Local anaesthetic techniques Flashcards
What is local anaesthesia?
Loss of sensation due to blockade of action of potential transmission in nerve fibres
reduced sensory input to the spinal cord and brain
What different methods can be used to apply local anaesthesia?
Epidural Topical Infiltration Perineural injection Regional blocks Epidural
How do local anaesthetics work?
Block voltage gated sodium ion channels and prevent the depolarisation of the nerve cell
Preventing an action potential from generating
What are the two types of anaesthetic block drugs?
Ester linkage Amide linkage (more popular)
What is the main determinant of potency for local anaesthetic drugs?
Lipid solubility
What is the main determinate that effects the duration of local anaesthetic drugs?
Protein binding
Highly bound drug stays within the lipoprotein of the nerve membrane longer
How does the pKa effect an anaesthetic drug?
Higher the pKa the less of the unionised base is present
only the unionised drug can penetrate the never membranes
Lower the pKa more of the ionised drug is available therefore the faster the onset
What happens to the local anaesthetic drugs in a acidic enviroment?
Higher proportion of the local anaesthetic is ionised
reduces the speed of onset
What is the pKa of lidocaine?
7.7 25% unionised at Ph 7.4
What is the pKa of bupivicaine?
8.1 15% unionised at pH 7.4
When may tissue be more acidic?
Inflammed tissue
Which nerve fibres are effected first once local anaesthesia is injected?
Unmylienated sensory fibres
Less distance between sodium channels less of them need to be blocked to prevent an action potential
A gamma fibres are blocked first
Motor nerves less sensitive
Why are local anaesthetic techniques helpful?
pre-emptive anlgesia
Multi-modal analgesia (reduces dosing of other drugs)
Reduces central sensitisation
Reduced requirement for maintenance analgesia
What are the characteristics of an ester local anaesthetic block (Procaine)?
Short duration
rapidly metabolised by plasma cholinesterases
What are the characteristics of an amide local anaesthetic drug?
Lidocaine bupivicaine
Liver metabolised
Longer duration of action
What does the addition of adrenaline with a local anaesthetic block cause?
Vasconstriction of blood vessels in the area
Prolonged onset of action
Prolonged duration
List the onset of action, the duration of action and the toxicity of lidocain
5-10 minutes
1.5-2 hours
Fairly toxic
Can be given intravenously
List the onset of action, the duration of action and the toxicity of bupivicaine
20-30 minutes 6-8 hours Cannot be given intravenously Good motor and sensory nerve separation Greater potential for cardiotoxicity
List the onset of action, the duration of action and the toxicity of Mepivicaine
slightly less toxic than lidocaine
useful in equine lameness diagnosis (less post-injection odema)
Longer duration of action 2-3 hours
What does systemic toxicity depend on?
Site of injection -vascualr sites lead to rapd absorption greater peak in plasma concentration -reduce dose at area high vascularity Drug used -Bupivicaine causes cardiotoxicity Speed of injection -only when giving iv Addition of adrenaline -local vascoconstriction
What cardiovascular effects can be caused by the administration of local anaesthetics?
hypertension tachycardia hypotension bradycardia cardiovascular collapse
What are the CNS effects caused by the administration of local anaesthetic technique?
Excitation followed by depression agitation shivering seizures depression
What are the local toxicity effects caused by the administration of local anaesthetics?
Haematoma (if needle penetrates blood vessel)
Nerve damage
Vasodilation
How can you reduce the risk on complications?
Small needle Dilute preparation 0.9% saline Draw back before administering Use appropriately sized syringe Calculate the safe maximal dose
What are the different characteristics of topical local anaesthetics, where can they be used and how are they applied?
Lidocaine is absorbed better than bupivicaine
cornea
Larynx
nasal passages
Skin
must be applied to the skin for 30-60 minutes underneath a dressing
How can you administer a local anaesthetic to the larynx if intubeaze is unavailable?
Drops of lidocaine onto the larynx
0.2-0.3ml
What local anaesthetic is mostly used for ocular surgery?
Procaine
Must be kept in the fridge
quick onset short duration of action
10-10mins
What can infiltration anaesthesia be used for?
close wounds
remove small growths
take biopsies
What dose can you use for infiltration anaesthesia and what considerations must you make?
5-10mg/kg dogs
less than 6mg/kg in cats
dilute with 0.9% saline
reduce by 30-40% in sick animals
How is a testicular skin block preformed?
2mg/kg of lidocaine Divided between each testicle Inject until the testicle swells slightly or 1mg/kg along the incision line
What is the dose for a mid-line incision local infiltration of anaesthetic?
1-2mg/kg
When can interpleural Local Anaesthetics be used?
Thoractomy chest drains Rib trauma Bupivicaine 1mg/kg, always dilute 0.9% saline cat- total voulme 5ml each side dog- 20kg 10ml each side Give first dose whilst anaesthetised Systemic analgesia Painful to inject Repeat every 8 hours
What other blocks can be preformed?
Digital nerve block
ring block (distal limb procedures)
Local infiltration around soft tissue lumps for removal
Why should dental blocks be used?
Dental procedures painful
will reduce maintenance of anaesthesia
improve pain management (up to 6 hours)
Improve speed of recovery
How do you preform an infraorbital nerve block and what tissues does it block?
Incisor, canine and premolar teeth nasal and buccal soft tissue Cranial -Don't insert needle deeply into canal -Elevate head slightly after injection Caudal -tissues rostral to the 1st molar -insert needle gently into the canal -apply pressure after injection -elevate head Care in cats and brachycephallic dogs canal is short
What does a maxillary nerve block do and how is it preformed?
Bone of the maxilla, soft and hard palates
soft tissues of the upper nose and lip
insert needle at the v notch just medial to the fourth maxillary premolar
What are the risks of a maxillary nerve block in cats?
Globe penetration due to the proximity of the ventral orbit to the tooth roots of the maxillary 4th premolar and the 1st molar
Blocking the infra-orbital nerve as it exits the infra-orbital canal is recommended
Describe what the infraorbital nerve block within the canal does
ipsilateral maxillay 2nd, 3rd and 4th premolar teeth
Canine and incisor teeth
soft issues of the skin, muzzle and upper lip
How do you preform an infraorbital block?
Use a 1ml syringe attached to a 27g needle
palpate the infraoritbal foramen
Retract the lip dorsally
The needle is held parallel to the hard palate
Advanced caudoventrally towards the foramen
The bevel is faced towards the bone
Aspiration is preformed to ensure no blood is drawn back
Local anaesthetic agent is slowly injected
What does the middle metal nerve block effect?
Ipsilaters canine and incisor teeth
Bone and soft tissue rostral to the 2nd mandibular premolar
How should you preform a middle mental nerve bloc?
Inject into the canal slowly can be painful
Apply digital pressure afterwards
How do you preform a mandibular nerve block?
Cannot enter the foramen directly
aim to block the nerve before it enters the canal
(may block inguinal never care in recovery)
Sedate animal
What drugs can you use for dental nerve blocks?
Lidocaine 2hours
Mepivicaine 2-3 hours
bupivicaine 6 hours
How much of the local anaesthetic drug should you administer in a dental block?
0.1-0.5ml dogs
0.1-0.3 cats
Small needles (23g-27g)
