Anaesthesia - Local Anaesthetics

Front 1

Why are local anaesthetics true analgesics?

Back 1

They are totally capable of stopping generation and ascension of nerve impulses -> nociception

Front 2

Advantages of local anaesthetics

Back 2

Only local disturbance of function

Rare systemic effects

Relatively safe

Full surgical conditions

Inexpensive

Minimal equipment

Easy deployment

Compatible with GA safety net

Flagged effect

Front 3

What is the flagged effect?

Back 3

Motor nerves are resistant to local anaesthetics.

If an animal can't move the part, it cannot feel it

Front 4

Local anaesthetic

Back 4

Drug that induces reversible local blockade of nerve conduction in a specific part of the body without altering consciousness

Front 5

Disadvantages of local anaesthetics

Back 5

Difficulties in application (know anatomy)

Know dose rates

Technique failure

Paralysis

CNS, cardiac toxicity

Tissue irritancy

Vasoconstriction

Neuraxial anaesthesia

Hypotension

Infection

Needle damage

Haematomas/abscesses

Front 6

Structure of local anaesthetics

Back 6

Hydrophobic ring and hydrophilic portions joined by ester or amide link

Front 7

Acidity?

Back 7

Weakly basic but form a weakly acidic solution

Front 8

Importance of ester linkage

Back 8

Hydrolysis in liver and plasma which may produce para-amino benzoic acid (allergic reactions)

Front 9

Importance of amide linkage

Back 9

Enzyme degeneration in liver

Elimination is slower

Front 10

Where are LAs eliminated?

Back 10

Kidney

Front 11

How are lipid solubility and potency related and why?

Back 11

Directly related

Due to ability to cross cell membranes

Front 12

What is the relation between protein binding and duration of action?

Back 12

Directly related

LAs act at proteinaceous sites

Front 13

What is the effect of adding vasoconstrictors?

Back 13

Decreases drug uptake and prolongs action

Front 14

Why is hyaluronidase added?

Back 14

Improves tissue dispersal and penetration

Front 15

What does injection pH determine?

Back 15

Onset time and poss. duration of action

(e.g. adding bicarbonate increases uncharged molecule proportion)

Front 16

What can affect the local tissue irritation levels and stinging?

Back 16

Speed of injection

Front 17

Which animals are particularly sensitive to procaine and why?

Back 17

Horses - low levels of plasma esterases

Front 18

How do preservatives and additives affect irritation?

Back 18

May cause greater irritation

Front 19

What may CNS and cardiovascular toxicity result from?

Back 19

Accidental IV injection

Unexpectedly rapid absorption from site

Front 20

Which signs come first?

Back 20

CNS then CV

Front 21

What is the order of CNS signs?

Back 21

Unexplained behavioural changes

Twitching

Recumbency

Opisthotonus

Convulsions

CNS depression (to coma)

Front 22

What are the CV effects?

Back 22

Negative dromotropy (cardiac conductance)

Arrythmias and cardiac arrest

Front 23

Which LA has the quickest onset of action time, shortest duration of action and the most tissue irritancy and systemic toxicity?

Back 23

Lidocaine

Front 24

Which technique may be used to avoid vasculature?

Back 24

Drawing back before injecting

Front 25

Name the ways in which local anaesthetics may be used

Back 25

Topical

Local infiltration

Conduction block

Inta-articular

IVRA

Neuraxial anaesthesia (extradural/intrathecal)

Regional anaesthesia/ specific nerve blocks

Front 26

Regional anaesthesia

Back 26

Total loss of sensation in an area

Front 27

Analgesia

Back 27

Sensation is not abolished

Front 28

Larnygeal desensitisation uses

Back 28

Topical anaesthesia - before tracheal intubation esp. pigs and cats

Sustain intubation in semi-conscious animals

Front 29

Corneal desensitization uses

Back 29

Proxymetacaine most common

Ocular examination, removal of foreign bodies (short operations)

Front 30

Disadvantage of corneal anaesthesia

Back 30

Prolonged use damages cornea - dry eyes, tear production decreases

Front 31

Cutaneous desensitization uses

Back 31

Wait 20-60 mins after depilation and application before percutaneous passage of a catheter

Front 32

What is EMLA?

Back 32

Eutectic mixture of local anaesthetics - prilocaine and lidocaine

Front 33

Intradermal bleb

Back 33

Faster and equally effective as topical application

Doesn't require depilation like EMLA

Less feasible in babies, ears of rabbits and pigs

Front 34

Interpleural topical anaesthesia

Back 34

Placed in interpleural space by thoracostomy tube (post-thoracotomy analgesia)

Front 35

tracheal topical anaesthesia

Back 35

Injecting through cricothyroid membrane during deep inhalation provides tracheal analgesia and suppresses laryngotracheal reflexes in response to bronchoscopy under inadequate anaesthesia

Front 36

Urethral

Back 36

Insertion of catheters -> analgesia and lubrication

Front 37

Peritoneal

Back 37

In people during laparoscopy and ovarian surgery -> analgesia

Front 38

Is local infiltration or regional anaesthesia easier to perform?

