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Diploma in Anaesthesia > Local anaesthetic agents > Flashcards

Local anaesthetic agents Flashcards

1
Q

What is general anaesthesia

A

Loss of sensation of the entire nervous system

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2
Q

What is local anaesthesia

A

Restricts loss of sensation to an area supplied by a limited number of nerves

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3
Q

Name and describe 4 routes of administration for local anesthetics

A
  1. Topical - cream, gel or spray for skin/mucous membranes
  2. Infiltration - for field blocks where superficial nerves are blocked locally using IV preparations
  3. Intravenously - Intravenous Regional Anaesthesia = Bier’s block for fractured wrist.
  4. Epidural or subarachnoid - regional anaesthesia - blocking spinal nerves
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4
Q

Which is more potent bupivacaine or lidocaine and why?

A

Bupivacaine is 4 x as potent as lidocaine because it is more lipid soluble

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5
Q

Why is bupivacaine 4 x as potent as lidocaine if it is 9 x the lipid solubility

A

Other factors also affect potency

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6
Q

Which form of the local anaesthetic drug is able to cross the cell membrane?

A

The unionized form. The more unionized, the more rapid the passage of the drug across the membrane and the faster the onset of the block.

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7
Q

Which form of the local anaesthetic drug is effective at blocking the voltage-gated sodium channels?

A

The ionized form

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8
Q

What is the pKa of a drug

A

this is the pH at which the ionized form of the drug equals the unionized form of the drug

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9
Q

What determines the proportion of any drug in the ionized form compared with the unionized form?

A
  1. Whether a drug is a weak acid or a weak base
  2. The pKa
  3. The pH of the environment
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10
Q

What % of lidocaine and bupivacaine are unionized at physiological pH of 7.4.

How does this relate to the respective onsets of action

A

Weak bases ionize at pH below their pKa
Lidocaine pKA = 7.9 —> 25% unionized at pH of 7.4
Bupivacaine p Ka = 8.1 —> 15% unionized at pH of 7.4

It correlates with the onset of action
Lidocaine –> Onset: 2 - 4 minutes
Bupivacaine –> Onset: 4 - 8 minutes

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11
Q

What can be added to increase the proportion of unionized drug

A

Sodium bicarbonate

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12
Q

What situation will cause for the regional pH to lower hence slowing the effect of the LA or rendering the LA ineffective?

A

Infected tissue/abscesses

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13
Q

What factors determine the duration of the local block?

A

Protein binding

Rate of removal from the site and subsequent metabolism

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14
Q

What can be added to lidocaine to increase its duration of action and what is the mechanism of this

A

Epinephrine 1: 200 000
Vasoconstriction reduces local blood flow to the site where the anaesthetic is infiltrated –> less lidocaine is cleared away.

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15
Q

Is the addition of adrenalin to bupivacaine clinically useful?

A

No. the duration of action of bupivacaine is determined mainly by tissue binding.

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16
Q

How does the addition of adrenalin to lidocaine alter the maximum dose of lidocaine that can be used safely?

A

Without adrenalin: 3mg/kg

With adrenalin: 7mg/kg

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17
Q

Does addition of adrenalin to bupivacaine allow for higher doses of bupivacaine to be administered safely.

A

No. Toxicity associated with bupivacaine is related to protein binding

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18
Q

How can local anaesthetic agents be classified in terms of chemical groups? How does this structural difference influence the pharmacokinetics of these agents?

A

There are two chemical groups

Amide local anaesthetics
- Amide linkage: N - C - C (middle C has double bonded O)

Ester local anaesthetics
- Ester linkage: C - C - O (middle C has a double bonded O)

Esterases are present in a variety of tissues and in the blood stream - Ester linkages are easily broken down and therefore have a shorter duration of action than amide LAs and are used less commonly for local blocks

Amidases are found in the liver

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19
Q

Which are more protein bound ester local anaesthetics or amide local anaesthetics and how does this effect the duration of action?

A

Amides are more protein bound which increases their relative duration of action

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20
Q

What is an amine?

A

An amine is a compound in which one or more of the hydrogen atoms in ammonia have been replaced by an organic functional group.

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21
Q

What is basic chemical structure of all local anaesthetics?

A

Lipid soluble ring joined to an amine-containing group by either an amide or and ester linkage

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22
Q

Name the amide local anaesthetic agents

A
  1. Lidocaine
  2. Bupivacaine
  3. Prilocaine
  4. Ropivacaine
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23
Q

What are the other names for lidocaine

A

Lignocaine, Xylocaine

24
Q

What is lidocaine pKa

A

7.9

25
Q

What is the partition co-efficient for lidocaine (relative lipid solubility)

A

3

26
Q

What is the % protein binding of lidocaine?

