Local anaesthetics Flashcards
How do local anaesthetics stop nerve conduction?
- By blocking the voltage-gated Na+ channels
What receptors does LA work on?
- Works on first order afferent receptors - doesn’t touch the central nervous system
What are the 3 connective tissue layers of a peripheral nerve?
- Epineurium (outer)
- Perineurium
- Endoneurium (inner)
Are the functions of type A-alpha nerve fibres? (2 points)
Sensory (proprioception)
Motor (skeletal muscle)
Are type A-alpha nerves myelinated or unmyelinated?
Myelinated
Which nerves do you not want LA to go near?
Type C fibres (autonomic (post-ganglion))
What type of nerve fibre is the most affected by LA?
A-delta fibres
What type of nerve fibre is the last one affected by LA?
- A-alpha fibres
Where does LA bind to?
A site in Na+ channels
What is LA’s action on Na+ channels? (3 points)
- LA blocks the channel and prevents Na+ influx
- This blocks the AP generation and propagation
- Block persists so long as sufficient number of Na+ channel are blocked
LA blocks Na+ channels in other excitable tissues e.g. the heart. What 2 effects can this have?
La can cause bradycardia and hypotension
What are the 3 organic components of LA?
- Aromatic region (hydrophobic)
- Ester or amide bond
- Basic amine side chain (hydrophilic)
What happens when the base of an LA is HCl?
Renders the amine base more water soluble
Is LA active or inactive in its ionised form?
Active
Can LA ONLY cross the membrane in its ionised or un-ionised form?
- Un-ionised
Why are small diameter axons more susceptible to LA block?
Due to the number of channels that are blocked - need to block proportionally (do not need to block all of the channels)
- SO, small diameter = less channels to block
Where are Na+ channels and K+ channels concentrated in a myelinated axon?
- At the nodes of Ranvier
What is meant by needing the ‘safety factor’ when injecting La to a myelinated axon?
If you block one NOR you might not be able to block the nerve completely as the jump is so strong it can bypass the block
- Need to block a greater area to have an effect (to block the AP, the LA needs to act on several NOR along the axon)
Why is the LA base present as hydrochloride?
To increase the solubility in aqueous solutions
For dental injections what are the solutions (in %)?
2%-4% solutions
What is the reducing agent in LA?
Sodium metabisulphide
Are preservative and fungiside’s included in LA preparations?
Yes
Are vasoconstrictors present in LA preparations?
Can be +/- having vasoconstrictors
In the UK, what sizes are the cartridges (in mls)?
1.8 or 2.2 mls
What are the 2 subtypes of LA?
- Esters
- Amides
What are 3 examples of LA that are esters?
- Cocaine
- Procaine
- Benzocaine
What are 4 examples of LA that are amides?
- Lignocaine (lidocaine)
- Prilocaine
- Articaine
- Bupivacaine
Are most LA’s vasoconstrictors or vasodilators?
Vasodilators - increased blood flow will increase the ‘wash-out’ of LA - this is why we may want to include a vasoconstrictor
To increase the duration of action, LA preparations often include a vaso-constrictor. Give 2 examples of these?
- Adrenaline
- Felypressin (synthetic vasopressin)
What do vasoconstrictors act on? (3 points)
- Vascular smooth muscle
- Adrenoreceptors
- ADH receptors (vasopressin)
Which adrenoreceptors do vasoconstrictors act on? (3 points)
- alpha receptors: vasoconstriction
- B2 receptors: vasodilation
- B1 receptors (cardiac muscle: Positive chronotropic effect (increase rate), positive inotropic effect (increase force))
Is adrenaline more effective on alpha or beta receptors?
- Equally effective on both
Does adrenaline increase or decrease CO?
Increases
What is adrenalines effect on mean arterial BP?
Overall, has little or no effect
Is noradrenaline more effective on alpha or beta receptors?
More effective on alpha receptors
Systemically, does adrenaline increase or lower total peripheral resistance?
Lowers
Given locally, does adrenaline have a vasoconstrictor or vasodilator effect?
Vasoconstrictor
Given locally, does noradrenaline have a vasoconstrictor or vasodilator effect?
Vasoconstrictor
Systemically does noradrenaline increase or lower total peripheral resistance?
Increases
Does noradrenaline increase or decrease CO?
Increases
Overall, does noradrenaline raise or lower mean arterial BP?
Raises
- This can result in a FALL in BP (paradoxical effect)
- Response of the body when we have an increase in arterial BP that is not necessary for compensation and the compensation is to drive to other areas to reduce TPR
How is LA ‘inactivated’?
It is ‘washed out’ from tissues by the blood supply
- (this is what is countered by the presence of a vasoconstrictor)
Ester types of LA are broken down by tissue esterases, what does this mean?
Means action of the LA is quite brief
Amide types of LA are broken down by liver amidases. What does this cause?
Causes longer duration of the action of LA
What are the different modes of administration of LA ? (6 points)
- Surface application (topical)
- Injection
- Local infiltration
- Regional nerve block
- Nerve root block (spinal, epidural)
- IV
What are the 2 doses of Lignocaine we can use for LA?
- 2% lignocain HCL
- 2% lignocaine HCL + 1:80,000 adrenaline
What are the 2 doses of Prilocaine we can use for LA?
- 4% prilocaine HCl
- 3% prilocaine HCl + felypressin (0.03U/ml )
How can you convert LA into % solutions as %’s are still widely used in clinical practice?
- x% solution = X mass/volume
e. g. 3% Prilocaine HCl solution
3% = 3g/100ml
= 30mg/1ml
A 2ml cartridge of 3% prilocaine HCl will contain 2x30 = 60mg prilocaine HCl
Why is adrenaline content expressed as a ratio instead of a % ?
Because only very small amounts of vasoconstrictor are present in LA preparations so can be confusing if it is given as a %
What is the max dose of Lignocaine?
Approx. 4mg per kg of body weight
What is the max dose of adrenaline in an LA preparation?
500ug (I think)
How much adrenaline does a cartridge of LA contain?
27.5ug
