Local Anaesthetics Flashcards
What is the function of local anaesthetics and how do they achieve this
To stop nerve conduction by blocking the voltage gated Na channels
Which part of the nerve pathway does LA block
Anaesthetises the receptor and the axon before it reaches the spinal dorsal horn or trigeminal nucleus
In what order are the different nerve fibres blocked by LA
As
C
Ab
Aa
Why may patients feel discomfort but not pain during injection of LA
Because of the activity of proprioceptors which are the last fibres to be anaesthetised
Describe the mechanisms of LA
LA binds to a site in the Na channel
LA blocks the channel and prevents NA influx
This blocks AP generation and propagation
The block persists so long as a sufficient number of Na channels are blocked
What effect does LA have on the heart
LA blocks Na channels in the heart as it is an excitable tissue
This can cause bradycardia and hypotension
What are the 3 components of a LA molecule
Aromatic region - hydrophobic
Ester or amide bond
Basic amine side chain - hydrophilic
How does a LA molecule present
Base.HCl
How does LA pass through the cell membrane
The active form - B.H+ - must breakdown so the base can diffuse through the membrane
Why must LA molecules be both hydrophilic and hydrophobic
Hydrophilic as they need to be soluble in water to move around in the tissues
Hydrophobic as they need to enter the membrane to work inside the cell
What will make it more difficult for LA to cross the cell membrane
In acidic areas with high H+ concentration, the dissociation of B.H+ will not happen quickly
How does diameter affect how an axon is susceptible to LA block
Na channels are distributed evenly so axons with a larger diameter will have more Na channels
This means you will need more LA to act on all these channels and it will take more time to occur
How does myelination of an axon affect LA block
Na channels are concentrated at the nodes of Ranvier where the axon is exposed to LA
Why is the term safety factor used when blocking myelinated axons with LA
If LA only acts upon a small region, the saltatory effect can jump across the anaesthetised area and produce and continue an AP
Must have a higher level of LA to act in several nodes to block the AP
What does a LA preparation include
LA base
Reducing agent (sodium metabisulphide)
Preservatives and fungicide
A vasoconstrictor
What may allergy from LA be due to
Most likely the a preservative or fungicide
May be the reducing agent
Rarely the LA
What are the different ester LAs
Cocaine
Procaine
Benzocaine
What are the different amide LAs
Lignocaine (lidocaine) Prilocaine Articaine Mepivacaine Bupivacaine Ropivacaine
How are the different types of LAs applied
Ester LAs are topical
Amide LAs are applied by injection
Why are vasoconstrictors used in LA
Most LAs are vasodilators and increased blood flow will increase wash out of LA
Vasoconstrictors increase duration of action of LA and as LA isn’t washed out, the concentration will be greater, so you can use less
What different vasoconstrictors are used in LA
Adrenaline
Felypressin (synthetic vasopressin)
Which receptors do vasoconstrictors act upon
Adrenoreceptors:
Alpha receptors - generate vasoconstriction
Beta 2 receptors - generate vasodilation
Beta 1 receptors in cardiac muscle - have a positive chronotropic and inotropic effect so increase heart rate and force
Vasopressin acts upon ADH receptors
How does adrenaline act as a vasoconstrictor
Locally it acts on alpha receptors causing vasoconstriction
How does noradrenaline act as a vasoconstrictor
Locally it has a vasoconstrictive effect as it acts on alpha receptors
Systemically it increases TPR as it acts more on alpha receptors
This increases cardiac output and raises the mean arterial BP
This results in a fall in BP
