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Primary Pharmacology > Local Anaesthetic Agents > Flashcards

Local Anaesthetic Agents Flashcards

1
Q

Structure of a local anaesthetic
Subclassification

A

Hydrophilic tertiary amine group linked to lipophilic aromatic group
Ester or amide based on the linking group

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2
Q

What are esters and amides

A

Ester -C with a =O and a -O-
Amide -NH-C- with a =O on the C

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3
Q

Where are local anaesthetics protonated

A

On the nitrogen in the amine side chain

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4
Q

Henderson hasselbach equation for la

A

pH = PKA + log10 LA/LAH+

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5
Q

Significance of protination in local anaesthetics

A

Only non ionised can pass through cell membrane
Only ionised has action

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6
Q

How are local anaesthetics stored in solution

A

With HCl to make acidic environment thus protinate the local and make it soluble in water

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7
Q

What do local anaesthetics block
How physically
What is the relevance of this for the type of block

A

Open fast sodium channels
Enter the channel itself from an intracellular direction
Use dependant - as only blocked when open

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8
Q

What is intracellular ph normally

A

7.1

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9
Q

Effect of molecular weight on local anaesthetic acitivty

A

No direct effect but large weight suggests increased size of side chain and thus increased lipid solubility

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10
Q

Effect of lipid solubility on local anaesthetics

A

Higher lipid solubility greater penetration of membranes so greater potency. Also increases rate of onset and duration of action
Also more toxicity and irritancy

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11
Q

Effect of PKA on local anaesthetics

A

Lower pKa, lower degree of ionisation at given ph so more rapid speed of onset and offset

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12
Q

Effect of ph on local anaesthetics

A

Acidosis increases proportion of ionised drug thus reduces amount available to cross membrane thus reduced potency

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13
Q

Effect of protein binding on local anaesthetics

A

Greater binding longer duration of action

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14
Q

Examples of Esther local anaesthetic
pKa for each
Protein binding for each

A

Amethocaine (tetracaine) - 8.5, 76%
Procaine - 8.9, 6%

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15
Q

Examples of amide local anaesthetics
pKa
Protein binding

A

Bupivicaine 8.1, 96%
Ropivicaine 8.1, 95%
Lidocaine 7.9, 64%
Prilocaine 7.9, 55%

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16
Q

Potency of local anaesthetics vs lidocaine

A

Amethociane, bupicvicaine, ropivicaine all 4x
Procaine 1/2
Priolcaine same

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17
Q

Lipid solubility (heptane partition coefficient) of local anaesthetics

A

Bupivicaine 27.5
Ropivicaine 6.1
Amethociane 4.1
Lidocaine 2.9
Prilocaine 0.9
Procaine 0.02

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18
Q

Another name for Amethociane

A

Tetracaine

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19
Q

How is Amethocaine presented

A

Gel 4% for topical anaesthetic to skin
Dilute solutions 1% for eyes

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20
Q

What is benzocaine
What is it used for

A

An ester local anaesthetic with no amine component so can’t be ionised thus low potency
Used in throat lozenges or onto mouth ulcers

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21
Q

What is cocaine
What is it used for

A

Naturally occurring ester
Topical anaesthesia of the nose

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22
Q

Why does cocaine not get absorbed systemically when given intranasally
Other advantage of this

A

Causes vasoconstriction limiting absorbtion also reducing bleeding

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23
Q

If it gets systemic how protein bound is cocaine
pKa
How is it eliminated

A

98%
8.7
Plasma and liver esterases

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24
Q

Why does cocaine cause a stimulatory response, consequence

A

Inhibits catecholamine reuptake
Euphoria, hyperthermia, vasoconstriction, hypertension, mydriasis, increased resp rate, increased metabolic rate, raised o2 consumption

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25
Q

What happens with very high levels of cocaine

A

Excitatory pathway blocking resulting in sedation, unconsciousness, resp depression and myocardial depression

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26
Q

Max dose of cocaine
Contraindication

A

1.5mg/kg
Porphyria

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27
Q

Onset and offset time of bupivicaine for large nerve block

A

Onset up to 60mins
Can last 48hrs

28
Q

Main side effect of bupivicaine

A

Myocardial depression

29
Q

Max dose of bupivicaine to avoid toxicity
When should it be avoided

A

2mg/kg
Don’t use IV
Avoid 0.75% in pregnancy

30
Q

How is bupivicaine metabolised

A

Metabolised to pipecolylxylidine and hydroxybupivicaine
Excreted in urine

31
Q

What is Levobupivicaine
Advantages
Max dose

A

S enatiomer of bupivicaine
Lower cardiotoxicity and CNS toxicity
Remains 2mg/kg and still not for iV

32
Q

Cardiotoxicity of lidocaine vs bupivicaine at equivalent CNS toxicity levels

A

Around 1/9th - acts as a 1b antiarrhythmic

33
Q

Lidocaine metabolism

A

Hepatic
Microsomes oxidases and amidases producing metabolites (ethylglycine, xylidide) that are excreted in urine

