Neuromuscular Blocking Agents Flashcards
What are the clinical indications for use of a neuromuscular blockade?
Relax skeletal muscles for surgical access
Facilitate control of ventilation
Facilitate tracheal intubation in cats/pigs
Ophthalmic surgery
Assist reduction of fresh dislocations and fractures?
Reduce amount of anaesthetic required?
Describe the physiology of the neuromuscular junction.
Motor neurone and muscle cell separated by synaptic cleft
Acetylcholine (ACh) released from nerve ending
Binds to post-synaptic nicotinic receptor
Two subunits must be bound
Results in muscle contraction
ACh rapidly hydrolysed by acetylcholinesterase within synaptic cleft
What must be available if neuromuscular blockades are used in a patient?
Facilities for ET intubation and IPPV!
What analgesic and anaesthetic effects do NMBs have?
None!
Describe onset/duration of the depolarising muscle relaxant Suxamethonium.
Fast onset
Duration 3-5 mins in cats, 20 mins in dogs
How is Suxamethonium broken down?
By pseudocholinesterase/plasma cholinesterase
What are the possible side effects of the depolarising muscle relaxant Suxamethonium?
Initial muscle fasciculation
Malignant hyperthermia
Increase serum potassium levels (care with urinary issues/burns)
Describe non-depolarising (competitive) muscle relaxants.
Compete with ACh for post-junctional binding sites
No initial muscle fasciculation
Relatively slow onset
Can be antagonised
Describe the non-depolarising muscle relaxant Atracurium.
Bis-isoquinolinium compound, mixture of 10 isomers
Not wholly hepatically metabolised (choice for renal/hepatic compromise)
Histamine release - give via slow IV
Stored in the fridge
What important isomers are associated with Atracurium?
Cisatracurium - only active isomer in Atracurium (more expensive to buy isolated)
Laudanosine - compound produced by metabolism
Describe the non-depolarising muscle relaxant Vecuronium.
Steroid compound
No histamine release
40-50% undergoes hepatic biotransformation
Powder - stable for 24hrs once reconstituted
What should we monitor when using NMBs?
IPPV resulting in effective ventilation
ET tube not kinked/dislodged
Breathing system connected to animal
Movement of thoracic wall
ETCO2 and SpO2
What are the signs of inadequate depth of anaesthesia?
Increase in pulse rate/BP/ETCO2
Salivation/lacrimation
Vasovagal response (bradycardia/hypotension/pallor)
Slight muscle twitching
Pupillary dilation
How can we monitor the efficacy of an NMB?
Peripheral nerve stimulator
Ulnar/peroneal/facial nerve
Train of four (4 electrical impulses applied to nerve over 2 second period - twitch strength should decrease/disappear)
Only monitors degree of NMB not depth of anaesthesia!
What factors can influence duration of an NMB?
Volatile agent
Hypothermia - prolong
Hepatic/renal insufficiency - prolong
Electrolyte/acid-base abnormalities
Muscle diseases e.g. myasthenia gravis - smaller doses required
Aminoglycoside antibiotics - prolong
Dose administered
