Local Anaesthetics Flashcards
What channels do local anaesthetics target?
Voltage gates Na channels
What order neurons do local anaesthetics work on?
1st order neurons
List the order of susceptibility of fibres to local anaesthetic (most to least)
A delta fibres - Most susceptible
C fibres
A beta
A alpha - least susceptible
What senses are blocked first by LA?
Adelta fibres anaesthetised first - (sensory) mechanoreception, thermoception, nociception and chemoreception.
What fibres aim to be targeted by LA and what fibres should be avoided? Why is this the case?
Adelta, C and Abeta fibres.
Aalpha fibres carry some motor function - do not require to be anaesthetised
Describe the mechanism of local anaesthetic.
LA binds to and blocks the Na channels in the injected site.
The blocked Na channels prevents the influx of Na ions.
LA is injected until sufficient numbers of Na channels are blocked to prevent the resting membrane potential from reaching the threshold.
(Not all Na channels have to be blocked)
What other tissues does LA work on? How would they reach this site?
Also block Na channels in cardiac muscle
If LA enters the circulation i.e. if needle penetrates a blood vessel.
What are the 3 components of a LA?
Basic amine side chain - hydrophilic
Ester OR amide bond
Aromatic region - hydrophobic
What are local anaesthetics presented as?
Hydrochloride
What form are LA’s?
Partly dissociated
What is the function of the partially dissociated form of LA’s?
Ionised - pharmacologically active
un-ionised - able to cross the axon membrane
So the LA is un-ionised when discussing across the membrane then binds to become ionised once in the axon and is able to become pharmacologically active.
Is anaesthetising 1 node of Ranvier enough?
No.
In myelinated axons the local currents can be so strong that they can bypass 1 blocked Na channel at a node of ranvier and regenerate the AP at the next node.
Have to let the LA diffuse across a greater area in myelinated axons.
Why are vasoconstrictors used in LA?
To prolong the effects of LA and allow a lower volume of drug to be used.
What are common vasoconstrictors used in LA?
Adrenaline
Felypressin
What receptors does adrenaline target?
Vascular smooth muscle receptors
Both alpha and beta.
Why does adrenaline have little/no effect on mean arterial blood pressure?
Since it lowers total peripheral resistance the increase cardiac output has minimal effect.
How are LA’s inactivated? (both esters and amides)
Washout by blood flow
Tissue esterase
Liver amidases (takes longer)
What component of LA causes the most issues with allergies?
Preservative
esp propylparaben
When should felypressin not be used as a vasoconstrictor? Why?
In pregnant women - induces labour
What are the two types of LA?
Ester
Amide - most common
