Neuromuscular blocking drugs used in anaesthesia are also known as muscle relaxants. By specific blockade of the neuromuscular junction, they enable light anaesthesia to be used with adequate relaxation of the muscles of the abdomen and diaphragm. They also relax the vocal cords and allow the passage of a tracheal tube. Their action differs from the muscle relaxants used in musculoskeletal disorders that act on the spinal cord or brain.
Patients who have received a neuromuscular blocking drug should always have their respiration assisted or controlled until the drug has been inactivated or antagonised. They should also receive sufficient concomitant inhalational or intravenous anaesthetic or sedative drugs to prevent awareness.
The following intravenous muscle relaxants are highlighted as essential knowledge by the RCEM Basic Sciences Curriculum for the MRCEM examination:
- Atracurium
- Suxamethonium
Atracurium
Atracurium is a non-depolarising neuromuscular blocker used to induce muscle relaxation and paralysis to facilitate intubation and controlled ventilation.
Atracurium competes with acetylcholine for nicotinic (N2) receptor binding sites at the post-synaptic membrane of the neuromuscular junction. This prevents acetylcholine from stimulating the receptors. Because the blockade is competitive, muscle paralysis occurs gradually.
The ‘intubating’ dose of atracurium is 0.3-0.6 mg/kg, and subsequent doses are one-third of this amount. Satisfactory intubating conditions are produced within 90 seconds of administration. There is a linear relationship between the dose, and the duration of action of atracurium is non-cumulative with repeated administration.
The duration of action of atracurium is prolonged by the following factors:
- Hypokalaemia
- Hypocalcaemia
- Hypermagnesaemia
- Hypoproteinaemia
- Dehydration
- Acidosis
- Hypercapnia
Histamine release may occur if doses >600 μg/kg are used. This can result in cutaneous flushing, hypotension and bronchospasm. Bradycardia has also been reported.
Suxamethonium
Suxamethonium is a depolarising neuromuscular blocker used to induce muscle relaxation and short-term paralysis, usually to facilitate endotracheal intubation.
Suxamethonium causes ‘persistent’ depolarisation by mimicking the effects of acetylcholine without being rapidly hydrolysed by acetylcholinesterase. It, therefore, inhibits the action of acetylcholine at the neuromuscular junction, and the propagation of an action potential is prevented.
The dose of suxamethonium is 0.5-2 mg/kg, and the usual single dose for an adult is between 50-100 mg intravenously. Infants and younger children are relatively resistant to suxamethonium and usually require a dose of 1-2 mg/kg. The onset of action occurs within 30 seconds, and the duration of action is 3-5 minutes.
Suxamethonium is contraindicated in patients with recent burns but can be given in the first 24 hours following the burn. It is also contraindicated in patients with spinal cord trauma causing paraplegia. It can be given immediately after the injury but should be avoided from approximately day 10 to day 100 after the injury.
Other contra-indications to the use of suxamethonium include:
- Severe muscle trauma
- Hyperkalaemia
- History of malignant hyperthermia
Recognised adverse effects of suxamethonium include:
- Bradycardia
- Hyperkalaemia
- Raised intracranial pressure
- Raised intraocular pressure
- Prolonged paralysis
- Anaphylaxis
- Malignant hyperthermia
Header image used on licence from Shutterstock
Thank you to the joint editorial team of www.mrcemexamprep.net for this ‘Exam Tips’ blog post.
