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

25
What is the partition co-efficient for lidocaine (relative lipid solubility)
3
26
What is the % protein binding of lidocaine?
65%
27
Describe the various presentations for lidocaine
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
How long does lidocaine take to work?
2 - 4 minutes
29
What is lidocaines duration of action?
1 - 2 hours (medium duration)
30
What are other uses for lidocaine?
Antiarrythmic - Rx persistent ventricular ectopics
31
Adverse effects of lidocaine?
Circumoral tingling, seizures, coma, dysrythmia
32
In what two forms is bupivacaine available
Racemic mixture (bupivacaine) or a single isomer (levobupivacaine)
33
What is the partition co-efficient of bupivacaine versus lidocaine
``` Lidocaine 3 Bupivacaine 28 (9 x lipid solubility) ```
34
What is the pKa of bupivacaine?
8.1
35
Protein binding of bupivacaine?
95%
36
What is the common trade name for bupivacaine?
Marcain
37
What are the different presentations of Marcain
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
List the uses for bupivacaine
1. Local nerve blocks | 2. Regional anaesthesia and analgaesia (espcially obstetric analgaesia)
39
What is the onset of action of bupivacaine
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
What is the duration of action of bupivacaine
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
What is the maximum dose of bupivacaine?
2mg/kg
42
What are adverse effects
CVS: Myocardial depression CNS: Convulsions --> coma levobupivacaine less toxic
43
Contraindications:
Established allergy/hypersensitivity SHOULD NOT BE USED FOR BIER's BLOCK
44
Name the ester local anaesthetics
Tetracaine Cocaine Procaine
45
How do ester LAs differ from amide LAs
Shorter duration | Higher incidence of hypersensitivity reactions
46
What can be used in children nervous of IV cannulation
1. Eutectic Mixture of Local Anaesthesia (EMLA) | 2. Ametop gel
47
What is the meaning of the word Eutectic?
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
How long before cannulation should EMLA/Ametop be applied?
30 - 60 minutes
49
What does EMLA contain?
Lidocaine 2.5% Prilocaine 2.5% Individual tubes contain 5g and come with occlusive dressings
50
Can EMLA be applied to mucous membranes?
Not useful --> Mucous membranes will be anaesthetized but absorption limits the duration of action --> systemic effects are not seen after a single application
51
How long does EMLA last?
Up to 5 hours
52
What is the problem with using EMLA
Causes vasoconstriction --> may make cannulation more difficult
53
Can EMLA be used over a large area in methaemoglobinaemia?
No. A metabolite can worsen the condition
54
What is the active ingredient in Ametop gel?
Tetracaine (amethocaine) 4% as a gel 1.5 g in indicidual tubes Thick layer plus occlusive dressing applied
55
How long does ametop take to work
30 mins but shorter duration the EMLA
56
Does ametop cause VC like EMLA
No
57
What side effect occurs more frequently with ametop versus EMLA
hypersensitivity (contains ester linked tetracaine) - urticaria/erythema