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Anaesthesia > Pharmacology of Local Anaesthetics > Flashcards

Pharmacology of Local Anaesthetics Flashcards

1
Q

Define local anaesthetic

A

Produces a reversible blockade of the propagation of the action potential, to anaesthetize a region of the body, w/ or w/o muscle paralysis, & are applied at target site.

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2
Q

What is the target site of a local anaesthetic?

A

Axons of A-delta and C fibres (sensory nerves)

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3
Q

Aδ fibres

Axon diameter, myelination, speed (m/s), fxn

A
  • axon diameter: 2-5 micrometres
  • Myelination: thinly myelinated
  • speed: 5-30 m/s
  • fxn: pain, temperature
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4
Q

C fibres

Axon diameter, myelination, speed (m/s), fxn

A
  • Axon diameter: 0.5-1
  • Myelination: Unmyelinated
  • Speed: 0.5-2 m/s
  • Fxn: pain, temperature
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5
Q

Aδ is what kind of pain?

A

Acute

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6
Q

C is what kind of pain?

A

Dull ache after injury

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7
Q

Locals reversible block the transmission of…

A

AP in the neuron axon by blocking voltage-gated Na+ channels

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8
Q

What is the nerve fibre’s susceptibility to local anaesthetic?

A

B > C = Aδ > Aγ > Aβ > Aα

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9
Q

What is the basic mechanism of action of local anaesthetics?

A
  1. Enters cell via membrane or Na+ channel
  2. reversibly binds to all Na+ channels from the inside of the cell
  3. Prevents nerve conduction
  4. block transduction at afferent nociceptor
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10
Q

The chemical structure of local anaesthetics

A

aromatic end = lipid solubility
Intermediate = Ester or Amide
Amine end = water solubility

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11
Q

Classification of local anaesthetics is based on if they have…

A

an ester or amide as an intermediate chain

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12
Q

What is the main ester-based local anaesthetic used in veterinary medicine? What species?

A

Procaine
Farm animal

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13
Q

What are the main amide-based local anaesthetics used in veterinary medicine?

A

Lidocaine, Mepivacaine (horses), bupivacaine, ropivacaine

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14
Q

lipophilicity of locals relates to the…

A

potency
effect on the onset of action
effect on duration

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15
Q

more lipophilic =

A

more potent

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16
Q

the pKa determines… and has an effect on…

A
  • speed of onset
  • Effect on the environment pH
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17
Q

protein binding is associated w/

A

duration of action
(only the unbound drug is active)

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18
Q

Extracellular pH is 7.4. This means it favours what form of local anaesthetic?

A

unionised form

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19
Q

Locals act at the … so…

A

locals act at the site of injection so slow absorption is desired

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20
Q

The absorption of locals is…

A

injection-dose-dependent

21
Q

Site of injection dependent absorption depends on…

A
  • physico-chemical properties
  • drug-tissue binding
  • tissue blood flow
  • intrinsic drug properties
22
Q

Explain the metabolism & excretion of amides.

A

Metabolised in liver (Cytochrome P450) –> water-soluble metabolites –> excreted by kidneys

23
Q

Explain the metabolism & excretion of Esters

A

Metabolised in plasma (pseudocholinesterase) –> water-soluble metabolites –> excreted via kidneys

24
Q

What are the three main influences of the formulation of locals on their site?

A
  • concentration/dose
  • adjuvants in some preparations
  • mixture of different locals in same syringe & injecting
25
Q

How does concentration/dose of formulation influence the site of action?

A
  • increases onset
  • increases efficacy
  • influences vol administered
  • has effects on epidural blocks (req high enough vol to get block level you want)
26
Q

Addition of epinephrine (& other vasoconstrictors) to local…

A

increases duration of action thus decreasing tissue blood flow

27
Q

Use of vasoconstrictors reduce…

A

blood perfusion by about 30%

28
Q

By reducing blood perfusion, vasoconstrictors cause what effects?

A
  • enhance conc of available drug in premises of neuronal tissue
  • increase duration of action
  • prolong analgesic effects
29
Q

Vasoconstrictors + locals should not be…

A
  • given IV –> will kill your patient
  • given in extremities –> necrosis risk
30
Q

What locals have anti-arrhythmic effects on the heart?

