Equine anaesthesia

Question 1

What are the causes of horses dying during or after anaesthesia?

Answer 1

Fractures in recovery (highest mortality)
Post anaesthetic myopathy (PAM)
Neuropathies and spinal cord malacia
Intra operative cardiac arrest (halothane induced myocardial sensitization to catecholamines?)
Respiratory obstruction (obligate nasal breathers - need nasal tubes for recovery)

Question 2

What are the risk factors of equine anaesthesia?

Answer 2

Increasing ASA grade
Increasing age and foals
Surgery type and position
Duration
Time of day, OOH provision
Agents used (premed/TIVA) - do not rely on ISO
Recovery quality

Question 3

Why can urine production increase after anaesthesia?

Answer 3

alpha-2-agonists interfere with ADH => increased urine production

Question 4

What should be done the night before an equine surgery?

Answer 4

Admit the night before - calm down and control environment
Complete physical exam - focusing on cardiopulmonary system
Blood sample - only if necessary (not in healthy animals)
Limit food
Remove shoes - make it safer for people and prevent damage to recovery box material (can tape up if owner does not want shoes removing)
Ensure horse is clean - reduce fomites going into sterile environment

Question 5

What should be done the morning of an equine surgery?

Answer 5

IV cannula- ALWAYS
Flush out mouth - prevent any foreign bodies e.g., food debris, from being pushed into lungs during intubation
Tail bandage/plait
Clip if possible
Weigh
Make an anaesthestic plan
Pre-medicate the horse

Question 6

What breathing system is used for equine surgery?

Answer 6

Circle circuit - only system that can provide enough oxygen

Question 7

Why is a ventilator necessary in equine surgery

Answer 7

Horses are prone to slowing their breathing under anaesthesia - no need for IPPV which is hard work for horses (big bag)

Question 8

What is the purpose of using IV fluids during anaesthesia

Answer 8

Prevent hypovolaemia

Question 9

What is the fresh gas flow rate used in equine surgery?

Answer 9

Start 6-8L/min
After 15min reduce to 3-4L/min

More efficient
Reduced to conserve ISO and oxygen

Question 10

What is the effect of acepromazine?

Answer 10

Anxiolytic (reduces anxiety)
Reduces catecholamines (↓cardiac arrest)
Reduce anaesthetic gas requirement
Improved recoveries
Improved tissue perfusion (vasodilation)

Question 11

When should pre-med acepromazine be given to horses?

Answer 11

IM 40minutes before sedation
IV 20minutes before sedation

Question 12

What is the effect of alpha 2 agonists?

Answer 12

Sedation for induction
Analgesia
Vasoconstriction and bradycardia

Question 13

What is the effect of opioids?

Answer 13

Analgesia
Improved sedation quality
Respiratory depression

Question 14

What are the typical premeds for an ASA 1+2 horse?

Answer 14

+/- Acepromazine (wait 20-40mins) + alpha 2 agonist +/- opioid
NSAID +/- Antibiotic

Question 15

How do you know a horse is ready to receive ketamine?

Answer 15

head down, wide based stance. Horses are insensible to surroundings

Question 16

How is equine anaesthesia induced?

Answer 16

IV ketamine
A benzodiazepine (midazolam) can be combined with the ketamine

Question 17

What can be used instead of an alpha 2 agonist in horses?

Answer 17

Guaifenesin can be given IV prior to ketamine instead of an alpha 2 agonist - muscle relaxant
Uncommon

Question 18

Once a horse is sedated and anaesthetised with ketamine - what are the next steps prior to surgery?

Answer 18

Intubate (Stretch neck)
Attach hobbles and move horse into position
Place on table and connect to anaesthetic machine
Maintain horse in recumbency on oxygen in ISO
Monitor
Place urinary catheter (prevent horses trying to stand too early as they need to stand to pee)
Can supplement anaesthesia with injectables for maintaining plane of anaesthesia (PIVA) e.g. lidocaine

Question 19

What is the risk of volatile anaesthetic agents in equine anaesthesia?

Answer 19

Hypotension - use lowest effective dose
Can cause myopathy

Question 20

What are the MAC values for ISO in horses?

Answer 20

1.3-1.6%

Question 21

What methods are there for monitoring equine anaesthesia?

Answer 21

Pulses
Reflexes and eye position (will tend to stay central due to ketamine) - palpebral reflex (want a slow sluggish blink when stimulated - do not want a fast blink or nystagmus)
Muscle tone
Machines: Multiparameter, pulse oximeters, blood gases

Question 22

How can TIVA be achieved in horses?