Back 38

Local infiltration as anatomical knowledge is less necessary

Front 39

Disadvantage of local infiltration

Back 39

Large quantity of local anaesthesia may be needed

Front 40

Line block

Back 40

Ensure a fine needle is used if possible, and infiltration is deep and long enough

May compromise wound healing

Front 41

Inverted L block

Back 41

Does not compromise wound healing

Avoids tissue distortion

Large volumes of anaesthetic required

Front 42

Ring block

Back 42

Injected on circumference of appendage in an attempt to block afferent sensory activity

Front 43

Why should adrenaline not be used in teat infiltration?

Back 43

Vasoconstriction -> necrosis and loss of organ

Front 44

IVRA

Back 44

IV injection of LA distal to a tourniquet applied around appendage to be operate on

Can be superimposed upon GA in distal limb ops.

Front 45

Method of IVRA

Back 45

Tourniquet applied slowly below tarsus or carpus usually - veins occluded before arteries

Vein identified -> skin prepared -> lidocaine solution injected -> analgesia in approx. 10 mins -> tourniquet removed -> alternative means of analgesia e.g. NSAIDs before tourniquet application

Front 46

What should you do if the tourniquet is applied proximal to tarsus?

Back 46

Two roles of bandage should be positioned between tourniquet and fossa between tibia and Achillies tendon -> distribution of tension required or arterial occlusion

Front 47

Which veins are often used?

Back 47

Radial vein (medial aspect)

Lateral aspect of lateral saphenous vein

Front 48

How are better results obtained?

Back 48

Catheter preplaced and limb exsanguinated using Esmarch's bandage (drives blood to proximal part of appendage)

Front 49

When is IVRA unsuitable?

Back 49

If limb is swollen with cellulites

Difficulty in finding vein

Increased risk of disseminating infection

Front 50

How should the adequacy of analgesia be confirmed before surgery begins?

Back 50

Pin prick to interdigital cleft (last place to have analgesia)

Front 51

Common specific nerve blocks

Back 51

Cornual, proximal paravertebral, distal paravertebral

Front 52

Where is the injection made in cattle for a cornual nerve block?

Back 52

2.5cm rostral to base of horn

Just lateral to temporal ridge of frontal bone

Front 53

Why is it advisable to infiltrate local anaesthetic around caudal base of horn in larger horn removal?

Back 53

Branches of first and second cervical nerves often supply sensory fibres to the horn

Front 54

Why is entry into frontal sinus painful?

Back 54

Nerve supply inaccessible to this region

Front 55

In goats, why should local anaesthetic be injected midway between lateral canthus of eye and lateral base of horn and midway between medial canthus and medial base?

Back 55

Horn has sensory supply from cornual branches of both zygomaticotemporal and infratrochlear nerves

Front 56

How much lidocaine should be used in cattle?

Back 56

3-10mls of 2% lidocaine

10mg/kg of lidocaine in goats

Front 57

Paravertebral nerve blocks in cattle and horses

Back 57

Cattle - abdominal surgery in standing cattle

Horses - rarely used because of small size of sublumbar fossa

Front 58

Where is it injected?

Back 58

In proximity to T13 and L1/2

(L3/4 provide some motor input to hindlimbs -> weakness)

Front 59

Proximal paravertebral nerve block method

Back 59

Area surgically prepared

L5 may be palpated and L1 identified as equal distance from L2 as L2 is from L3

Insertion of needle 5cm away from midline, above cranial edge of transverse process of L1

10-15mls of 2% lidocaine injected -> blocks ventral branch

Further 5mls blocks dorsal nerve branch if needle withdrawn 2.5cm

Front 60

What is a successful block indicated by?

Back 60

Scoliosis (spine curved with concavity on unblocked side)

Heat and coat may stand on end

Absence of pin-prick response

Front 61

Advantages of PV technique

Back 61

More efficient than inverted L or line block - smaller volume for greater area

Leaves drug remote from wound margins

Accurate injection produces analgesia of all muscle layer and peritoneum

Front 62

Distal paravertebral nerve block

Back 62

Identify ends of transverse processes of L1, L2, l4

Needle inserted parallel and ventral to L1 transverse process

15ml of 2% lidocaine injected in fan-shaped pattern

Needle inserted dorsal to transverse process and another 15ml injected

Blocks T13 nerve

Same with L2 and L4 to block L1 and L2 nerve

Front 63

Auriculopalpebral nerve branches from and supplies

Back 63

Facial nerve

Motor fibres to eyelids

Front 64

Purpose of auriculopalpebral block

Back 64

Eyelid akinesia -> ocular examination and relaxation for ocular surgery

Front 65

Why does this block not produce analgesia?

