Local Anaesthetics Flashcards
What is an LA?
an agent used to block the conduction of sensory information, including pain, in a localised area or region to facilitate surgical intervention or other painful procedures
What do LAs produce?
anaesthesia and analgesia
Why does cocaine cause psychotropic effects?
due to the blocking of dopamine reuptake
What are common LAs and what are their properties?
- cocaine – medium rate of onset, medium duration and little use due to psychotropic effects and potential for abuse
- lidocaine – rapid onset and medium duration
- bupivacaine – slow onset, long duration and mainly used for epidural
What is the mechanism of action of LAs?
they block Na+ channels and therefore sensation; without sodium influx, the nerve cannot generate or propagate the electrical signal, which means that the sensation of pain or any other feeling is not transmitted to the brain
What must LAs be?
amphiphilic i.e. has both hydrophilic (charged) and hydrophobic (uncharged) properties
How do LAs reach the site of action?
by penetrating the hydrophobic lipid rich nerve sheath and axonal membrane in an unionised form
What does use-dependent block mean?
the blocking effect of the anaesthetic is stronger when the nerve is active, because the sodium channels are more frequently open when the nerve is firing
What is use-dependent hydrophilic block?
when the LA enters the sodium channel in its hydrophilic form and blocks the channel from the inside
What does use-independent block mean?
the LA can block the sodium channel whether or not the nerve is actively firing
What is use-independent hydrophobic block?
when LAs in their non-charged, fat-soluble (hydrophobic) form can block sodium channels without relying on the channels being open or the nerve being active
What form of LA can cross the lipid-rich cell membrane?
uncharged (hydrophobic) form
What happens to LAs in a more alkaline environment?
they are more in the uncharged form, which can easily penetrate the nerve cell membrane
What happens to LAs in a more acidic environment?
they are more in the charged (hydrophilic) form, making it harder to cross the nerve cell membrane and reduces effectiveness
What is the duration of LAs dependent on?
the rate of diffusion and absorption away from the site
