Equine anaesthesia workshop & lecture Flashcards
How long should you fast a horse for?
Long fasting leads to post op pain and colic because gut motility decreases, now horses are left with small hay net overnight so they can keep grazing
Is acepromazine licensed for use in horses in the UK?
yes
When would you avoid using acepromazine
Breeding stallions, pregnant mares, animals with existing severe emotional excitation, foals
List the 3 licensed (UK) alpha 2 agonists for horses
Xylazine, Detomidine, Romifidine
What are the differences between the 3 alpha 2 agonists?
Xylazine is shortest acting (useful for short procedures & sedation), Detomidine has slightly longer duration, Romifidine has longest duration
What induction agent is most used in horses in the UK?
Ketamine
Which volatile agents are licensed for use in horses?
Isoflurane
Describe in a few sentences the differences between doing an equine GA in a paddock/field compared to doing an equine GA in an equine hospital, e.g. logistics/safety/reasons
Paddock:
- horses are not very well behaved so won’t let you do much
- need to be heavily sedated so can’t do standing
Owen is a 2-year-old well behaved colt but a little excitable, that the owner would like castrating. The owner would like this procedure done ‘standing’ and is happy to bring Owen to the vet practice for the procedure to be undertaken. Owen weighs 300Kg
- Devise a sedation protocol
- Do you place an IV cannula?
- Describe which local anaesthetic you would like to use, how you would inject it, suggest a suitable dose/volume
- Even when done standing be prepared to drop horse in case of emergency (e.g. bleeding)
Sedation: a-2 agonist (Detomidine or Romifidine) + opioid (Butorphanol – some analgesia but mainly enhances sedation (prevents kicking), Buprenorphine also option (more expensive & takes longer to act))
Can top up sedation if needed - Yes, in case horse starts moving, Cannula facing up (against flow of blood) can cause clot if left in for >24h so often see vets do cannula facing down but in this case either would work. Usually 12-16G cannula
- Lidocaine or Mepivicaine (longer acting)
Intratesticular (10-15ml per testicle) (volume into body of testicle, spermatic cord & skin as you come out)
Procaine not useful as you can only give 5ml & it can change color of fur
- Dancer is a very valued delightful mare that is intermittently lame on both her hind limbs. She requires a GA for bilateral stifle arthroscopy. She weighs 480Kg.
- Devise a suitable anaesthetic protocol, include sedation/premedication, induction of anaesthesia drugs and maintenance. Work out doses.
- What are the advantages of combining midazolam with the ketamine for induction?
- After giving the ketamine – how will you ensure Dancer becomes recumbent safely. Will you remain in the induction box with her?
- Do you advise any local blocks are administered? If so what? If not, what is your reasoning?
- Acepromazine (calms her down), then wait 30-45 min, then give a-2 agonist (e.g. romifidine) +- opioid, once she is well sedated give ketamine
- Ketamine causes increased muscle tone, can give midazolam to relax her a bit more (helps with intubation & procedure)
- Can stay in & press horse against wall & slide her down but risk is that horse can come onto you, other options are squeeze door, 1 person with halter to guide her down or leave her to come down herself
- Many vets don’t give because they think loss of sensation can make recovery longer
Advise to inject opioid or alpha-2 into joint which doesn’t cause as much weakness
In what species is anaesthetic death the highest
Equine
esp. in emergency colic cases
What are the equine anaesthesia risk factors?
try to not use to much ISO
Why do horses die during or after anaesthesia?
Fractures in recovery (highest mortality)
Post anaesthetic myopathy (PAM) (due to something done by surgeon perioperatively)
Neuropathies and spinal cord malacia
Intra operative cardiac arrest (halothane induced myocardial sensitization to catecholamines? Less common now that we use more ISO)
Respiratory obstruction (obligate nasal breathers – often use nasal tubes for recovery)
Why can urine production be increased after anaesthesia?
because a-2 agonist interferes with ADH production
Describe standing sedation in horses
Some surgical procedures can be performed with horse standing e.g. castration, sarcoid removal
Sedatives & analgesics & LA combinations can be used
How could you sedate this horse for castration?
This horse was sedated with romifidine and butorphanol IV, and then had two intratesticular injections of lidocaine, and IV NSAID
How do you prepare a horse for GA the night before surgery?
Admit the night before (to calm down & control environment)
Complete physical exam (esp. cardiopulmonary system)
- Murmurs?
Blood sample (not needed in healthy animals)
Starve?