34
Q

How does priolcaine toxicity compare to lidocaine
Why
What can it be used for as first choice

A

Less toxic
High tissue fixation and rapid metabolism
IV regional anaesthetic eg biers block

35
Q

How is Prilocaine metabolised

A

Liver lungs and kidneys to o-toluidine then hydroxytoludine

36
Q

What is the issue with priolcaine metabolism

A

O-toluidine causes oxidation of ferrous (2+) Fe to ferric (3+) resulting in methaemoglobinaemia

37
Q

Effects of methaemoglobinaemia
Who is most at risk

A

Cyanosis
SpO2 fixes at 85% no matter true sats

Kids - under developed metabolic pathways and hb more easily oxidised

38
Q

Treatment of methaemoglobinaemia

A

If necessary methylene blue 1-2mg

39
Q

Triggers for methaemoglobinaemia

A

Priolcaine (O-toluidine)
Emla
Methylene blue in high concentrationS (>7mg/kg)

40
Q

What enatomer is Ropivicaine

A

99.5 S enatomer

41
Q

Difference between Ropivicaine and bupivicaine

A

Less lipid soluble and thus less readily penetrates myelin sheaths causing more C fibre block than a fibre potentially sparing motor function
Half as caridotoxic as bupivicaine

42
Q

What protein does Ropivicaine bind to

A

Mainly alpha 1 acid glycoprotein

43
Q

Max dose Ropivicaine

A

3mg/kg

44
Q

What does emla stand for
What does it contain

A

Eutectic mixture of local anaesthetic
2.5% Prilocaine and 2.5% lidocaine

45
Q

What is a eutectic mixture
Relevance to emla
What else is in emla

A

A mix in which the consitiuant components are in proportions that the freezing point is as low as possible with the consituants freezing simultaneously
Allows consituants not to be in aqueous solution ie in their pure form and not ionised
Carboxypolymethylene and sodium hydroxide resulting in an oil water emulsion

46
Q

Specific gravity of heavy bupivicaine
Specific gravity of csf

A

1.026
1.005

47
Q

What is specific gravity

A

The density of a solution relative to the maximum density of water (that occurs at 4oC)

48
Q

Effect of adding adrenaline to local anaesthetic

A

Vasoconstriction resulting in
Less systemic uptake thus less toxicity
Increased duration
Reduced surgical bleeding

49
Q

Issues with adrenaline in local anaesthetic

A

Systemic uptake of adrenaline, esp with other agents that cause arrhythmia eg halothane
Ischemia if end arteries effected too severely

50
Q

Max concentration of adrenaline in local anaesthetic

A

<1:200,000

51
Q

How much adrenaline would you put in 20ml local to make adrenaline 1:200,000

A

0.1ml of 1:1000

52
Q

What is felypressin
Relevance to local anaesthetics

A

Octopeptide derived from ADH
Direct acting vasopressor that is safe to use with halothane

53
Q

What is hyaluronidase
Relevance to local anaesthetics

A

Causes temporary depolymerisation of hyaluronic acid
Facilitates spread of drug through connective tissues following sc or IM administration

54
Q

How can acid base physiology be modified to increase local anaesthetic potency

A

Alkalisation of the solution increases tissue ph, more non ionised LA so more enters nerves

55
Q

What additives can be given with local anaesthetics that have analgesic activity

A

Opioids
Clonidine
Ketamine

56
Q

When is 7.5% levobupivicaine contraindicated

A

Obstetrics

57
Q

Why does the same dose of levobupivicaine have increased activity than bupivicaine
How much

A

13% more
Dose of Levo expressed as mg of base compound
Dose of bupivicaine expressed as mg of hydrochloride salt (thus heavier)

58
Q

How are bupivicain and levobupivicaine excreted differently

A

Bupivicaine - metabolised in liver 16% unchanged in urine
Levo - non excreted unchanged, main metabolite excreted in urine

59
Q

Preparations of lidocaine

A

Plain solution 0.5,1,2%
Solutions with adrenaline 1:200,000
Gel 1, 2% with Chlorhexidine for urethral
Cream 4% for topical
Solution for pharynx, larynx and trachea 4%
Spray for airway anaesthesia 10%

60
Q

Max dose lidocaine

A

3mg/kg
7 with adrenaline

61
Q

Metabolism lidocaine

A

70% dealkalation in liver
10% unchanged in urine

62
Q

Max doses Prilocaine

A

6mg/kg
8.5mg/kg with felypressin

63
Q

Presentations of Prilocaine

A

Plain 1,4%
With felypressin 3%

64
Q

Presentations of Ropivicaine

A

Plain solution 2,7.5 and 10mg/ml
100 and 200ml bags at 2mg/ml for epidural

65
Q

Max dose Ropivicaine

A

250mg
150mg for C-section under epidural
Cumulative dose 675mg/24o

Primary Pharmacology (18 decks)

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