Equine Anesthesia Flashcards
What are some factors that increase the risk of death during equine anesthesia?
- Emergency laparotomies: so all colic cases
- Surgeries lasting 2+ hours
- Dorsal recumbency
- Out of Hours surgery
- Mares in their last trimester of pregnancy
- < 1 month old
- > 12-14 years old
- Lack of premedication
What are some factors that can reduce the risk of death during equine anesthesia?
- Use of Acepromazine (alpha 2 agonist): is an anti-arrhythmic drug
- Use of TIVA: total intravenous anesthesia
- Age b/w 2-7 years old
- Use of blood pressure monitoring system
What are considered to be the 4 main causes of death during equine anesthesia?
1) Cardiac arrest
2) Fractures
3) Abdominal complications
4) Post-anesthetic myopathy
You have a horse with reduced circulation (e.g. colic) or a neonatal foal, or is a breeding stallion, can you use Acepromazine?
No
ACE should not be used in horses with reduced circulating volume or neonatal foals
Foals rely on their heart rate to maintain their blood pressure due to poorly developed vasomotor control
In breeding stallions, it can cause penile prolapse + phimosis
An otherwise healthy horse presents for routine surgery, are pre-operative blood tests mandatory?
No
A horse presents for colic or is 14+ years old, what is the minimum blood test parameters needed before going for surgery?
- PCV
- TP
- Electrolyte profile
A horse presents for surgery, but hasnt been signed out of the food chain. What does this mean?
The horse is still considered a food-producing animal, and therefore we are limited in what drugs we can use on this horse
A horse received a substance from the “essentials” list, what is the with-holding period?
6 months
What legal document will tell you if a horse has been signed out of the food chain?
Its passport
What is the normal HR for adult horses?
20 - 40 bpm
You notice a resting 2nd degree AV block when assessing a horse, what can be done next?
- Trot the horse and see if it disappears
If it doesn’t, consider speaking with a cardiologist
What is the normal RR for an adult horse?
8 - 12 breaths/ min
You notice a resting grade 1-3 murmur on the left side while assessing a horse, what do you suspect and what can be done next?
This is likely functional mitral regurgitation, that goes away during intense exercise and doesn’t affect performance
- Consult a cardiologist to confirm
You notice a resting grade 4 murmur on the right side while assessing a horse, what do you suspect and what can be done?
This is likely functional tricuspid regurgitation, that doesn’t affect performance
- Consult a cardiologist to confirm
How long do you withhold food from a horse before surgery?
Why do we withhold food from horses before surgery?
2-4 hours is usually enough
Can be up to 12 hours
We withhold food because there will be less material in the GI tract, better venous return, and better ventilation as there is less weight pressing on the diaphragm from the intestines
What is involved in the pre-operative preparation of an equine patient?
- Withhold food for 2-4 hours
- Give tetanus toxoid, antibiotics and NSAIDs if indicated
- Obtain an accurate weight
- Groom/cleaned
- Clip and prep surgical area
- Tie up/ bandage tail
- Pick out feet and remove shoes
- Rinse mouth
- Place an IV catheter
Formulate an anesthetic plan for a healthy 3-year-old horse, that is requiring routine castration in the field using general anesthesia rather than standing
Premed: Acepromazine + Romifidine + Morphine
Induction: Ketamine + Diazepam
Maintenance: Ketamine boluses + Isoflurane
What are the commonly used premedications in equine anesthesia?
- Acepromazine: tranquilizer
- Xylazine (alpha2 agonist)
- Detomidine (alpha2 agonist)
- Romifidine (alpha2 agonist)
+ an opioid (Butorphanol, Buprenorphine or Morphine)
Note: Morphine is the agent of choice! (0.01 mg/kg IV)
What are the effects of the alpha2 agonists in equine premed?
- Profound Sedation
- Analgesia
- Muscle relaxation
+ Hyperglycemia, diuresis and bradycardia
A horse presents with colic, which alpha2 agonist would be ideal in the premedication protocol for this horse and why?
- Xylazine
Short duration of action (20 mins)
Provides good visceral analgesia
Between the alpha2 agonists used in equine med, which one provides the least amount of ataxia?
- Romifidine
Between the alpha2 agonists used in equine med, which one provides the most amount of ataxia?
- Detomidine
When inducing a horse for surgery, what is a common combination protocol ?
Ketamine 2.2-3 mg/kg + 200mg
+ ONE of the following:
- Diazepam 0.05mg/kg OR
- Midazolam 0.05mg/kg OR
- Thiopental 0.3mg/kg OR
- Guaifenesin 50-100mg/kg
Which general anesthetic agents can be used in horses?
