Equine anaesthesia

Question 1

Does inhalation or IV anaesthesia carry the greatest risk of death?

Answer 1

inhalation because these are most often used for long duration anaesthesia which are the most risking.

Question 2

What must be considered when anaesthetising a horse?

Answer 2

In Europe, horses are considered food animals.

Question 3

What do you do if you want to do a procedure under sedation and LA but the horse is too excited?

Answer 3

Horse will not sedate well –> come back another day or use GA

Question 4

Name 2 drugs that can be used to sedate a horse.

Answer 4

• PHENOTHIAZINES - Acepromazine

- ALPHA-2 AGONISTS - xylazine, detomidine and romifidine

Question 5

Outline acepromzaine as a sedative - effects, ROA, combinations, side effects, CIs

Answer 5

• on own produces limited sedation
• calms horse and makes other procedures possible (clipping)
• ROA = IV, IM, oral and transmucosal
• COMBINE - opioids to give better sedation
• EFFECTS - hypotension
• CONTRAINDICATIONS - hypovolaemia, stallions (priapism risk), competition (abuse)

Question 6

What is the effect of acepromzaine on volatile agent

Answer 6

considerably reduces dose and easier to maintain stable anaesthesia

Question 7

What are the mainstay of equine sedation?

Answer 7

alpha 2 adrenoceptaor agonists (xylazine, detomidine and romifidine)

Question 8

Outline alpha 2 agonists as a sedative - effects, side effects and ROA

Answer 8

• deep sedation is dose related to a maximum
• horse can still kick under deep sedation (adding a low dose opioid reduces this risk)
• SIDE EFFECTS (major) - bradycardia, decreased CO due to a biphasic BP change, hyperglycaemia, increased urine productine, gut atony, ecbolic (xylazine especially)
• ROA - IV or IM or sublingual gel (detomidine)

Question 9

What are the differences between the alpha 2 agonists? 3

Answer 9

1. ) extent of ataxia - romifidine the least
2. ) duration of action - short xylazine, detomidine and romifidine (longest)
3. ) cost

Question 10

Is medetomidine licensed in horses?

Answer 10

NO - nevertheless it is used widely as an infusion during anaesthesia. Given on its own –> severe ataxia –> falling

Question 11

Can all alpha 2 agonists be given with opioids in horses??

Answer 11

Yes (beware increased ataxia with romifine and) and/or to horses which have already received low doses of acepromazine (a phenothiazine).

Question 12

T/F: opioids alone sedate horses

Answer 12

False (however a low dose of an opioid increases the depth of sedation of both phenothiazines and alpha 2 agonists), reduces touch response but can increase ataxia. This is not an analgesic dose.

Question 13

T/F all sedation (except Mg) is banned in equine competition

Answer 13

True

Question 14

What sedative can be used for horse box rest?

Answer 14

Out of competition, low dose acepromazine can be very effective without obvious sedation. In USA - a very long acting phenothiazine. Alpha-2 agonists tend to sedate too much. Reserpine used to be used, nasty side effects.

Question 15

What should you consider when doing local anaesthesia of horse limbs?

Answer 15

consider if you want full weight on affected limb

Question 16

Outline different analgesics in horses - 4

Answer 16

• local vs. epidural
• NSAIDs
• Opioids
• alpha 2 agonists

Question 17

Outline NSAID use

Answer 17

excellent analgesia, especially as swelling often causes pain. High dose (flunixin) can mask surgical colic. BEWARE overdose toxicity is mainly colitis/chronic ulceration –> diarrhoea

Question 18

Outline opioids use

Answer 18

• controversion - balance analgesia and excitement properties.
• BEWARE - gut effects can cause impacted colic, sedative reduce or prevent excitement effects

Question 19

OUtline examples of opioids and ROA

Answer 19

• BUTORPHANOL - L, widely used, limited analgesia
• PETHIDINE (meperidine) L, antispasmodic for gut so useful in spasmodic colic,
• ROA = ONLY by IM injection - occasional dramatic anaphylactic reactions if given IV.

Question 20

Outline alpha 2 agonist use

Answer 20

as for other species good analgesia but associated with sedation which may not be wanted, useful for anaesthesia and epidural protocols

Question 21

Outline epidural analgesia

Answer 21

• easy to perform caudal analgesia - care (especially local and xylazine) –> ataxia, recumbency. For non-surgical a combination of detomidine and morphine is popular (detomidine works quickliy, morphine slow onset - several hours but long action. Morphine epidurally –> pruiritis..

Question 22

How are longer surgeries maintained under anaesthesia?

Answer 22

volatile agents is most usual, although recently a few TIVA methods are becoming practicable.

Question 23

How long does a horse need to be starved?

