Non-depolarising muscle relaxants are commonly used during anaesthesia to provide relaxation for surgery, to allow mechanical ventilation and they are also regularly used in intensive care. This article describes the mechanisms by which the drugs work and also the differences between specific drugs.
Mechanism of action
Non-depolarising muscle relaxant drugs (NDMRD) compete with acetyl choline (ACh) molecules released at the neuromuscular junction to bind with the ACh receptors on the post synaptic membrane of the motor endplate. They therefore block the action of ACh and prevent depolarisation (or activation) of the muscle contraction process. Muscle groups differ in their sensitivity to muscle relaxants; ocular muscles responsible for opening and moving the eyes are the most sensitive followed by the muscles of the jaw, neck, limbs, intercostals and abdomen. The diaphragm is the least sensitive muscle, which is why patients undergoing surgery sometimes hiccup or breathe as an early sign that the relaxants are wearing off.
Non-depolarising muscle relaxant drugs also act on presynaptic receptors interfering with the entry of calcium which causes an inhibition in the release of ACh. Other drugs such as the aminoglycoside antibiotics (eg gentamicin) and volatile agents may also effect this mechanism and increase sensitivity to relaxants.
A variety of relaxant drugs are in use in different parts of the world. All produce profound muscle paralysis but have varying effects on the autonomic nervous system. None of the drugs cross the blood brain barrier as they are water soluble, polar molecules and therefore have no effect on the central nervous system. All non depolarising drugs should be used with care in patients suspected to be suffering with myasthenia gravis or myasthenic syndrome as patients with these conditions are extremely sensitive to their effects.
General considerations in the use of muscle relaxants
Muscle relaxants are principally used to provide good muscular relaxation for surgery. When they are used respiration must be controlled via an endotracheal tube. A few general guidelines for the use of relaxants are listed below:
1. Always be certain that you will able to ventilate the patient by face mask before paralysing them.
2. If a rapid onset of action is required then suxamethonium should be used as it acts more quickly than any of the non-depolarising drugs. If a short duration of paralysis is required suxamethonium is most suitable and may be given in repeated doses provided atropine is administered prior to the second dose of suxamethonium to avoid bradycardia.
3. Non-depolarising muscle relaxants take about one and a half to two minutes to act and you should allow time for relaxation to develop before attempting intubation.
4. The supplemental dose should be about 25% of the initial dose. Never attempt to reverse the relaxation until at least 15-20 minutes after the last dose of relaxant was given.
5. Never extubate a patient until you are certain that the paralysis has been reversed and they have adequate muscle strength to protect their airway and breathe. One way of testing this is to assess whether they are able to lift their head off the pillow for 5 seconds. Ensure that breathing is of adequate depth and frequency.
6. It takes some time before the larynx is able to protect the airway and so the patient is best placed in the lateral position for recovery.
7. If a nerve stimulator is available it can be used to monitor the degree of relaxation. However it is not essential and relaxants can be safely be used without a nerve stimulator by careful observation of clinical signs.
8. When muscle relxants are administered awareness is always a danger since a paralysed patient cannot move in response to pain. It is therefore essential to ensure that the depth of anaesthesia is adequate.
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