Neuromuscular Blockers

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Neuromuscular blockers

Definition

Neuromuscular blocking agents are a class of drugs primarily indicated for use as an adjunct to anesthesia. Neuromuscular blocking drugs relax skeletal muscles and induce paralysis.

Purpose

Neuromuscular blockers are indicated for a wide variety of uses in a hospital setting, from surgery to trauma care. In surgery, they are used to prepare patients for intubation before being placed on a ventilator and to suppress the patient's spontaneous breathing once on a ventilator.

Description

Neuromuscular blockers relax skeletal muscle tone by blocking transmission of key neurotransmitters through the neuron receptors at the neuromuscular junction (NMJ). They are divided into two major categories, depolarizing and non-depolarizing neuromuscular blockers, corresponding to the manner in which they exert their therapeutic effect. Depolarizing neuromuscular blocking agents mimic the effects of the neurotransmitter acetylcholine (ACh) and change the interaction between ACh and neuron receptors. Blockade occurs because membranes surrounding the neuromuscular junction become unresponsive to typical ACh-receptor interaction. Non-depolarizing neuromuscular blockers bind to receptors to prevent transmission of impulses through ACh neurotransmitters.

Neuromuscular blockers are primarily used in a clinical or hospital setting. In the United States, they are known by several generic and brand names, including atracurium (Tracurium), cisatracurium (Nimbex), doxacurium (Neuromax), mivacurium (Mivacron), pancuronium (Pavulon), pipecuronium (Arduan), rocuronium (Zemeron), succinylcholine (Anectine), tubocurarine, and vecuronium (Norcuron).

A physician will decide which neuromuscular blocking agent, or combination of neuromuscular blocker and other type of anesthesia, is appropriate for an individual patient. During surgical anesthesia, neuromuscular blockers are administered after the induction of unconsciousness, in order to avoid patient distress at the inability to purposefully move muscles. Neuromuscular blockers can be used on pediatric patients.

Recommended dosage

Neuromuscular blocking agents are most often administered though an intravenous (IV) infusion tube. Typically, the time in which the medicines begin to exert their effects and duration of action are more predictable when neuromuscular blocking agents are administered via IV. Dosages vary depending on the neuromuscular blocking agent used and the duration of action desired. The age, weight, and general health of an individual patient can also affect dosing requirements.

Depolarizing and non-depolarizing agents are grouped together into three categories based on the time in which they begin to exert their anesthetic effects, causing muscle relaxation or paralysis and desensitization, and the duration of those effects (duration of action). Short-acting neuromuscular blockers begin to work within 30 seconds to two-and-a-half minutes and have a typical duration of action ranging from five to twenty minutes. Short-acting agents include mivacurium, rocuronium, and succinylcholine. Intermediate-acting agents exert their effects within two to five minutes and typically last for twenty to sixty minutes. Atracurium, cisatracurium, pancuronium, and vecuronium are intermediate-acting neuromuscular blockers. Long-acting neuromuscular blocking agents take effect within two-and-a-half to six minutes and last as long as 75–100 minutes. Doxacurium, pipecuronium, and tubocurarine are long-acting neuromuscular blocking agents.

The duration of action of any neuromuscular blocking agent can be prolonged by administering smaller supplemental (maintenance) doses via IV following the initial blockade-creating dose.

Precautions

Each neuromuscular blocking agent has its own particular precautions, contraindications, and side effects. However, many are common to all neuromuscular blockers. Neuromuscular blocking agents may not be suitable for persons with a history of lung diseases, stroke , increased intracranial pressure, increased intraocular (within the eye) pressure as in glaucoma, liver or kidney disease, decreased renal function, diseases or disorders affecting the muscles, angina (chest pain ), and irregular heartbeats and other heart problems. Neuromuscular blockers are not typically used on patients with recent, severe burns, elevated potassium levels, or severe muscle trauma. There is an increased risk of seizure in patients with seizure disorders such as epilepsy .

Neuromuscular blockers can be administered to patients who have suffered a spinal cord injury resulting in paraplegia (paralysis) immediately following the injury. But further use of neuromuscular blockers is typically avoided 10–100 days after the initial trauma.

Patients who are obese or have increased plasma cholinesterase activity may exhibit increased resistance to neuromuscular blocking agents. Some cholinergic stimulants that act as cholinesterase inhibitors , including medications used in the treatment of Alzheimer's disease , may enhance neuromuscular blockade and prolong the duration of action of neuromuscular blockers.

With careful supervision, neuromuscular blocking agents can be used in pediatric patients. However, rare but serious complications such as bradycardia (decreased heart rate) are more likely to develop in children than in adults.

Placental transfer (passing of the medication to the fetus) of neuromuscular blocking agents is minimal. Histamine release is associated with neuromuscular blocking agents tubocurare and succinylcholine. Complications such as bronchospasm, decreased blood pressure, and blood clotting problems could arise in patients especially sensitive or susceptible to changes in histamine levels.

Side effects

In some patients, neuromuscular blockers may produce mild or moderate side effects. Anesthesiologists (specialists in administering anesthesia and treating pain) may notice a slight red flushing of the face as neuromuscular blockers are administered to the patient. After completion of the surgical procedure, headache , nausea, muscle soreness, and muscle weakness are the most frequently reported side effects attributed to neuromuscular blockers. Most of these side effects disappear or occur less frequently after a few hours or days.

With depolarizing neuromuscular blocking agents, fasciculations (involuntary muscle contractions) may occur before the onset of muscle relaxation or paralysis. Some patients report generalized muscle soreness or pain after taking a neuromuscular blocking agent that causes fasciculations. Women and middle-aged patients reported this side effect more frequently.

Other, uncommon side effects or complications associated with neuromuscular blockers can be serious or may indicate an allergic reaction. As neuromuscular blockers are most frequently used in trauma, surgical, and intensive hospital care, physicians may be able to counteract the following side effects or complications as they occur:

bradycardia
cessation of breathing
severe bronchospasm
prolonged numbness in the extremities
extended paralysis
jaw rigidity
skeletal muscle atrophy or trauma
impaired blood clotting
severe decrease in blood pressure
chest pain or irregular heartbeat

Interactions

Neuromuscular blocking agents may have negative interactions with some anticoagulants, anticonvulsants (especially those also indicated for use as skeletal muscle relaxants), antihistamines, antidepressants, antibiotics, pain killers (including non-prescription medications) and monoamine oxidase inhibitors (MAOIs).

Cholinergic stimulants, some insecticides, diuretics (furosemide), local anesthetics, magnesium, antidepressants, anticonvulsants, aminoglycoside antibiotics, high estrogen levels, and metoclopramide (Reglan) may affect the duration of action of neuromuscular blocking agents.