer, it eventually wins the battle for the spaces on the receptor and the NMBs are swept away in the blood stream. They are passed through the liver and are metabolized and eliminated from the body.
Pharmacologic Properties of Two Common NMBs
1 minute or less
TIME TO MAXIMAL EFFECT
less than 3 minutes
DURATION OF ACTION
METABOLISM AND ELIMINATION
Liver metabolism with excretion in bile and urine
No significant effect on heart rate of blood pressure. Duration of action may be prolonged in kidney failure
May increase heart rate. May cause histamine release with allergic-like reaction (relatively rare)
Obviously, these substances are extremely dangerous and should only be used by qualified professionals, such as anesthesiologists.
They should not be used on patients who
- are conscious
- are not fully monitored (oxygen level, EKG, blood pressure, etc)
- haven't received adequate oxygen prior to being put under anesthesia whenever possible (trauma patients and some special other cases exist)
- are not attended by a doctor with the ability to manage the airway of an unconscious, paralyzed patient
- don't need to have muscle relaxation for their procedure
Risks and Considerations of Using Neuromuscular Blockers
These medications are safely used millions of times per day in the appropriate setting. When neuromuscular blockers are given, breathing will stop. Other medications MUST be given first to induce unconsciousness. These medicines should NOT be used on conscious patients.
Also, because changes in heart rate and blood pressure can occur, these vital signs must be continually monitored throughout the anesthetic.
Knowing that breathing will stop, it is standard practice to provide oxygen before a patient goes to sleep to fill up the reserve of oxygen in the lungs. The doctor will then be able to safely provide oxygen through a mask, then a breathing tube while assisting breathing.
Only doctors who are able to control the delivery of oxygen and have the proper training and experience to do so, should administer these medicines. Usually, this task is relatively easy and routine. Other time, unexpected difficulties can arise. It is crucial that these medicines only be used in a setting and by people- equipped and trained to handle this situation.
And, as with other medicines used during surgery, the medicines should only be used when needed. Some procedures don't require a breathing tube or full muscle relaxation. In these anesthetics, a neuromuscular blocker may not be used.
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