Local anaesthetics Flashcards
What do Local anaesthetics do
Local anaesthetics stop nerve conduction by blocking the voltage-gated Na+ channels
What tissue layers form the diffusion barries of a peripheral nerve
Epineurium (outer)
Perineurium (middle)
Endoneurium (inner)
What is the order of axon susceptibility to LA block
A delta
C fibres
A beta
A alpha
Why might a patient still feel discomfort and pressure during an extraction after LA
As the A alpha axons as last to be anaesthetised and therefore the propriceptors may not have been blocked fully yet during the procedure
How does LA work
LA binds to a site in the Na+ channel
LA blocks the channel and prevents Na+ influx
This blocks action potential generation and propagation
Block persists so long as a sufficient number of Na+ channels are blocked
What is a risk of using LA
Local anaesthetics block Na+ channels in other excitable tissue, e.g. heart muscle
LA can cause bradycardia and hypotension
What is LA made up of
Aromatic region (hydrophobic)
Ester or amide bond
Basic amine side chain (hydrophilic)
When can LA cross the membrane
When un-ionised (remove H from B.HCL)
What is the breakdown of B.HCL to be diffusible
B.HCL -> B.H+ + Cl- (B.H+ is active but non-diffusible)
B.H+ -> B. + H+ (B. is diffusible but non active)
How is the B. re-ionised to become active once inside the membrane
H+ passes through Na+ channel and then rejoins with B.
Why are small diameter axons more susceptible to LA block
As the number of Na+ channels are smaller
How is a myelinated axon anaesthetised
At the nodes of ranvier which have high conc of sodium channels
What is the safety factor on myelinated axons
To effectively block the AP, the LA needs to act on several nodes of ranvier as the saltory conduction is strong enough to bypass just one node of ranvier and continue propogation of the AP
What is the reducing agent in LA
Sodium metabisulphide
What are the 2 families of LA
Esters
Amides
Example of an ester LA
Benzacaine
What Amide LA’s do we use
Lidocaine
Prilocaine
Articaine
What preparation is used to make LA last longer
Using a vaso-constrictors (as most LA are vaso-dilators which will ‘wash out’ LA) such as adrenaline and felypressin before injecting LA
What is a benefit of using Vaso-constrictors
Can use less anaesthetic
Reduces spred/wash away effect
What receptors do vaso-constrictors act on
Vascular smooth muscle receptors
Adrenoreceptors
ADH receptors
What effect does adrenaline have to cause vasoconstriction
Adrenaline is equally effective on alpha and beta receptors
Given locally, it has a vasoconstrictor effect (action on alpha receptors)
If given systemically, it lowers Total Peripheral Resistance (beta > alpha)
Adrenaline increases Cardiac Output
Overall, adrenaline has little or no effect on mean arterial BP
What effect fors noradrenaline have to cause vaso-constriction
Noradrenaline (NA) is more effective on alpha than on beta receptors
Given locally, it has a vasoconstrictor effect (alpha receptors)
Systemically, it increases TPR (alpha > beta)
NA increases Cardiac Output
Overall, NA raises mean arterial BP
This can result in a FALL of BP (how)
Why do we not use noradrenaline
Causes an increase in Mean arterial BP which can then result in a fall in BP
What causes LA to ‘wear off’ or become inactive
Ester types broken down by tissue esterases
- Action is quite brief
Amide types broken down by liver amidases
- Longer duration of action
What are the administration routes of LA
Surface application (‘topical’)
Injection
Local infiltration
Regional nerve block
Nerve root block (‘spinal’, ‘epidural’)
Intravenous
What are the compositions of LA used in dental injections for Lidocaine
2% lignocaine HCl
2% lignocaine HCl + 1:80,000 adrenaline
What is the prilocaine composition used in dental injections
4% prilocaine HCl
3% prilocaine HCl + felypressin (0.03U/ml)
What does 3% of prilocaine mean
3g / 100ml
=30mg / 1ml
Therefore a 2ml cartridge of prilocaine will contain 60mg of prilocaine HCL
How is LA conc expressed when using a vaso-constrictor
Expressed as ratios due to the dilute nature of the solution
E.g. 1:80,000
1 part adrenaline in 80,000 parts of liquid
What is the maximum dose of adrenaline
500ug
