LA symposium 2 - Local anaesthetics Flashcards
(35 cards)
How do local anaesthetics stop nerve conduction
by blocking voltage-gated Na channels
which receptor in the nerve pathway does LA work on
first order afferent receptor (we don’t interfere with CNS)
Why will the nerve axon closest to the LA injection site be the first to be anaesthetised
it’s closer and the number of membranes that it has to cross is the same as axons further in the bundle
what very important characteristic does LA need to be to cross membranes in peripheral nerves
lipophilic
what impact does fat within the peripheral nerve have on the LA
allows it to stay longer
Nerve axons differ in their susceptibility to block by LA, what is the order that the fibres block in
- A delta
- C
- A beta
- A alpha
Why might a dentist tell a patient the tooth will be numb but you might feel pressure
- because nerve axons differ in their susceptibility to block by LA
- A alpha fibres blocked last are involved in proprioception
What are the functions of the different types of nerve axon
A alpha
- sensory (proprioception)
- motor (skeletal muscle)
A beta
- sensory (mechanoreception)
A gamma
- motor (muscle spindles)
A delta
- sensory (mechano-, thermo- noci- and chemo-receptors)
C
- sensory (noci-, thermo- and chemo- receptors)
- autonomic (post-ganglionic)
What is the mechanism of action 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
- block persists so long as a sufficient number of Na channels are blocked
what effect does LA have on the heart and why
- can cause bradycardia and hypotension
- LA block Na channels in other excitable tissue e.g. heart muscle
what are the components of LA
3 components:
- aromatic region (hydrophobic)
- ester or amide bond
- basic amine side chain (hydrophilic)
Why are LA’s presented as hydrochloride (B.HCl)
renders the amine base more water soluble (so increases solubility)
What form do LA’s have to be to cross the membrane
partly dissociated
- active in ionised form
- can cross membrane only in un-ionised form
Why are small diameter axons more susceptible to LA block
because of number of channels which are blocked. Small diameter axon has less channels to block
How do myelinated axons impact susceptibility to LA
- more channels to block
- Na+ channels (and K+ channels) are concentrated at the nodes of ranvier
- so need safety factor
why do we need safety factor
- on myelinated axons the local currents are strong enough to flow past the blocked region, and to regenerate the AP at the next node of ranvier
- to block the AP, the LA needs to act on several nodes of ranvier along the axon
How should LAs be prepared
- LA base present as hydrochloride, to increase solubility in aqueous solution
- for dental injections, 2-4% solutions
- reducing agent (sodium metabisulphide)
- preservative(s) and fungicide
- +/- vasoconstrictor
When people say they are allergic to LA what do they usually mean
- that it’s the preservative/ reducing agent that they are allergic to
- check manufacturer
- possible but rare to be allergic to the actual LA
what are the 2 types of LA
- esters
- amides
What is the ester that we normally use
Benzocaine (topical LA)
What amides do we use
- Lignocaine (lidocaine)
- prilocaine
- articaine
- bupivacaine (surgical procedures)
Why are vasoconstrictors added to most LAs
- most LAs are vasodilators
- increased blood flow will increase ‘wash-out’ of LA
- to increase duration of action, LA preparations often include a vaso-constrictor
what are used as vasoconstrictors
- adrenaline
- felypressin (synthetic vasopressin)
What do vasoconstrictors act on
receptors on vascular smooth muscle
- adrenoreceptors (alpha and beta)
- ADH receptors (vasopressin)
