Local anaesthesia Flashcards
Describe the stages of the generation of APs
- depolarising stimulus - na+ channels open, na+ enters the cell
- inactivation - na+ channels close, K+ channels open, K+ leaves the cell
- cell refractory state - na+ channels restored to resting state but k+ channels still open so cell is refractory
- resting state - na+ and k+ channels restored to resting state
What are the 3 components in the structure of a local anaesthetic?
- aromatic region - very lipid soluble/hydrophobic
- amine side chain - hydrophilic
- ester or amide bond eg
- cocaine - ester
- lidocaine - amide
What is a special LA that doesn’t obey the usual structural law and what is different about it’s structure
Benzocaine that doesn’t have a basic amide group so has a weaker potency
What is the hydrophillic mechanism of action of LAs on VGSC
- drug remains in equilibrium between ionised and unionised forms (bc LAs are weak bases)
- unionised form can pass across membranes but cannot have any action
- ionised form is needed to have an action but cannot pass across membranes
- this pathway is use dependent as the channels need to be open for the cation drug to access the VGSCs.
What is the hydrophobic MoA of LAs on VGSCs
Lipid soluble drugs can access the hydrophobic pathway and drop into the channel even when the channel is closed - not use dependent
What are the effects of LAs on the body
- prevent generation and conduction of APs
- do not influence resting membrane potentials
- may influence
a. channel gating - hold an inactivated state in a channel
b. surface tension - lower surface tension - selectively block
a. smaller diameter fibres eg nociceptive pain fibres
b. non myelinated fibres - pain fibres are often small and unmyelinated
What is the pH and pka of LAs and what effect does this have
They are weak bases and so are mostly ionised and less pass into the axons of neurones. As they have high pKa they are use pH dependent
What is different with how LAs react to infected tissues
Infected tissues are normally slightly acidic so the LA is less effective as more will be ionised
What are routes of administration of LA
Surface anaesthesia - spray or powder form
Infiltration anaesthesia - sc injection
IV regional anaesthesia - IV injection distal to a pressure cuff
Nerve block anaesthesia - injection
Spinal anaesthesia - intrathecal (subarachnoid space)
Epidural anaesthesia - injection into epidural space
when are surface anaesthesias used
Sore throat relief - high conc can lead to systems toxicity
when are infiltration anaesthesia used
Post surgery sutra LA analgesia - used in minor surgeries and SC injection directly into tissues
why is adrenaline co-administered with an SC injection of anaesthesia?
- slow down diffusion of LA away from site of injection - lower conc of LA needed
- reduce systemic toxicity
when are iv regional anaesthesias used
trigger finger repair, used in limb surgery
can cause systemic toxicity if cuff released prematurely
when is a nerve block anaesthesia used, where is this injected?
Tooth extraction or an injection close to nerve trunks eg dental nerves - usually co administered with a vasoconstrictor
when are spinal anaesthesias - intrathecally used? Where is this injected?
Sub arachnoid space injection used in hip replacement - with an injection close to spinal roots