Back 65

Doesn't provide sensory afferents to cornea or palpebral areas

Front 66

Auriculopalpebral block injection

Back 66

Dorsal point of zygomatic arch - 5-8ml of 2% lidocaine

Front 67

What preventative measure should be taken?

Back 67

Lubrication of conjunctival sac with bland opthalmic treatment -> blinking not possible

Front 68

Supraorbital block

Back 68

Injected at supraorbital foramen for anaesthesia of forehead and middle portion of upper eyelid -> akinesia

Same volume of lidocaine

Front 69

Infraorbital block

Back 69

Anaesthetise upper lip and nostril

Injected as infraorbital nerve emerges from foramen

3-5ml of 2% lidocaine

Front 70

Mental block

Back 70

Anaesthetise lower lip

As mental nerve leaves foramen

3-5ml

Front 71

Retrobulbar block

Back 71

Needle inserted through eyelid either dorsal or ventral to globe

15ml of 2% solution

Too far caudal - dura mater of optic nerve could be penetrated

-> subarachnoid injection -> meningitis

Front 72

Common in small animal practice neuraxial anaesthesia

Back 72

Intercostal

Mandibular

Brachial plexus

Infraorbital

Retrobulbar

Front 73

Intercostal anaesthesia

Back 73

Used after intercostal thoracotomy before wound closure -> visceral side of nerve

Caudal border of each rib, avoiding intercostal artery and vein

0.5ml per nerve, bupivicaine and adrenaline, analgesia 12-20 mins after injection for 3-7 hrs

Front 74

Brachial plexus

Back 74

Blunted spinal needles used, 10 ml of 50:50 lidocaine and bupivicaine injected at shoulder joint parallel to vertebral column towards costochondral junction

Up to 30 mins onset

Front 75

Infraorbital

Back 75

3-5mls of local mixture - nasomaxillary area corresponding to canine tooth

Front 76

Mandibular

Back 76

Ipselateral mandible and teeth

Lidocaine:bupivicaine

Long needle medial to horizonal amus close to palpated foramen

Front 77

Neuraxial anaesthesia

Back 77

Injected around spinal cord

Front 78

Extradural anaesthesia

Back 78

Injected into extradural space

Front 79

Subarachnoid anaesthesia

Back 79

Into CSF

Front 80

Which reflex is abolished with a sacro-coccygeal extradural?

Back 80

Ferguson's - abdominal wall straining initiated on stretching of birth canal

Front 81

Sacro-coccygeal uses

Back 81

Tail amputations, Caslicks's operation (prevent windsucking - queefing), removal of vaginal or rectal neoplasia, epiziotomy ( cut between vagina and anus) etc.

Front 82

Injection method

Back 82

Pump the tail to dorsiflexion to find most mobile intercoccygeal space

Sacrococcygeal space just cranial to this

Surgically prepare site

Insert needle 15 degrees to vertical

Fill needle hub with LA

Pop through interarcuate ligament

Aspiration of LA in hub - absolute absence of resistance - loss of tail tone

1-2ml LA + air bubble

Front 83

What does it mean if the air bubble deforms upon injection?

Back 83

Too much resistance - no in correct place

Front 84

Dose

Back 84

0.2-0.4mg/kg of 2% solution

Front 85

Why is there no pop in horses?

Back 85

No interarcuate ligament

Front 86

What is xylazine?

Back 86

alpha-2 agonist

Front 87

Lumbrosacral extradural

Back 87

Anaesthesia/analgesia of both hind limbs and abdomen

(morphine = no limb paralysis)

Front 88

How should a dog be positioned for this injection?

Back 88

Lateral or sternal recumbency with straight vertebral column

Hind limbs extended forward

Front 89

For which dogs is this difficult?

Back 89

Splinted or cast legs

Joint mobility disease present

Front 90

Injection method

Back 90

Prepare most dorsal palpable points of tuber coxae (imagine line connecting iliac prominences)

Caudal to this is depression, flag with scalpel blade incision

Pass needle perpendicular to skin

Pop as needle passes through interarcuate ligament

May be tail movement, should be loss of resistance to injection

Front 91

What will a combination of lidocaine, bupivicaine and morphine produce?

Back 91

Rapid onset of action (lidocaine) and long duration of action

Front 92

Precautions for neuraxial anaesthesia

Back 92

Sterile field

CSF aspirated -> one third of dose should be given slowly or abort attempt

May cause damage to nerves

Bloody tap -> reposition needle