Equine anaesthesia Flashcards
What are the causes of horses dying during or after anaesthesia?
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)
What are the risk factors of equine anaesthesia?
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
Why can urine production increase after anaesthesia?
alpha-2-agonists interfere with ADH => increased urine production
What should be done the night before an equine surgery?
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
What should be done the morning of an equine surgery?
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
What breathing system is used for equine surgery?
Circle circuit - only system that can provide enough oxygen
Why is a ventilator necessary in equine surgery
Horses are prone to slowing their breathing under anaesthesia - no need for IPPV which is hard work for horses (big bag)
What is the purpose of using IV fluids during anaesthesia
Prevent hypovolaemia
What is the fresh gas flow rate used in equine surgery?
Start 6-8L/min
After 15min reduce to 3-4L/min
More efficient
Reduced to conserve ISO and oxygen
What is the effect of acepromazine?
Anxiolytic (reduces anxiety)
Reduces catecholamines (↓cardiac arrest)
Reduce anaesthetic gas requirement
Improved recoveries
Improved tissue perfusion (vasodilation)
When should pre-med acepromazine be given to horses?
IM 40minutes before sedation
IV 20minutes before sedation
What is the effect of alpha 2 agonists?
Sedation for induction
Analgesia
Vasoconstriction and bradycardia
What is the effect of opioids?
Analgesia
Improved sedation quality
Respiratory depression
What are the typical premeds for an ASA 1+2 horse?
+/- Acepromazine (wait 20-40mins) + alpha 2 agonist +/- opioid
NSAID +/- Antibiotic
How do you know a horse is ready to receive ketamine?
head down, wide based stance. Horses are insensible to surroundings
How is equine anaesthesia induced?
IV ketamine
A benzodiazepine (midazolam) can be combined with the ketamine
What can be used instead of an alpha 2 agonist in horses?
Guaifenesin can be given IV prior to ketamine instead of an alpha 2 agonist - muscle relaxant
Uncommon
Once a horse is sedated and anaesthetised with ketamine - what are the next steps prior to surgery?
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
What is the risk of volatile anaesthetic agents in equine anaesthesia?
Hypotension - use lowest effective dose
Can cause myopathy
What are the MAC values for ISO in horses?
1.3-1.6%
What methods are there for monitoring equine anaesthesia?
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
How can TIVA be achieved in horses?
Top-up bolus injections (ketamine e.g. 0.5mg/kg)
Continuous rate infusions (‘triple drip’ in equine anaesthesia – useful for field anaesthesia)
How is the anaesthesia caused by TIVA different to that of inhalant gas in horses?
Produces a much diminished anaesthesia stress response compared with inhalation agents and is therefore considered as a physiologically superior method of anaesthesia
What is the ‘triple drip’ in equine anaesthesia?
guaifenesin - muscle relaxant
Ketamine
Detomidine - alpha 2 agonist
How can PIVA be used in horses?
What are the common problems seen during equine anaesthesia?
Hypotension
Hypoxaemia
Hypercapnia
Bradycardia
Tachycardia
Neuropathy
Eye problems
How can hypotension lead to myopathy post-operatively in surgery?
pressure in muscle exacerbated by hypotension and position during surgery
How an hypotension be improved in equine anaesthesia?
Use positive inotrope (dobutamine) - increases BP
Reduce isoflurane
Use PIVA
Increase fluids
Describe spinal cord malacia in equine anaesthesia
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
How does hypoxaemia occur in equine anaesthesia?
Animal on its back squishing lungs (V/Q mismatch)+ reduced RR and CO by drugs => low O2 delivery
How can hypercapnia be improved in equine anaesthesia?
Reduce the volatile agent (isoflurane)
Instigate ventilation
How can hypoxaemia be improved in equine anaesthesia?
Reduce ISO
Ventilate/IPPV - increases resp/tidal volume
Tilt table if possible
Give beta agonist (down ETT) .g., salbutamol
Why does hypercapnia occur in equine anaesthesia?
The animal is turned on its back, RR is reduced (by drugs and position), so CO2 rises in the body
What should be done if bradycardia occur during equine anaesthesia?
Address cause e.g., volatile agent, opioid, alpha 2 agonist etc.
Use buscopan, atropine or glycopyrrolate
What should you do if a horse displays tachycardia during anaesthesia?
Depth not adequate - Ketamine bolus IV, consider % volatile agent (increase?)
More analgesia
Check arterial blood flow (hypercapnia?)
Treat for hypotension
Why does neuropathy occur in equine anaesthesia?>
poor positioning +/- hypotension
Describe eye problems in equine anaesthesia
Usually when horse has been in lateral recumbency
Must protect lowermost eye with padding
Lubrication for eye
Describe the process of recovery from equine anaesthesia
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
What is the optimum anaesthesia recovery environment for horses?
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)
What are signs of upper airway obstruction in equine anaesthesia?
Noisy breathing following tracheal extubation
Nostril flaring on inspiration
Abdominal respiratory effort
Exaggerated thoracic excursion
Absence of airflow at the nostrils
What are the signs of laryngeal obstruction in equine anaesthesia?
Dorsal displacement of soft palate
Epiglottic retroversion
How can laryngeal obstruction be dealt with during equine surgery?
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
How can nasal oedema be dealt with in equine surgery?
Common usually resolves as horse stands
Use nasopharyngeal tubes, or phenylephrine or both
Can recover horse to standing with ET tube in place