Answer 22

Top-up bolus injections (ketamine e.g. 0.5mg/kg)
Continuous rate infusions (‘triple drip’ in equine anaesthesia – useful for field anaesthesia)

Question 23

How is the anaesthesia caused by TIVA different to that of inhalant gas in horses?

Answer 23

Produces a much diminished anaesthesia stress response compared with inhalation agents and is therefore considered as a physiologically superior method of anaesthesia

Question 24

What is the ‘triple drip’ in equine anaesthesia?

Answer 24

guaifenesin - muscle relaxant
Ketamine
Detomidine - alpha 2 agonist

Question 25

How can PIVA be used in horses?

Answer 25

Question 26

What are the common problems seen during equine anaesthesia?

Answer 26

Hypotension Hypoxaemia Hypercapnia Bradycardia Tachycardia Neuropathy Eye problems

Question 27

How can hypotension lead to myopathy post-operatively in surgery?

Answer 27

pressure in muscle exacerbated by hypotension and position during surgery

Question 28

How an hypotension be improved in equine anaesthesia?

Answer 28

Use positive inotrope (dobutamine) - increases BP Reduce isoflurane Use PIVA Increase fluids

Question 29

Describe spinal cord malacia in equine anaesthesia

Answer 29

young, shire (big, heavy) breeds Horse sits like a dog in recovery Issue with blood supply to caudal part of spinal cord intra-operatively Euthanasia required

Question 30

How does hypoxaemia occur in equine anaesthesia?

Answer 30

Animal on its back squishing lungs (V/Q mismatch)+ reduced RR and CO by drugs => low O2 delivery

Question 30

How can hypercapnia be improved in equine anaesthesia?

Answer 30

Reduce the volatile agent (isoflurane) Instigate ventilation

Question 30

How can hypoxaemia be improved in equine anaesthesia?

Answer 30

Reduce ISO Ventilate/IPPV - increases resp/tidal volume Tilt table if possible Give beta agonist (down ETT) .g., salbutamol

Question 30

Why does hypercapnia occur in equine anaesthesia?

Answer 30

The animal is turned on its back, RR is reduced (by drugs and position), so CO2 rises in the body

Question 31

What should be done if bradycardia occur during equine anaesthesia?

Answer 31

Address cause e.g., volatile agent, opioid, alpha 2 agonist etc. Use buscopan, atropine or glycopyrrolate

Question 32

What should you do if a horse displays tachycardia during anaesthesia?

Answer 32

Depth not adequate - Ketamine bolus IV, consider % volatile agent (increase?) More analgesia Check arterial blood flow (hypercapnia?) Treat for hypotension

Question 33

Why does neuropathy occur in equine anaesthesia?>

Answer 33

poor positioning +/- hypotension

Question 34

Describe eye problems in equine anaesthesia

Answer 34

Usually when horse has been in lateral recumbency Must protect lowermost eye with padding Lubrication for eye

Question 35

Describe the process of recovery from equine anaesthesia

Answer 35

Volatile agent switched off Reconnected to hoist Head supported Positioned in RLR in recovery box (if dorsal or RLR on table), LLR if LLR on table Pull dependent limb forwards Demand valve can be used to stimulate breathing Nasal tubes (obligate nasal breather) Extubate when swallowing or just before Exit recovery box, watch, can recover with ropes outside

Question 36

What is the optimum anaesthesia recovery environment for horses?

Answer 36

Quiet Dark Empty bladder Analgesia Allow time for anaesthetic drug elimination Sedation -> low dose alpha 2 agonists. Given once in recovery and breathing spontaneously (romifidine) or once signs of reduced anaesthestic depth occur (xylazine)

Question 37

What are signs of upper airway obstruction in equine anaesthesia?

Answer 37

Noisy breathing following tracheal extubation Nostril flaring on inspiration Abdominal respiratory effort Exaggerated thoracic excursion Absence of airflow at the nostrils

Question 38

What are the signs of laryngeal obstruction in equine anaesthesia?

Answer 38

Dorsal displacement of soft palate Epiglottic retroversion

Question 39

How can laryngeal obstruction be dealt with during equine surgery?

Answer 39

Select appropriate ET tube size and insert gently Leave ET tube in place for recovery from laryngeal surgery Be prepared to intubate if obstruction occurs Emergency tracheostomy

Question 40

How can nasal oedema be dealt with in equine surgery?

Answer 40

Common usually resolves as horse stands Use nasopharyngeal tubes, or phenylephrine or both Can recover horse to standing with ET tube in place