Answer 23

controversial - classically 8-12 hours - probably 2-4 hours sufficient.

Question 24

Where should you place an IV catheter as a premed?

Answer 24

jugular vein

Question 25

Outline preoperative preparation - 6-

Answer 25

- starvation - place IV catherter - jugular - pre-op analgesia as relevant - remove shoes - AB/anti-tetanus (as relevant) - wash out mouth

Question 26

Name 3 potential IV accidents

Answer 26

- SC injection of irritant drugs - intracarotid injection - air embolism - jugular thrombosis (esp guiaphenesin).

Question 27

Outline a normal premed protocol

Answer 27

Acepromazine (long acting and reduces anaestheic death by half) followed LATER by (as part of induction combination) an alpha 2/ butorphanol PLUS further analgesia as required.

Question 28

Name 3 ways a horse can be controlled during anaesthetic induction

Answer 28

- free fall - padded gate - tilting tables

Question 29

What is the most common IV induction technique?

Answer 29

Ketamine based techniques

Question 30

Outline ketamine IV induction

Answer 30

hallucinogenic and convulsant - must be given in combination with agents which reduce this effect (alpha 2 agonists usually). The sedative (xylazine, detomidine or romifidine) is given IV and allowed to take full effect (2-5 minutes). Ketamine then given IV with or without a benzodiazepine (diazepam or midazolam). The horse becomes recumbent in 1-3 minutes.

Question 31

What should you do when a horse has gone down with ketamine?

Answer 31

wait until the recumbent horse relaxes (1-2 minutes later) before attempts are made to position the horse.

Question 32

When does the influence of ketamine disappear after induction?

Answer 32

approximately 15-30 minutes after induction - the duration of action of the alpha 2 agonists used then governs the ease with which subsequnet anaesthesia is kept stable.

Question 33

What is the tiletamine-zolazepam combination used for?

Answer 33

to induce anaesthesia in horses sedated with xylazine.

Question 34

Outline the hypnotic method of anaesthesia induction

Answer 34

- propofol, alfaxalone or thiopental - large doses needed rapidly, cost - strictly experimental. - thiopentane very irritant if given outside the vein (always give via a catheter, dose depends on premeds used, generally used following an alpha 2 premedication). Can be used in combination with chloral hydrate or with guaiphenesin).

Question 35

Describe thiopentane pharmacokinetics

Answer 35

- IV administration ,blood levels are reduced by redistribution, so although small doses are short acting, larger doses are cumulative --> prolonged sedation. - higher doses --> longer and worse recoveries

Question 36

What do GG and GGE stand for?

Answer 36

Guiaphenesin (a centrally acting mm relxant, it is NOT an anaesthetic

Question 37

Outline the use of guiaphenesi

Answer 37

- always combine with an anaesthetic agent - for induction, give by infusion until horse becomes ataxic, then anaesthesia is induced with thiopentone or ketamine. Alternatively for induction, thiopentane or ketamine can be mixed with GG then the horse is infused with this mixture till the horse becomes recumbent. - very long lasting (mm weakness after an hour) --> lower dependence on volatile agents

Question 38

Describe ETT intbation in the horse

Answer 38

Easy - horse laryngeal reflexes are weak. - blind method - cuffed tubes (25mm for 450-500kg horse, 20mm for ponies, 30 mm for very large warmbloods and heavy breeds) - very expensive and easily damage - horses don't usually obstruct prior to intubation but colics may regurgitate

Question 39

What volatile agent is licensed for use in the horse?

Answer 39

isoflurane is the only licensed agent but not all makes of isoflurane are licensed.

Question 40

Outline isoflurane in the horse

Answer 40

- onset and recovery rapid - very fst recovery can be poor (sedation in recovery can improve this) - more of a respiratory depressant than halothane --> IPPV often required - hypotension worse than tahothane - less cardiac depressing than halothane

Question 41

Although halothane is no longer available, what are its properties?

Answer 41

- slow but good quality recovery - less respiratory depressing so horses can usually maintain spontaneous respiration - PROBLEMS - reduce myocardial contractility, reduced CO, low BP and poor perfusion

Question 42

Name 2 new volatile agents

Answer 42

Desflurane and sevoflurane

Question 43

What are desflurane and sevoflurane's properties?

Answer 43

may well prove preferable to isoflurane for equine anaesthesia becase of their low solubility, therfore fast indction, ease of stabilising anaesthesia and rapid recovery. - Neither are licensed for equine use

Question 44

Outline the use of NO in horses

Answer 44

- controversial - NO diffuses into gut spaces causing increased pressure on the lungs and further worsening the RR - only use if arterial blood oxygen tension can be monitored.

Question 45

Describe the breathing circuit used in horses

Answer 45

must be rebreathing (i.e. circuit) incorporating 'absorbant' to absorb the expired CO2, most economical too.

