Local Anaesthetics Flashcards Preview

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Flashcards in Local Anaesthetics Deck (27):

What are local anaesthetics used for?

+ When loss of consciousness is neither necessary or desirable

+ As an adjunct to surgery to avoid high-dose general anaesthetics

+ Post-operative analgesia

+ For major surgery, with sedation


What are the two different bonds in LAs?

+ Amide (most commonly used)

+ Ester (used extremely rarely)


What are examples of amide-containing anaesthetics?

+ Lidocaine (lignocaine): medium-acting; rapid onset

+ Prilocaine: medium-acting; no vasodilation

+ Bupivacaine / Levobupivacaine: long-acting; slow onset

+ Articaine: short-acting; rapid onset


What are examples of ester-containing anaesthetics?

+ Tetracaine: long-acting; very slow onset

+ Chloroprocaine: medium-acting

+ Benzocaine: atypical mechanism of action


Why are amide-containing anaesthetics used more commonly than ones containing esters?

Esters are more unstable and can be metabolised into compounds associated with allergic reactions


How do LAs work/what is their mechanism of action?

Work by reversibly blocking voltage-gated Na+ channels:
- stabilises excitable membranes
- prevents membranes from being depolarised


What is the effect of LAs on peripheral nerves?

They consist of fibers of different function, diameter and insulation - all can be blocked by LAs but at different rates


What is order of loss, due to the effect of LAs?

1. Pain
2. Temperature
3. Proprioception
4. Skeletal muscle tone


What factors influence the probability that a local anaesthetic will block an impulse?

+ Diameter of fibre:
- smaller nerve fibers blocked more easily than large fibers

+ Myelination status:
- myelinated fibers blocked more easily than unmyelinated

+ Length of nerve exposed to drug

+ Length of time exposed to drug

+ Concentration of drug


What are the different states of the voltage gated sodium channels?

+ Resting
+ Open
+ Inactivated


What are features of LAs regarding the voltage-gated sodium channels?

Local anaesthetics are weak bases:
- can exist as neutral or protonated
- pH dependent


How do LAs block the voltage-gated sodium channels?

Block when IONISED via an intracellular binding site


Cannot pass through membrane when IONISED


What is the difference between the hydrophobic and hydrophilic pathways?

Hydrophobic: no use-dependence

Hydrophilic: use-dependent


What does pKA have to do with LAs?

As almost all LAs are weak bases, the pKA is the pH at which 50% of drug is ionised
- most LAs have pKA values 8-9


What is the Henderson-Hasselbalch equation?

pKa - pH = log10 x ( [BH+] / [B] )


What are the features of LAs that show use-dependence?

+ Bind to open air or inactive channels

+ Related to frequency of neuronal firing
- faster onset in faster firing neurones


What is the duration of action of LAs dictated by?

Rate of removal
- blood flow
- action of plasme esterases (ester-linked LAs only)
- hydrophobicity of drug


What is an effect caused by the majority of LAs?

- except cocaine (vasoconstriction) and prilocaine


What is the effect of vasoconstriction on duration of action/rate of removal?

↓ blood flow to area ∴ ↓ rate of removal

Vasoconstriction (adrenaline)
- can ↑ duration ~2x
- can also ↓ bleeding during surgery
- risk of ischaemic damage at extremities


What are the routes of administration for LAs?

SURFACE: nose, mouth, bronchial tree, cornea, UT

INFILTRATION: injection into tissues to reach nerve branches/terminals

NERVE BLOCK: small or large regional block by injection around nerve

INTRAVENOUS REGIONAL: double-cuff method to contain LA to a limb

EXTRADURAL (epidural) (particulary bupovacaine): used in thoracic, lumbar and sacral regions

SUBARACHNOID (intrathecal): drug injected into the subarachnoid space (CSF)


What is a Eutectic Mixture of Local Anaesthetics (EMLA) and what is it used for?

Mixture of lignocaine and prilocaine used for dermal anaesthesia


What are features of EMLAs?

Melting point of mixture is lower than that of the individual chemicals
∴ higher concentration of both can be used
∴ increased chance more will get across skin


What causes unwanted side affects of LAs?

Entry into systemic circulation


What are the side effects of LAs on the CVS?

+ Dysrhythmias
+ Sudden fall in blood pressure due to effects on heart and vasculature


What are the side effects of LAs on the CNS?

+ Restlessness
+ Tremors
+ Convulsions
+ Respiratory centre depression
+ Death


Why do LAs have sosuch a range of side-effects

They are non-specific to nerves


What does the route of LA administration help to do?

It helps to localise the effect