- Isoflurane is licensed
- Sevoflurane is on the essentials list
Once a horse is connected to the breathing circuit, what is the recommended initial fresh gas flow? (L/min)
What is the initial vaporizer settings for isoflurane? (%)
- 6 - 10 L/min
This is because you need to denitrogenate such a large breathing system and administer the inspired volatile agent in the shortest time possible - Iso = 4 - 5%
Once a horse is stabilized on the surgical table, what level of oxygen fresh gas flow can be used?
And what vaporizer setting can be used for Isoflurane?
- 1L/100kg (or less if using monitoring equipment)
- Iso = 1 - 3% (Usually around 1.5%)
When can you administer an NSAID to a horse undergoing anesthesia, and what NSAID might you use?
Give before surgical procedure
- Flunixin meglumine 1.1 mg/kg
- Phenylbutazone 2.2 mg/kg
While alpha2 agonists are used for induction of a horse for anesthesia, you can also use them as a CRI
What rates of Detomidine, Romifidine and Xylazine are used?
Detomidine = 10mcg/kg/hr
Romifidine = 40 mcg/kg/hr
Xylazine = 1mg/kg/hr
What is the benefit of using alpha2 agonists as a CRI during general anesthesia?
- Provides excellent analgesia
- Reduces the MAC of the inhalent volatile agent
- Improve recovery quality
What is the best way to monitor depth of anesthesia in a horse?
- Palpebral reflex: should be “just” present or sluggish
- Perineal reflex/ anal tone: should be absent
- Blood pressure: MAP should be kept above 70 mmHg, and any rapid changes in BP should be addressed
- Pulse Ox
- Capnography
- ECG + HR
- MM
You have been maintaining a horses anesthesia out in the field with boluses of ketamine, but you notice the horse has a strong palpebral reflex as well as ear and lip twitches, is this horse adequately anesthetized?
Possibly
Ketamine does not eliminate the cranial nerve reflexes and therefore palpebral, gag and ear/lip twitches are normal
Check BP, and perineal reflex to ensure adequate anesthesia
You notice a horse is under anesthesia and it has a rapid nystagmus, does this horse have adequate anesthesia?
NO!
This is a sign that the horse is about to move/ wake up
No nystagmus should be seen, along with no limb movement, swallowing or anal tone
You notice a horse under anesthesia has hypotension (MAP < 70 mmHg), why does this happen and what can be done to improve it?
- Very common side effect of using volatile agents
- They cause myocardial depression and peripheral vasodilation
Tx:
1) Reduce the vaporizer setting
2) Increase IV fluids
3) Use Positive Inotropes like Dobutamine
4) and as a last resort, use Vasoconstrictors like Noradrenaline or Phenylephrine
You notice a horse under anesthesia is experiencing cardiac arrest, why does this happen and what can be done to improve it?
- Cause is unknown, unexpected, unpredictable and the horse could have been 100% healthy
Tx:
1) CPR
You notice a horse under anesthesia is experiencing hypoventilation, why does this happen and what can be done to improve it?
- Need an arterial blood gas to diagnose
- Commonly happens, leading to respiratory acidosis
Tx:
1) Reduce dose of anesthetic drugs
2) Intermittent Positive Pressure Ventilation
You notice a horse under anesthesia is experiencing hypoxemia, why does this happen and what can be done to improve it?
- V/Q mismatch
- More likely to occur in dorsal recumbency, than in lateral because in dorsal the abdominal viscera pushes against the diaphragm and thus stents its movement
Tx/ Prevention:
1) Tilt table/ elevate head so viscera doesn’t push on diaphragm
2) Fast the patient before surgery
3) Use Mechanical ventilation
4) Recruitment maneuver: fill lungs to 40 mmH2O and hold to allow for oxygen to be absorbed
5) Increase CO/ Blood pressure
6) Administer ACE as a premed: improves arterial oxygen retention
7) Administer B2 agonist (e.g. Salbutamol) to cause bronchodilation
You notice a horse recovering from anesthesia is experiencing myopathy (lameness, restlessness, sweating, tachycardia and pain), why does this happen and what can be done to improve it?