Question 46

What is the minimal acceptable monitoring with halothane anaesthesia? 6

Answer 46

- DEPTH - reflexes have variable reliability, response to surgery, CV measurements (esp BP) and inspired and the end tidal level of the volatile agent - HR AND RHYTHM - ARTERIAL BP - direct pressure can be measured via a cannula in the facial artery and an inexpensive aneroid manometer - ETCO2 and VOLATILE ANAESTHETIC AGENT - most useful monitor of all - pulse oximetry (peripheral BF and Hb saturation) - arterial blood gas and acid base concentrations

Question 47

What are the 2 main positions for sx

Answer 47

- lateral and dorsal recumbency - in both you must have the head raised, don't overstretch the head/neck as you can stretch the recurrent laryngeal nn --> laryngeal paralysis

Question 48

Describe lateral recumbency for - 4

Answer 48

- under front leg pulled forward - upper limbs supported parallel to table - preferably HLs pulled back - don't pull limbs over abdomen (prevents venous return and leads to upper limb myopathy)

Question 49

What shouldn't you do to a horse in dorsal recumbency for sx

Answer 49

DO NOT LOCK stifles on HL

Question 50

Define PIVA

Answer 50

Partial intravenous anaesthesia

Question 51

What is the use of infusions to improve analgesia known as?

Answer 51

Supplementary analgesia or as Partial Intravenous Anaesthesia (PIVA)

Question 52

What options are available for intra-operative analgesia? 6

Answer 52

- Local analgesic blocks - best - NSAIDs - work post-op but don't work intra-operatively - OPIOIDS - controversial, don't decrease MAC in the horse, high doses cause stimulation in the anaesthetised horse. - ALPHA 2 AGONISTS - intermittent injection or infusion - LIDOCAINE - reduces MAC - reduces isoflurane but combined CV effects not improved versus isoflurane so? must stop infusion 20 minutes before end of anaesthesia or poor recovery - KETAMINE - probably most effective

Question 53

List the 6 main anaesthetic complications when using volatile agents

Answer 53

- hypotension - hypercapnia - hypoxaemia - cardiac arrhythmias - cardiac arrest - muscle and nerve damage

Question 54

Outline hypotension as a complication of volatile agents - 5

Answer 54

- minimum BP at 30 minutes after induction - BP improves after, especially once surgery starts - improvement due to increase in systemic vascular resistance (peripheral perfusion becomes poor) - Severe (MAP inadequate coronary perfusion --> cardiac arrest - Non-directly lethal (MAP hypoxic mm damage

Question 55

How can hypotension due to anaesthesia be treated? What shouldn't you use?

Answer 55

- reduce volatile agents, replace with injectables - large volumes of fluids (Hartman's) * Inotropic agents* to improve CO, notably DOBUTAMINE by infusion but slow this if HR falls when BP rises. Care if an anti-cholinergic was included in the pre-meds as a marked tachycardia follows. - Don't use vasoconstrictors such as phenylephidrine (it will increase BP at the expense of peripheral perfusion --> CO falls)

Question 56

How do you treat hypercapnia

Answer 56

IPPV but this reduces CO by increasing intrathoracic pressure BUT the significance of hypercapnia is not known.

Question 57

Define hypercarbia

Answer 57

the same as hypercapnia, an increase in the level of CO2 in the blood (normal is 40mmHg in the horse)

Question 58

Describe 2 stages of hypercapnia/hypercarbia

Answer 58

- moderate (50mmHg or 7kPa) --> increases CO and BF | - very high (75 mmHg or 10kPa) --> muscle tremor (horse appears to be awakening)

Question 59

T/F; despite breathing high O2 concentrations, arterial oxygen levels may fall, particularly in horses in lateral recumbency.

Answer 59

FALSE - despite breathing high O2 concentrations, arterial oxygen levels may fall, particularly in horses in DORSAL recumbency.

Question 60

How can hypoxaemia be treated?

Answer 60

DIFFICULT - ask surgeon to be quicker! - IPPV doesn't significantly improve oxygenation - occasionally the use of a salbutamol inhaler improves oxygenation - improve CO (to provide more O2 to the tissues)

Question 61

Why do horses become hypoaxaemic under GA?

Answer 61

Main factor is V/Q mismatch in lngs --> blood shunts (worsened by low CO, further lowered by IPPV). Mismatching is worse the larger the horse.

Question 62

Does hypoxia in horses under GA matter?

Answer 62

A lot of evidence suggests horses tolerate hypoxia very well with minimal problems. A reason for this: 1.) the Hb dissociation curve in the horse is shifted to the left versus the dog so the Hb is fully saturated at arterial oxygen tensions lower than other species.