A

65%

27
Q

Describe the various presentations for lidocaine

A
  1. In solution 1% or 2% with/without adrenalin 1:200 000.
  2. Gel: 2% gel for catherization
  3. Metered spray: 10mg/dose
  4. Topical 4% solution
28
Q

How long does lidocaine take to work?

A

2 - 4 minutes

29
Q

What is lidocaines duration of action?

A

1 - 2 hours (medium duration)

30
Q

What are other uses for lidocaine?

A

Antiarrythmic - Rx persistent ventricular ectopics

31
Q

Adverse effects of lidocaine?

A

Circumoral tingling, seizures, coma, dysrythmia

32
Q

In what two forms is bupivacaine available

A

Racemic mixture (bupivacaine) or a single isomer (levobupivacaine)

33
Q

What is the partition co-efficient of bupivacaine versus lidocaine

A 
Lidocaine 3
Bupivacaine 28 (9 x lipid solubility)
34
Q

What is the pKa of bupivacaine?

A

8.1

35
Q

Protein binding of bupivacaine?

A

95%

36
Q

What is the common trade name for bupivacaine?

A

Marcain

37
Q

What are the different presentations of Marcain

A

Marcain Spinal 0.5% Heavy Solution for Injection

  • bupivacaine 0.5% (5mg/ml)
  • Glucose 8% (80mg/ml)

Marcain Spinal 0.5% Solution for Injection
- bupivacaine 0.5% (5mg/ml

38
Q

List the uses for bupivacaine

A
  1. Local nerve blocks

2. Regional anaesthesia and analgaesia (espcially obstetric analgaesia)

39
Q

What is the onset of action of bupivacaine

A

ROUTE AND DOSE DEPENDENT

Epidural: Up to 17 minutes to spread to T6 dermatome (Scott 1980)

Infiltration: Fast (Barash 2009); Dental injection: 2 to 10 minutes

Spinal: Within 1 minute; maximum dermatome level achieved within 15 minutes in most cases

40
Q

What is the duration of action of bupivacaine

A

ROUTE AND DOSE DEPENDENT

Epidural: 2 to 7.7 hours (Barash 2009)

Infiltration: 2 to 8 hours (Barash 2009); Dental injection: Up to 7 hours

Spinal: 1.5 to 2.5 hours (Tsai 2007)

41
Q

What is the maximum dose of bupivacaine?

A

2mg/kg

42
Q

What are adverse effects

A

CVS: Myocardial depression
CNS: Convulsions –> coma
levobupivacaine less toxic

43
Q

Contraindications:

A

Established allergy/hypersensitivity

SHOULD NOT BE USED FOR BIER’s BLOCK

44
Q

Name the ester local anaesthetics

A

Tetracaine
Cocaine
Procaine

45
Q

How do ester LAs differ from amide LAs

A

Shorter duration

Higher incidence of hypersensitivity reactions

46
Q

What can be used in children nervous of IV cannulation

A
  1. Eutectic Mixture of Local Anaesthesia (EMLA)

2. Ametop gel

47
Q

What is the meaning of the word Eutectic?

A

Eutectic refers to a gel state that occurs in a mixture of different agents that WOULD NOT OCCUR with each component alone at a given temperature (room temp for EMLA)

48
Q

How long before cannulation should EMLA/Ametop be applied?

A

30 - 60 minutes

49
Q

What does EMLA contain?

A

Lidocaine 2.5%
Prilocaine 2.5%

Individual tubes contain 5g and come with occlusive dressings

50
Q

Can EMLA be applied to mucous membranes?

A

Not useful –> Mucous membranes will be anaesthetized but absorption limits the duration of action –> systemic effects are not seen after a single application

51
Q

How long does EMLA last?

A

Up to 5 hours

52
Q

What is the problem with using EMLA

A

Causes vasoconstriction –> may make cannulation more difficult

53
Q

Can EMLA be used over a large area in methaemoglobinaemia?

A

No. A metabolite can worsen the condition

54
Q

What is the active ingredient in Ametop gel?

A

Tetracaine (amethocaine) 4% as a gel

1.5 g in indicidual tubes

Thick layer plus occlusive dressing applied

55
Q

How long does ametop take to work

A

30 mins but shorter duration the EMLA

56
Q

Does ametop cause VC like EMLA

A

No

57
Q

What side effect occurs more frequently with ametop versus EMLA

A

hypersensitivity (contains ester linked tetracaine) - urticaria/erythema

Diploma in Anaesthesia (73 decks)

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