Due to the constant pressure from the weight of the horse on its own muscles leading to poor perfusion
- Many factors contribute: Low BP, Large horses, body position, inadequate padding, long anesthesia (2+ hours), and venous stasis
- Lateral recumbency: triceps are usually affected
- Dorsal recumbency: gluteal/ longissimus dorsi and hindlimb adductors are usually affected
Prevention:
1) Maintain MAP > 70mmHg
Tx:
1) IV Fluid therapy: to flush the myoglobin out of the blood
2) NSAIDs + Alpha2 agonists: for sedation and analgesia
3) ACE: keep horse calm
4) Recovery with a sling
5) Physiotherapy
You notice a horse recovering from anesthesia is experiencing neuropathy, why does this happen and what can be done to improve it?
- Due to the same reasons as myopathy, leading to a non-painful nerve paralysis
- Facial nerve: head collar buckles pressing on the facial nerve, provide padding or remove them!
- Radial nerve: forelimb should be pulled forward and the upper limb parallel to the floor
- Femoral nerve: Poor positioning, hindlimbs must be parallel to the floor
You notice a horse recovering from anesthesia is experiencing an unusual calm and is unable to move its tail and hindlimbs, why does this happen and what can be done to improve it?
- Etiology is unknown
But believed to be due to a reduced perfusion and blood supply to the spinal cord, leading to ischemia + spinal cord malacia.
It is non-painful, but is fatal in all cases
Requires a PM for definitive diagnosis
You notice a horse recovering from anesthesia fractures its leg, why does this happen and what can be done?
Thought to be due to age-related loss of bone density in older horses
Mares seem to be affected more
NOT associated with recovery quality
- Euthanasia is recommended every time. It is unfair to have the horse recover from surgery and then go through recovery with a fractured leg/ have to undergo surgery AGAIN
When should Ketamine and Lidocaine CRI’s be discontinued during a horse’s surgery?
20-30 mins prior to recovery
What alpha-2 agonist is often used as a CRI to improve equine recovery, and why?
- Romifidine
Has the least ataxic effect and therefore horses are most successful standing up on their first attempt, leading to a lower risk of fractures or dislocations
What is the purpose of Recovery Sedation in equine medicine (post-surgery)
Give examples of drugs used for Recovery Sedation
Recovery sedation allows for the patient to eliminate the volatile anesthetic before attempting to stand
Patients without recovery sedation are known to have very violent recoveries that can put staff and themselves at risk of injury
e. g.
- Alpha-2 agonists: Romifidine, Xylazine, Detomidine
- Acepromazine
- Opioids: Butorphanol
One issue with equine recovery post-anesthetic is respiratory obstruction due to nasal edema, how can this be managed?
- Placement of nasal tubes to maintain patency of the nasal airways
- Use of nasal spray phenylephrine
remember: horses are obligate nasal breathers and require a nasal airway in order to oxygenate
What are the benefits of using TIVA (total intravenous anesthesia) in equine anesthesia?
- No special equipment needed
- Can be used in the field and in the OR
- Has minimal CVS and RS depression
- Recovery quality tends to be good
- Lower morbidity and mortality associated with TIVA
- Better safety to personnel as noxious volatile agents are not being used
What are the disadvantages of using TIVA (total intravenous anesthesia) in equine anesthesia?
- All of the drugs used have to be metabolized in the liver and therefore there may be competition for metabolism in the liver
- The maximum duration of anesthesia that can be achieved using TIVA is 90 mins, thus any anesthesia expected to be longer than this NEEDS inhalant anesthesia
- It is more difficult to control anesthetic depth
Give examples of TIVA’s used in equine anesthesia
1) Triple Drip: Guaifenesin + Alpha-2 agonist (Xylazine or Detomidine/ Romifidine) + Ketamine
How does Guaifenesin act on the Central Nervous System?
- Mechanism is largely unknown but suspected to be a GABA agonist
- Will bind to specific inhibitory neurotransmitter receptors in the brain and spinal cord that are activated by GABA
- This will depress impulse transmission in the neurons of the spinal cord without impairing breathing
- Providing muscle relaxation and some sedation
What is the dose and rate used for Guaifenesin?
Dose: 25 - 100 mg/kg IV
Caution: NEVER exceed 200 mg/kg
Rate: 2 - 3 ml/kg/hour, which can be decreased to 0.7 - 1 ml/kg/hour
What is the toxic dose of Guaifenesin, and what signs will you look out for in case of toxicity?
Toxic Dose: 200+ mg/kg
Signs of toxicity = loss of palpebral reflex, respiratory arrest, cardiac arrest, and death
Is there an antidote to Guaifenesin?
No
Guaifenesin’s toxic dose is 200 mg/kg, however, it is advised to keep the dose between 25 - 100 mg/kg
Why is this?
Dosing greater than 100 mg/kg has side effects including:
- Tremors or rigidity, even though it’s a muscle relaxant…
- Apnea
Where is Guaifenesin metabolized and excreted?
