Local anaesthetic techniques Flashcards
(44 cards)
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