Remove shoes (weapons that can cause damage to people if horse goes down or move during surgery/recovery, can also damage recovery box)
Ensure horse is clean (to reduce fomites going into sterile environment)
How do you prepare a horse for GA the morning of surgery?
IV cannula- ALWAYS
Flush out mouth (so there is no grass stuck in teeth that gets pushed down into lungs during intubation)
Tail bandage/plait
Clip if possible
Weigh
Make anaesthestic plan
Pre-medicate the horse
Apart from the horse, what else do you prepare before GA?
Prepare monitoring equipment
Prime anaesthetic circuit with isoflurane
Change soda lime
Check ET tubes
Label & prepare drugs
Fluid therapy to prevent low BP
Leak test anaesthetic machine & ventilator & set TV
Prepare table
What type of circuit do you use during equine GA?
Rebreathing circuit- CIRCLE
Fresh gas flow rate?
- Start 6-8L/min
- After 15min reduce to 3-4L/min
What are the benefits of using acepromazine has a premed in horses
Anxiolytic
Reduces catecholamines (↓cardiac arrest)
Reduce anaesthetic gas requirement
Improved recoveries?
Improved tissue perfusion (vasodilation)
What are the risks of using acepromazine has a premed in horses
Penile prolapse
Hypovolaemia
When do you start premed in horses?
IM 40minutes before sedation
IV 20minutes before sedation
What is the effect of a-2 agonists as a premed in horses?
Sedation for induction
Analgesia
Vasoconstriction and bradycardia
Which one? (depends on duration of procedure)
What is the effect of opioids as a premed in horses?
Analgesia
Improved sedation quality
Respiratory depression
When should horses be premedicated?
In induction box
Give enough time to take ffect
How do you know when a horse is ready for induction?
head down, wide based stance, insensible to surroundings
What is a typical equine premed for ASA I-II
What is the main induction drug for horses?
Ketamine
What is commonly combined with ketamine for induction?
A benzodiazepine (e.g. midazolam)
What additional drug can be used for muscle relaxation?
Guaifenesin (instead of a-2 agonist)
What are the next steps after inducing a horse?
- Intubate (ET tubes, cuffs checked, gag, lubricant, straighten head)
- Attach hobbles & move horse
- Place on table & connect to anaesthetic machine
- Maintain horse in dorsal or lateral recumbency on O2 in iso
- Monitor as usual, place arterial line for ABP, ECG, Et CO2
- Catheterise (urinary) (horses need to stand to pee & if they wake up with full bladder they may try to stand too early)
- Connect IV fluids
- Can supplement anaesthesia with injectables for maintaining plane of anaesthesia (PIVA) e.g. lidocaine
What is the most commonly used volatile anaesthetic in horses?
Isoflurane (only licensed one in horses)
What is the main issue with inhalant anaesthesia?
Hypotension (which can lead to myopathy)
Need lowest dose that can maintain anaesthesia
Use end tidal isoflurane concentration & monitoring parameters to see how much is in horse
What is the MAC of ISO & SEVO in horses?
How can you monitor a horse during anaesthesia?
If ketamine was used eye tends to stay central
Eye signs: palpebral reflex
Muscle tone: can feel tension in neck as horse starts to get lighter
Horses have arterial line during surgery to check blood gases
How can TIVA be achieved in horses?
Top-up bolus injections (ketamine e.g. 0.5mg/kg)
or
Continuous rate infusions (‘triple drip’ in equine anaesthesia – useful for field anaesthesia)
What is the benefit of TIVA
Produces much diminished anaesthesia stress response compared with inhalation agents & is thus considered physiologically superior method of anaesthesia
What is the triple dip of equine TIVA
What is the purpose of PIVA?
What are some common problems seen during equine anaesthesia?
What is the target mean arterial pressure (MAP) to avoid post-anaesthetic myopathy?
> 70 mmHg
How can hypotension be managed?
Reduce volatile agent
increase IV fluids
use positive inotrope (dobutamine) (makes heart pump harder to increase BP)
Use PIVA
What causes myopathy and what are the signs?
Swollen brachial muscle & dropped knee/fetlock, sweating, tachycardic & very painful
Caused by pressure in muscle & exacerbated by hypotension & position during surgery (esp. lateral recumbency)
What is spinal cord malacia?
usually in young or heavy breeds
Horse sits like dog & cant get up
Possibly caused by hypotension which causes blood supply of caudal spine to be damaged
Fatal
Common in dorsal recumbency
What are causes of hypoxaemia in horses under GA?
