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Principles of Science BVetMed 3 > Local anaesthesia and analgesia > Flashcards

Local anaesthesia and analgesia Flashcards

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1
Q

Define local anaestehsia

A

Implies that the local anaesthetic is used as the only means of anaesthesia - a cow that has a standing c-section performed under local anaesthesia

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2
Q

Define locla analgesia

A

to provide extra analgesia as a supplement to GA> Can be used to provide multi-modal analgesia, in association with e.g. opioids and/or NSAIDs

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3
Q

what is the triad of GA?

A
  • narcosis (unconsciousness)
  • analgesia (antinociception)
  • mm relaxation
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4
Q

How can local analgesia be acheived? 3

A
  • local anaesthetic drugs
  • opioids
  • alpha 2 agonists
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5
Q

Pharmacology - local anaesthetics

A
  • drugs that reversibly block the transmission of action potentials along an axon
  • interfere with the action of sodium channels
  • cocaine first to be used in 1800s
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6
Q

What is a differential block?

A

= not all nn types are equally sensitive to local anaesthetics

  • B fibres most sensitive (sympathetic)
  • then Adelta fibres (sensory - PAIN)
  • least sensitive Abeta and Aalpha fibres (motor and proprioceptive)
  • sensitivity of C fibres (unmyelinated overlaps)
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7
Q

D/t differential block, which order are sensations lost?

A
  1. pain
  2. cold
  3. warmth
  4. touch
  5. deep pressure
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8
Q

Name 2 types of local anaesthetic

A
  • esters

- amides

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9
Q

Outline ester pharmacology

A
  • rarely used
  • metabolised by plasma pseudocholinesterases
  • allergenic
  • e.g. procaine (LICENSED), tetracaine
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10
Q

Outlien amide pharmacology

A
  • metabolised in liver by amidases
  • allergic reactions rare
  • e.g. lidocaine, bupivicaine, ropivicaine (n.b. a drug with two ‘i’s within the word are amides)
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11
Q

What does lipid solubility determine?

A

potency - axonal membranes are predominantly lipid

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12
Q

What does protein binding determine?

A

duration of action - bind to protein (sodium channels) within axonal membranes

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13
Q

What does pKa determine?

A

the speed of onset (must diffuse across axon sheath in uncharged base form)

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14
Q

What is pKa?

A

the dissociation constant (i.e. when 50% of drug is ionised)

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15
Q

Does lidocaine (pKa 7.9) or bupivicaine (pKa 8.1) work fastest?

A

Lidocaine because more of the drug is non-ionised nearer blood pH (7.4)

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16
Q

Which local anaesthetics can be used in large animals?

A
  • Farm/food - procaine only licensed (cattle only)
  • Horses - all can be used (if licensed or on positive list, reports of skin/SC swelling after lidocaine, mepivacaine mainly used)
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17
Q

Absorption - local aaesthetics

A
  • local anaesthetics are unusual in that they are applied directly to the site of action
  • systemic absorption also partly controls duration of action
  • vasoconstrictor (adrenaline) can be added to reduce systemic absorption (reduce local BF –> increases duration of action)
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18
Q

When might you have local anaesthetic toxicity?

A
  • accidental intravascular injection (always draw back on syringe before injecting)
  • overdose (always check weight and calculate maximum dose)
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19
Q

3 main complications of local anaesthesia

A
  • Nerve damage (low incidence in humans, no intraneural injection as this destroys nn)
  • Systemic toxicity (ensure right dose and draw back)
  • Local toxicity (very rare)
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20
Q

Features of CNS toxicity d/t local anaesthesia

A
  • sedation
  • tremors
  • seizures
21
Q

Outline local anaesthesia effects on CV toxicity

A
  • direct action on heart and peripheral vasculature
  • indirect action by blocking autonomic nn fibres
  • worst with bupivacaine
  • ultimate toxicity is CV arrest
22
Q

Name 2 miscellaneous toxicities d/t local anaesthetics

A
  • allergy

- methaemoglobinaemia

23
Q

Describe methaemoglobinaemia

A

Fe2+ in haemoglobin oxidised to Fe3+ –> cannot bind and carry O2 –> results in cyanosis (especially prolocaine = EMLA cream), also benzocaine, lidocaine and procaine

24
Q

ROA - local anaesthesia

A
  • topical
  • regional
  • spinal/ epidural
  • intra-articular
  • IVRA
  • systemic
25
Q

Name 2 local anaesthetics as eye drops

A
  • Proparacaine (proxymetacaine)
  • tetracaine (amethocaine)
  • (can slow corneal healing)
26
Q

Define EMLA cream

A

= Eutectic mixture of local anaesthetics

  • lidocaine and prilocaine
  • used for venous catheterisation
  • absorbed across intact skin
  • apply 30 minutes before (occluded)
27
Q

What do you use infiltrative anaesthesia for?

