Muscle relaxants Flashcards
What are the indications for muscle relaxants?
Patient (risk of regurg) - Obese, pregnant, hx of reflux, hiatus hernia, unstarved, bowel obstruction, delayed gastric emptying.
Anaesthetic - ability to control gasses required (ventilation), resp disease, abnormal positioning required, ETT placement is easier.
Surgical - microsurgery (NB no movement), muscle relaxation required (eg laparotomy, ortho), ECT therapy
What must be given before any general muscle relaxant?
A hyponotic agent (avoid awareness!)
Apart from muscle relaxants, how else can muscle relaxation be acheived?
- Regional anaesthesia (spinal, epidural, nerve block)
2. High dose volatile
What are the two categories of neuromuscular blockers?
Depolarising
Non-depolarising
Three sites at which action potential can be blocked to achieve muscle relaxation
- Nerve membrane
- Neuromuscular junction
- Muscle membrane
How do depolarising NMBs work?
Similar structure to ACh. Non-competitive binder of ACh receptors. These agents act by depolarizing the sarcolemma of the skeletal muscle fiber. This persistent depolarization makes the muscle fiber resistant to further stimulation by ACh. There are two phases to the depolarizing block. During phase I (depolarizing phase), they cause muscular fasciculations (muscle twitches) while they are depolarizing the muscle fibers. Eventually, after sufficient depolarization has occurred, phase II (desensitizing phase) sets in and the muscle is no longer responsive to acetylcholine released by the motoneurons. Not reversible.
How do non-depolarising NMBs work?
Competitive binding of ACh receptors. DO NOT depolarise membrane simply prevent ACh from binding. Reversed by giving high dose of ACh.
What is ED(95) for NMBs?
The dose of NMB required to paralyse 95% of normal poeple.
What is the intubating dose of NMBs?
2 x ED(95)
Intubating dose of NMBs leads to…
- Loss of muscle tone (cannot maintain airway)
- Inability to breath or cough
- Loss of protective reflexes (esp. laryngeal)
What factors potentiate/prolong the action of muscle relaxants?
- Drugs (Aminoglycosides[eg. Genta] and inhalational anaesthetics)
- Electrolytes (decreased Ca, increased Mg, and change in K)
- Acidosis
- Temperature (Hypothermia = depolarisers, hyperthermia = non-depolarisers)
- Inherited muscle abnormalities (Myasthenia gravis, dystrophies etc)
Most common NMB depolariser
Suxamethonium (ultrashort-acting)
NMB used in very short cases (short acting)
Mivacurium
Most commonly used NMBs in healthy patients
Vecuronium (intermediate acting)
Rocuronium (intermediate acting)
Best NMB to use in patients with renal failure
Cisatracurium (intermediate acting)
[can use atracurium but it can cause histamine release and hypotension]
Long acting NMB used in cardiacs and long cases
Pancuronium
When is suxamethonium contraindicated?
Any condition with high potassium as it causes K to be released from cells.
1. Renal failure
2. Massive tissue injury (burn or crush injury)
3. Disuse of muscle (paraplegic, stroke, end stage AIDS)
Careful in conditions with decreased pseudocholiesterase activity:
1. Hypothermia
2. Liver disease
3. Pregnant women
4. Certain drugs
Possible side effects of suxamethonium
- Scoline pains
- Hyperkaleamia (an increase of about 0.5 is seen)
- Bradycardia (ACh-like structure increases parasympathetic tone)
- Malignant hyperthermia
- Scoline apnoea (different pseudocholineesterase structure)
What are the treatment options for scoline apnoea?
Give FFP. This will contain the enzymes needed to degrade suxamethonium. However, risk of infection and transfusion related acute lung injury (TRALI)
Therefore, sedating and ventilating for several hours waiting for spontaneous resolution is preferential.
How is suxamethonium metabolised?
By plasma- or psuedocholinesterases (synthesised by the liver) present in the plasma.
Two different structure of non-depolarising NMBs
benzyl-isoquinolines
amino-steroids
Why do non-depolarisers have no CNS effect?
They are highly ionised and water soluble. Tightly bound to plasma protiens.
Therefore, no penetration of lipid barriers.
When choosing intubation dose of non-depolarising NMB what must be taken into account?
Patients lean body mass
What is the likely cause of a sudden deterioration in a patient’s haemodynamic status following administration of a NMB?
Anaphylaxis