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MD2001 > Local Anaesthetics > Flashcards

Local Anaesthetics Flashcards

1
Q

What are local anaesthetics used for?

A

+ 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

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2
Q

What are the two different bonds in LAs?

A

+ Amide (most commonly used)

+ Ester (used extremely rarely)

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3
Q

What are examples of amide-containing anaesthetics?

A

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

+ Prilocaine: medium-acting; no vasodilation

+ Bupivacaine / Levobupivacaine: long-acting; slow onset

+ Articaine: short-acting; rapid onset

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4
Q

What are examples of ester-containing anaesthetics?

A

+ Tetracaine: long-acting; very slow onset

+ Chloroprocaine: medium-acting

+ Benzocaine: atypical mechanism of action

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5
Q

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

A

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

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6
Q

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

A

Work by reversibly blocking voltage-gated Na+ channels:

  • stabilises excitable membranes
  • prevents membranes from being depolarised
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7
Q

What is the effect of LAs on peripheral nerves?

A

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

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8
Q

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

A
  1. Pain
  2. Temperature
  3. Proprioception
  4. Skeletal muscle tone
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9
Q

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

A

+ 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

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10
Q

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

A

+ Resting
+ Open
+ Inactivated

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11
Q

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

A

Local anaesthetics are weak bases:

  • can exist as neutral or protonated
  • pH dependent
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12
Q

How do LAs block the voltage-gated sodium channels?

A

Block when IONISED via an intracellular binding site

HOWEVER,

Cannot pass through membrane when IONISED

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13
Q

What is the difference between the hydrophobic and hydrophilic pathways?

A

Hydrophobic: no use-dependence

Hydrophilic: use-dependent

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14
Q

What does pKA have to do with LAs?

A

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

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15
Q

What is the Henderson-Hasselbalch equation?

A

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

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16
Q

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

A

+ Bind to open air or inactive channels

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

17
Q

What is the duration of action of LAs dictated by?

A

Rate of removal

  • blood flow
  • action of plasme esterases (ester-linked LAs only)
  • hydrophobicity of drug
18
Q

What is an effect caused by the majority of LAs?

A

Vasodilation

- except cocaine (vasoconstriction) and prilocaine

19
Q

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

A

↓ blood flow to area ∴ ↓ rate of removal

Vasoconstriction (adrenaline)

  • can ↑ duration ~2x
  • can also ↓ bleeding during surgery
  • risk of ischaemic damage at extremities
20
Q

What are the routes of administration for LAs?

A

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)

21
Q

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

A

Mixture of lignocaine and prilocaine used for dermal anaesthesia

22
Q

What are features of EMLAs?

A

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

23
Q

What causes unwanted side affects of LAs?

A

Entry into systemic circulation

24
Q

What are the side effects of LAs on the CVS?

A

+ Dysrhythmias

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

25
Q

What are the side effects of LAs on the CNS?

A 
\+ Restlessness
\+ Tremors
\+ Convulsions
\+ Respiratory centre depression
\+ Death
26
Q

Why do LAs have sosuch a range of side-effects

A

They are non-specific to nerves

27
Q

What does the route of LA administration help to do?

A

It helps to localise the effect

MD2001 (52 decks)

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