Ventilation-perfusion (V/Q) mismatch
O2 failure
reduced RR due to drugs
What parameters indicate hypoxaemia
Low SpO₂% on pulse oximeter
Low SaO₂ on blood gas analysis
PaO₂ < 60 mmHg
What should you do if a horse develops hypoxaemia during anaesthesia?
- Check the SpO₂% reading
- Ensure pulse oximeter is working properly - Take arterial blood gas sample
- Confirm PaO₂ is lower than expected (should be 4-5x FiO₂%) - Reduce iso/sevoflurane concentration
- High inhalant levels suppress respiration - Ventilate the horse (IPPV)
- Increase RR or TV - Ensure adequate circulating blood volume
- Give IV fluids if needed - Tilt surgical table (head up slightly if possible)
- Helps redistribute blood flow. - Administer b-agonist via ET tube (e.g., salbutamol)
- Improves oxygenation.
8.Finish surgery quickly
What is the expected PaO₂ in a horse on 100% oxygen?
400-500 mmHg
What is hypercapnia?
High CO₂ levels in blood, seen on capnograph & blood gas
Why does hypercapnia occur during equine anaesthesia?
horse is on its back, respiratory rate is reduced due to drugs & positioning, leading to CO₂ retention
When is mild hypercapnia tolerated?
It can promote spontaneous breathing in recovery increases oxygen dissociation in tissues
How can hypercapnia be managed?
Reduce volatile anaesthetic agent
Initiate ventilation
What factors should be considered when evaluating bradycardia?
Breed/fitness of horse
Pre-anaesthesia heart rate
Blood pressure & oxygen delivery
What are the potential causes of bradycardia?
Volatile agent effects
Increased vagal tone
Opioids
Alpha-2 agonists
Toxaemia
Hypoxia
How can bradycardia be treated if it persists?
Address cause first, then if it still persists:
Administer Hyoscine N-butylbromide (Buscopan); other options include atropine or glycopyrrolate.
What factors should be evaluated with tachycardia?
Breed/fitness of horse
Pre-anaesthesia heart rate
Arterial blood pressure
Reflexes (nystagmus, palpebral reflex)
What are the possible causes of tachycardia?
Sympathetic stimulation (nociception)
Hypercapnia
Acid-base disturbances
Drug reactions
Hypotension
How is tachycardia managed?
Administer ketamine bolus IV
Adjust volatile anesthetic agent (increase FGF to speed up effect)
Provide additional analgesia
Check arterial blood gases for hypercapnia
Treat hypotension if present
What are the causes of neuropathy in equine anesthesia?
Poor positioning
Hypotension
When does neuropathy typically become evident?
Upon recovery, may resemble myopathy but is less painful
What are the common sites of neuropathy?
Radial nerve
Facial nerve (if the head collar is left on during surgery)
Why are horses prone to eye problems under anesthesia?
may be in lateral recumbency for long periods.
Ketamine reduces blink reflex, leading to corneal drying
How can eye problems be prevented?
Padding under dependent eye
Frequent lubrication.
What equipment can be used to assist in breathing during recovery?
Demand valve to stimulate breathing
Nasal tubes (as horses are obligate nasal breathers)
When should ET tube be removed?
When horse swallows or just before
What are the key steps in positioning a horse for recovery?
Position in RLR (if dorsal/LR on table)
LLR if LLR on table
Pull dependent limb forward
What factors contribute to an optimal recovery environment?
Quiet and dark area
Empty bladder
Adequate analgesia
Time for anesthetic drug elimination
Sedation (low-dose alpha-2 agonists)
What sedatives are used in recovery?
Romifidine (given once breathing spontaneously)
Xylazine (given when signs of reduced anesthetic depth occur)
What are the optimal recovery timings in horses?
What are the signs of upper airway obstruction post-extubation?
Stridor or stertor
Nostril flaring on inspiration
Abdominal breathing effort
Exaggerated thoracic movement
Lack of airflow at nostrils
What are the common causes of airway obstruction?
Nasal passage or laryngeal obstruction
Dorsal displacement of soft palate
Epiglottic retroversion
Laryngeal swelling post-surgery
How can laryngeal obstruction be prevented?
Use appropriately sized ET tubes
Insert ET tube gently
Be prepared for re-intubation if necessary
Emergency tracheostomy if severe
What causes nasal oedema post-anaesthesia?
Prolonged recumbency leading to congestion in nasal passages
How can nasal oedema be managed?
Nasopharyngeal tubes
Phenylephrine spray
Recovering horse to standing with ET tube in place