- Metabolized in the liver, into Glucuronide
- Excreted in the urine
You are using Guaifenesin as a CRI, you notice the horse is blinking, has lacrimation, is swallowing, and has a nystagmus
Is this normal? What can be done?
This is normal for Guaifenesin anesthesia for the horse to appear light
This is NOT urgent
- Assess the HR, RR and position of the eyes to gain a true indication of depth of anesthesia
Can Guaifenesin be used in the pregnant mare?
Yes, it does cross the Placenta but no depression is seen in the foal
Also, does NOT lead to premature delivery or abortion
However, Ketamine must be used concurrently to provide analgesia and unconsciousness
You are using Guaifenesin as a CRI and notice one of the horse’s limbs is slowly moving, is this normal? What can be done?
- Slow movement of one limb (depending on the surgery) is normal and NOT urgent
- Assess HR, RR, and eye position to ensure adequate depth of anesthesia
You are using Guaifenesin as a CRI and notice the horse moves its head or more than one of its limbs, is this normal? What can be done?
- This indicates a top-up is needed, the horse is too light
Why are foals prone to Bradycardia and Hypotension?
Their PSNS is developed, however their SNS is not fully developed
You are presented with a foal that is younger than 1 month old, which Induction agent should you avoid?
Alpha-2 agonists
You are presented with a foal that requires sedation, what drug combination is often adequate for a foal?
Opioid + Benzodiazepine
You are presented with a foal that requires sedation for surgery, what is the protocol for sedating a foal that was brought with its mare?
- No fasting needed unless specifically indicated
- Keep the mare and foal together for as long as possible
- Sedate the mare: Alpha-2 agonist +/- Acepromazine +/- Butorphanol
- Induce the foal with the dam present to keep both calm: Diazepam + Ketamine IV OR Propofol
- Intubate foal
Can NSAIDs be used in foals for analgesia?
Yes, Carprofen can be used in foals > 6 weeks old
Caution: there is a high risk associated with GI ulceration and therefore GI protectants MUST be given as well
Other risks include: renal toxicity and platelet dysfunction
A horse presents with colic and is clearly in pain, what analgesia protocol can be used?
- Alpha-2 agonist: Xylazine
+ Opioid: Morphine
+ NSAIDs: Flunixin
You are presented with a colic case, you have provided adequate analgesia to control pain (Xylazine + Morphine + Flunixin), what is done next?
Once analgesia is provided
- Place a NG tube to relieve pressure in the stomach
- Administer Hypertonic saline through multiple IV catheters: 1 - 2 whole bags are usually needed (can give up to 5 bags)
- Administer antibiotics before anesthesia
What PreMed, Induction, and Maintenance agents are commonly used in colic cases?
Premed: Low dose Xylazine (0.3-0.5 mg/kg) + Morphine (0.2 mg/kg)
Induction: Ketamine + Diazepam
Maintenance: Inhalant (Sevo/Iso) + Lidocaine CRI (loading dose = 1.5 mg/kg over 15 mins, CRI maintenance = 25-100 mcg/kg/min)
Caution: Avoid Acepromazine!
A colic case presents, can Acepromazine be used as part of the premed?
No
Causes vasodilation, and in these horses their CVS is usually compromised with poor contractility, hypotension, and arrhythmias, therefore vasodilation can be fatal
A colic horse is under anesthesia for exploratory laparotomy, you notice the horse is becoming hypocalcemic, what can be done to correct this?
This is very common
- Calcium gluconate 10%: 6-10 ml/kg over 20 mins
A colic horse is under anesthesia for exploratory laparotomy, you notice the horse is becoming hypomagnesemic, what can be done to correct this?
- Magnesium sulphate: 25mg/kg IV over 20 mins
A colic horse is under anesthesia for exploratory laparotomy, you notice the horse is becoming hypokalemic, what can be done to correct this?
This is very common
- Potassium Chloride, but at a maximum rate of 0.5 mEq/kg/hour
A colic horse is under anesthesia for exploratory laparotomy, you notice the horse is becoming acidotic (pH <7.2), what can be done to correct this?
This is very common
- Sodium bicarbonate + extra ventilation to get rid of the extra CO2
Is sedation required during a donkey/mule recovery post-anesthesia?
No
Unlike in horses who require sedation for a smooth recovery, donkeys/mules recover slower and more controlled
What parameters can be used to assess the depth of anesthesia in a donkey/mule?
- BP is the most sensitive indicator + HR + RR
Eye signs and palpebral reflex is less reliable
