Muscle Relaxers: How They Work

Muscle relaxers are drugs that cause relaxation of the striated muscle.

Classification of muscle relaxants

A common classification is the one in which the relaxants are divided into central and peripheral. The mechanism of action of these two groups is differentiated by the level of effect on the synapses. Central muscle relaxers affect the synapses of the spinal cord and the medulla oblongata. Peripheral muscle relaxers affect directly the synapses, which transmit excitation to the muscle. In addition to the above groups, there is a classification that divides muscle relaxants depending on the nature of the effect.

Central muscle relaxants have not been disseminated in anesthesia practice. But peripheral drugs are actively used to relax the skeletal muscles.

There are the following groups:

  • depolarizing muscle relaxants;
  • antidepolarizing muscle relaxants.

There is also a classification based on the duration of action:

  • ultrashort - last 5-7 minutes;
  • short - less than 20 minutes;
  • average - less than 40 minutes;
  • long-acting - more than 40 minutes.

The depolarizing muscle relaxers are short-circuiting: succinylcholine, ditilin. Preparations of short, medium and long-term action are mainly nondepolarizing muscle relaxants. Drugs of a short action: Mivacurium. Medium-acting: atracurium, rocuronium, cisatracurium. Long-acting drugs: tubocurarine, orphenadrine, pipecuronium, baclofen.

How do muscle relaxants work?

Nondepolarizing muscle relaxants are also called nondepolarizing or competitive. This name fully describes their mechanism of action. Nondepolarizing muscle relaxants compete with acetylcholine in the synaptic space. They are tropic to the same receptors. But acetylcholine is destroyed under the influence of cholinesterase in a few milliseconds. Therefore, it is unable to compete with muscle relaxants. As a result of this action, acetylcholine is not able to affect the postsynaptic membrane and cause the process of depolarization. The chain of the neuromuscular pulse is interrupted. The muscle is not excited. To stop blockade and restore conduction, it is necessary to introduce anticholinesterase drugs, for example, proserin or neostigmine. These substances will destroy cholinesterase, acetylcholine will not disintegrate and will be able to compete with muscle relaxants. Preference will be given to natural ligands.

The mechanism of action of depolarizing muscle relaxers is to create a permanent depolarizing effect, lasting about 6 hours. The depolarized postsynaptic membrane is unable to receive and carry out nerve impulses, the chain of signal transmission to the muscle is interrupted. In this situation, the use of anticholinesterase drugs as an antidote will be erroneous, since the accumulating acetylcholine will cause additional depolarization and enhance neuromuscular blockade. Depolarizing relaxants have basically an ultrashort effect.

Sometimes muscle relaxants combine the actions of depolarizing and competitive groups. The mechanism of this phenomenon is unknown. It is suggested that antidepolarizing muscle relaxants have an aftereffect in which the muscular membrane acquires a persistent depolarization and becomes insensitive for a while. As a rule, these are preparations of a longer duration
How to use muscle relaxants?

The first muscle relaxants were alkaloids of some plants or curare. Then their synthetic analogs appeared. It is not entirely correct to call all muscle relaxants curare-like substances since the mechanism of action of some synthetic drugs differs from that of alkaloids.

The main area of application of muscle relaxers was anesthesiology. Currently, clinical practice cannot do without them. The invention of these substances allowed to make a huge leap in the field of anesthesiology. Miorelaxants allowed to reduce the depth of anesthesia, better control the work of the body's systems, created conditions for the introduction of endotracheal anesthesia. For most operations, the basic condition is a good relaxation of the striated muscles.

The effect of muscle relaxants on the functioning of body systems depends on the selectivity of the effect on the receptors. The more selective the drug, the less side effects on the part of the organs it causes.

In anesthesiology, the following muscle relaxants are used: succinylcholine, dithiline, moldaine, myvacurium, cisatracurium, rocuronium, atracurium, tubocurarine, mivacurium, pipecuronium and others.

In addition to anesthesiology, muscle relaxers have found application in traumatology and orthopedics for muscle relaxation in the direction of dislocation, fracture, and also in the treatment of diseases of the back, ligamentous apparatus.

Side effects of muscle relaxants

Muscle relaxants can cause an increase in heart rate and pressure rise. Succinylcholine has a twofold effect. If the dose is small, it causes bradycardia and hypotension, if large - opposite effects.

Relaxants of the depolarizing type can lead to hyperkalemia if the patient's potassium level was initially elevated. This phenomenon occurs in patients with burns, large injuries, intestinal obstruction, tetanus.

In the postoperative period, long-term muscle weakness and pain are undesirable effects. This is explained by the persistent depolarization. Long-term recovery of respiratory function can be associated with both muscle relaxants, hyperventilation, airway obstruction, or an overdose of decararizing drugs (neostigmine).

Succinylcholine is able to increase the pressure in the ventricles of the brain, inside the eye, in the skull. Therefore, its use in the relevant operations is limited.

Muscle relaxants of depolarizing type in combination with drugs for general anesthesia can cause a malignant rise in body temperature. This is a life-threatening condition that is difficult to manage.

Common drugs and their doses
  • Tubocurarine. The dose of tubocurarine used for anesthesia is 0.5-0.6 mg / kg. Administer the drug slowly, within 3 minutes. During the operation, maintenance doses of 0.05 mg / kg are administered fractionally. This substance is a natural curare alkaloid. It has a tendency to decrease pressure, it causes significant hypotension when used in high doses. The antidote for Tubocurarine is Proserin;
  • Dithilin. This drug belongs to the depolarizing type of relaxants. It has a short but powerful effect and creates a well-balanced relaxation of muscles. The main side effects: prolonged apnea, lifting blood pressure. There is no specific antidote. Listenon, succinylcholine, and miorelaxant have a similar action.
  • Diplatcin. It's a nonpolarizing muscle relaxant, which lasts about 30 minutes. A dose sufficient for one operation is 450-700 mg. No significant side effects were observed with its use;
  • Pipecuronium. The dose for anesthesia is 0.02 mg / kg. It lasts for 1.5 hours. Unlike other drugs, it is more selective and does not affect the cardiovascular system;
  • Esmeron (rocuronium). The dose for intubation is 0.45-0.6 mg / kg. The effect lasts up to 70 minutes. Bolus doses during the operation of 0.15 mg / kg;
  • Pancuronium. This drug is also known as Pavulon. The recommended dose for anesthesia is 0.08-0.1 mg / kg. A maintenance dose of 0.01-0.02 mg / kg is administered every 40 minutes. The drug has multiple side effects of the cardiovascular system since it is a non-selective drug. It may cause arrhythmia, hypertension, tachycardia. The drug significantly affects the intraocular pressure. Pancuronium can be used for Caesarean section operations since it does not penetrate well through the placenta.

All these drugs are used exclusively by anesthesiologists-resuscitators in the presence of specialized respiratory equipment. You can buy meds online at 

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Tuesday, 11 December 2018

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