A

Lidocaine, Procaine

31
Q

What locals can CAUSE lethal arrhythmias if given in high doses?

A
  • Bupivacaine
  • Ropivacaine
32
Q

Explain Purkinje Fibre AP

A
  • Phase 4: resting membrane potential
  • Phase 0: rapid depolarization phase: opening of the fast Na+ channels
  • Phase 1: repolarization “notch“: inactivation of the fast Na+ channels and movement of K+ and Cl- ions
  • Phase 2: “plateau” phase: inward movement of Ca2+ and outward movement of K+
  • Phase 3: the “rapid repolarization“: Ca2+ channels close, K+ channels still open
33
Q

Local anaesthetics used an anti-arrhythmics are what class? What does it do to the AP of the purkinje fibres?

A

Class I agent (membrane-stabilising agents)
Interferes w/ Na channels (affects phase 0)

34
Q

Lidocaine

Safety profile, Onset, Duration of action, route, Uses, Cautions

A
  • Safety Profile: high
  • Onset: 5-10 mins, MM faster
  • Duration: 1-2 hrs
  • Route: IV
  • Uses: local infiltration, epidural, spinal anaesthesia, desensitisation larynx at intubation, class 1b anti-arrhythmic, MAC-sparing properties, prokinetic
  • Caution: do not use before area of FB b/c will cause more motility, lidocaine stings on injections
35
Q

Mepivacaine

Safety profile, Onset, Duration of action, route, Uses, Cautions

A
  • Safety profile: high
  • Onset: quick
  • Duration: similar to lidocaine (1-2 hrs)
  • Uses: local infiltration, epidural anaesthesia, Dx anaesthesia in horses
  • Cautions: ?
  • Route: ?
36
Q

Bupivacaine

Safety profile, Onset, Duration of action, route, Uses, Cautions

A
  • Safety profile: Low (know your toxic dose for P)
  • Onset: 10-30 mins
  • Duration: 4-6 hr
  • Route: NOT IV
  • Uses: local infiltration, epidural, spinal anaesthesia

Levobupivacaine has a higher safety profile (less cardiotoxic)

37
Q

Ropivacaine

Safety profile, Onset, Duration of action, route, Uses, Cautions

A

Onset/Duration: similar to bupivacaine (10-30 mins, 4-6 hrs)
Higher safety profile than bupivacaine
Use: lower motor blockade at clinical doses, similar to bupivacaine?

38
Q

local anaesthetics can be found in what types of preparations?

A
  • single use vials for injection
  • sprays for topical
  • drops for topical
  • creams for skin
  • lidocaine skin patches
39
Q

What are the signs of systemic toxicity caused by locals?

A
  1. CNS (sedation, nystagmus, lip-smacking, tonic-clonic seizures, coma)
  2. CV (bradycardia, arrhythmias, hypotension, cardiac arrest)
  3. Respiratory (depression)
40
Q

What are signs of toxicity in a local region?

A

tissue irritation of neurons, skeletal muscle

41
Q

What are other forms of toxicity seen with locals?

A
  • allergic rxns
  • methaemoglobinaemia
42
Q

What is the order of presentation of toxicity signs?

A
  1. CNS signs, may not be observable in anaesthetized patients
  2. respiratory system depression
  3. cardiovascular system depression
43
Q

When it comes to toxicities with Bupivacaine, what do you need to remember?

A

It causes cardiovascular depression 1st which can cause arrhythmias that are deadly

44
Q

What are toxic doses of Lidocaine

A

8 -16 mg/kg
6 mg/kg in dogs, 3mg/kg in cats

45
Q

What are toxic doses of bupivacaine?

A

5 mg/kg
do not inject more than 3 mg/kg in dogs/cats

46
Q

What is the toxicity level of ropivacaine in dogs/cats

A

3 mg/kg

47
Q

local toxicities of local anaesthetics is mainly seen with…

A

Procaine (ester-types)

48
Q

How do you treat local anaesthetic toxicity?

A
  • symptomatic/palliative: CV &/or resp support, steroids for allergic rxn, seizure coontrol (Diazepam, Propofol)
  • Lipid rescue (Intralipid emulsion)

Anaesthesia (24 decks)

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