Question 63

Outline how cardiac arrhythmias occur as a consequence of anaesthesia

Answer 63

horses are particularly sensitive to vagal stimulation whilst under inhalation anaesthesia and any such stimulation may precipitate a cardiac arrest with no prior bradycardia. Severe bradycardia sometimes occurs for no apparent reason.

Question 64

How can cardiac arrhythmias be prevented? 1 What is the problem with this?

Answer 64

anticholinergics BUT tachycardia and tachyarrhythmias are seen following the use of anti-cholinergics when animals are toxic, short of fluid, have adrenaline and sometimes following the use of non-anaesthetic IV agents (e.g. TMPS).

Question 65

How can tachyarrhythmias be treated? 3

Answer 65

1. remove the cause 2. reduce arterial CO2 3. if above fail --> anti-arrhythmic agents

Question 66

Why might cardia arrest occur due to anaesthesia?

Answer 66

- hypotension (severe where MAP <40 mmHg as this causes inadequate coronary perfusion) - can also occur in well monitored horses with good arterial pressures and blood gases in which there is no reason for vagal reflexes.

Question 67

How can cardiac arrest be detected?

Answer 67

- capnograph very useful - a primary cardiac arest is often followed by apparent signs of lightening of anaesthesia, hyperventilation, nystagmus and jerking movements which continue until brain death occurs and resuscitation is impossible at this stage.

Question 68

How do you resuscitate a horse following cardiac arrest? 4

Answer 68

- switch off volatile agent - external cardiac massage (jump on chest and land on knees, not practical to be fast enough really) - provide IPPV (even if horse appears to be breathing) - after instituting cardiac massage use drugs to restart the heart (epinephrine, atropine)

Question 69

What are the causes of paralysis of different nerves (facial, radial/obturator, spinal)

Answer 69

- facial - fall at induction or pressure during sx - radial or obturator paralysis - if all limbs are adducted - spinal damage - sporadically, usually young horses in dorsal recumbency * prolonged anaesthesia not needed for this damage, aetiology unknonw*

Question 70

Outline equine post-anaesthetic myopathy

Answer 70

- major cause of post-op morbidity and can be a cause of mortality - due to reduction in perfusion through hypotension adn weight of the animal

Question 71

How can you reduce the risk of equine post-anaesthetic myopathy?

Answer 71

- padding and positioning - mm damage likely to occur in any anaesthetic which exceeds 2 hours. - improve CO (keep MAP <70mmHg or higher if surgery is likely to be long) - reduce surgical time if possible

Question 72

What are the advantages of total intravenous anaesthesia (TIVA)?

Answer 72

BP better maintained (not necessarily CO). Johnson's survey showed no cardiac arrests either BUT all short anaesthetics.

Question 73

What are the different methods for TIVA?

Answer 73

- SHORT duration anaesthesia-induction agents + top ups - MEDIUM anaesthesia - infusion of induction agents (i.e. alpha 2 agonist/ ketamine) - LONG anaesthesia - currently only experimental work because very expensive. Combinations include: 1. propofol + analgesic (alpha 2 agonist and/or ketamine) 2. midazolam/ketamine/alpha 2 agonist (reverse midazolam at end of anaesthesia with flumazenil but reversal of the alpha 2 agonist is not usually necessary)

Question 74

How should you position the horse for recovery?

Answer 74

- surgery in dorsal recumbency - place in left lateral recumbency (i.e. right lung up) - surgery in lateral recumbency - position depends on whether hypoxia or mm damage is the most serious risk. Leaving it on the same side reduces hypoxia. Turning it over reduces mm damage.

Question 75

When should the ETT be removed?

Answer 75

controversial. perhaps as soon as horse is in the recovery box to prevent damage to the tube (v expensive) OR until horse is on its feets

Question 76

Can hypoxia occur in recovery?

Answer 76

Yes - once the horse is no longer breathing 100% O2, hypoxia can be severe. Oxygen can be administered by nasal tube.

Question 77

What determines the quality and speed of recovery?

Answer 77

which induction agents were used and on the depth of anaesthesia at the end of surgery. slow recoveries are advisable. very rapid recovers (e.g. ketamine followed by isoflurane) are usually violent. more sedative agent (xylazine) can be given to slow recovery.

Question 78

When is post-op analgesia most effective?

Answer 78

if given before pain commences - adequate analgesia will improve the quality and often speed of recovery.

Question 79

What should be done post-op before the horse is removed from the table for comfort?

Answer 79

catheterise the bladder

Question 80

Differentiate myopathy from spinal cord malacia

Answer 80

- both complications seen post-op - MYOPATY - a reperfusion injury, very painful - SPINAL CORD MALACIA - totally painless, descending SC damage, usually results in euthanasia