A
  • lidocaine +/- adrenaline
  • lowest possible concentration
  • fine (24g) needles
  • don’t use with adrenaline in tissues supplied by end arteries (e.g. tails, ears)
28
Q

Name 2 field blocks

A
  • line blocks
  • inverted L block
    (give skin, SC, mm)
29
Q

What do you block in a paravertebral nn block?

A

T13, L1, L2 dorsal and ventral branch

30
Q

Effectiveness - paravertebral anaesthesia

A
  • hyperaemia
  • lateral curvature with convexity to blocked side
  • test with needle
31
Q

Problems - paravertebral anaesthesia -4

A
  • fat animals
  • mm spasm
  • damage to aorta
  • infection
32
Q

How do you do the cornual nn block in cattle and goats?

A
  • CATTLE: cornual branch of lacrimal nn (halfway b/w lateral canthus and horn bud)
  • GOATS: also cornual brach of intratrochlear nn (halfway b/w medial canthus and horn bud)
33
Q

What do you use for a cornual nn block in cattle/goats?

A
  • Sedation: xylazine or detomidine

- Local analgesic: procaine

34
Q

Name 4 peripheral nn blocks of CN and what they anaesthetise

A
  • Mandibular nn (teeth and lower lip)
  • Infraorbital nnn (upper lip and nose)
  • Supraorbital and auriculopalpebral nn (eye)
  • Retrobulbar block (eye enucleation)
35
Q

List examples of TL nn blcoks

A
  • Cervical palpebral (shoulder, brachium)
  • Axillary approach to brachial plexus (elbow distally)
  • Radial, ulnar, median and musculocutaneous (elbow distally)
  • Radial, ulnar and median (carpus distally)
  • IVRA (Bier’s block, below tourniquet)
36
Q

List examples of HL nn blocks and what they do

A
  • lumbar and sacral plexus block (whole limb)
  • femoral/ sciatic (stifle distally)
  • IVRA (Bier’s block, below tourniquet)
37
Q

What might you use neurostimulation and ultrasound for?

A

to detect location of nn to align needle to block

38
Q

Advantages - epidural analgesia

A
  • relatively simple technique
  • good analgesia intra and post-op
  • decreased stress response to anaesthesia and sx
39
Q

What drugs can be used for epidural anaesthesia?

A
  • local anaesthetics (SA)
  • opioids (SA, LA)
  • alpha 2 agonists (LA)
  • ketamine, NSAIDs
40
Q

Indications - epidural analgesia

A
  • abdominal and hind quarter sx in SA under light GA
  • standing sx in farm animals and horses
  • post-op analgesia for above sx or injuries
41
Q

Where do you do epidural anaesthesia in small vs. large animals?

A
  • SMALL: lumbosacral site, in association with GA

- LARGE ANIMALS: ‘caudal’ site, sacro-coccygeal or Co1-Co2, standing animal, prevent straining during parturition

42
Q

Contraindications - Epidural analgesia - 5

A
  • sepsis
  • infection at injection site (risk of internalising infection)
  • coagulopathy
  • hypotension/ hypovolaemia
  • distorted anatomy (use ultrasound)
43
Q

Complications - epidural analgesia

A
  • nn damage
  • pruiritus
  • urinary retention (ask close observation)
  • motor dysfunction
  • accidental vascular injury
  • haematoma formation
  • infection
  • hypotension
  • respiratory depression d/t cranial spread
44
Q

Indications - IVRA (Bier’s block)

A

analgesia for sx of distal limb

45
Q

Method - IVRA (Bier’s block)

A

exsanguinate limb distal to sx site - apply tourniquet - inject 2-3ml 1% lidocaine into distal vein - effect dependent on tournique - limit 2 h d/t ischaemia of distal tissue

  • don’t use bupivicaine or ropivacaine or your patient will die!!!
  • TL or HL
46
Q

T/F: lidocaine can be used IV

A

True

47
Q

Actions of systemic local anaesthetics (lidocaine)

A
  • anti-arrhythmic properties (class 1b - for ventricular arrhythmias)
  • analgesia
  • prokinetic (equine colic)
  • free radical scavenger
48
Q

What tends to be the first side effect sieen with systemic local anaesthetics (lidocaine)?

A

usually CNS side effects seen first

49
Q

When should you be particularly aware of local anaesthetic toxicity in SA?

A

with repeated blocks/administration

Principles of Science BVetMed 3 (110 decks)

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