28 – Equine Field Anesthesia

Question 1

What are 5 considerations for field anesthesia?

Answer 1

1. Procedural
2. Environmental
3. Patient
4. Equipment
5. Personnel

Question 2

Procedural considerations

Answer 2

• Nature of injury?
• Standing sedation and local block OR general anesthesia?
• Complexity of surgery?
• Expected duration?
  o Top ups
  o Triple drip: combo of drugs, longer duration and action (medicine will accumulate and contribute to ‘rough’ recovery)
• *only SIMPLE procedures of SHORT DURATION should be attempted away from the ‘relative safety’ of well-equipped and well-staffed hospital

Question 3

What are the optimal environmental conditions?

Answer 3

• Soft ground
• Flat or gentle gradient

Question 4

What should be avoided for environmental conditions?

Answer 4

• Man made or natural hazards
  o Barbed wires fences
  o Farm equipment
  o Ponds
  o Streams
  o Ravines

Question 5

What are the 4 points for patient considerations?

Answer 5

1. Thorough history and physical examination
2. Stabilization
3. Weight estimation
4. Owner consent

Question 6

Thorough history and physical examination (patient considerations)

Answer 6

• CV and respiratory systems
• Gut motility
• Pain

Question 7

Stabilization (patient considerations)

Answer 7

• Hemorrhage
• Fasting (minimum 6 hrs)

Question 8

Weight estimation (patient considerations)

Answer 8

• *Drug dosages require an accurate weight
• Tape
• Calculation

Question 9

What is the MINIMUM anesthesia equipment?

Answer 9

• IV catheters
• sufficient amount of drugs
• soft ropes with an appropriate halter
• small towels: covering/protecting eyes
• Oxygen supplementation

Question 10

Oxygen supplementation

Answer 10

• Horses very prone to hypoxemia during GA (V/Q MISMATCH!)
• E-tank with nasal insufflation
  o Full E-tank (2200psi) holds 625L of O2

Question 11

Personnel considerations

Answer 11

• You are responsible for safety of the horse, but ALSO safety of owner and every other individual!
• *wide range

Question 12

What are some sedation/premedication’s for horses?

Answer 12

1. Acepromazine
2. Alpha2 agonists

Question 13

Acepromazine (sedation/premedication)

Answer 13

• Useful as ‘pre-med’ often in combo with other drugs
• Used alone for non-painful procedures
• Sufficient time should be allowed for it to work (*30-40min)
• Provides ANTI-ARRHYTHMIC EFFECT
• Vasodilation and hypotension does occur (alpha1 antagonism)
• Penile prolapse! (AVOID IN BREEDING STALLIONS)
• Use to decrease mare’s stress if working on the foal

Question 14

Alpha2 agonists (sedation/premedication)

Answer 14

• MORE reliable sedation
• Analgesia and muscle relaxation (some ataxia)
• *equipotent doses of 3 different alpha2 agonists used in horses
• *DO NOT USE TRUST WHEN USED ALONE=HORSES CAN STILL KICK

Question 15

What are the 3 alpha2 agonists used in horses?

Answer 15

• Xylazine (20 mins)
• Detomidine (40 mins)
• Romifidine (40 mins)

Question 16

Standing sedation vs pre-anesthetic medication doses of alpha2 agonists

Answer 16

• Standing: low dose
• Pre-anesthetic medication: high dose

Question 17

Alpha2 agonists have CV effects with IV administration

Answer 17

• Bradycardia does NOT require Atropine in horses and ruminants
  o Wait a few minutes and HR will increase (need high enough HR to get ketamine to brain to have a SMOOTH induction)
• Also don’t use atropine in horses and ruminants as it decreases GI motility
• *do not titrate in large animals=by pass excitement phase

Question 18

Alpha2 agonists with opioids

Answer 18

• *butorphanol
• Morphine: use a longer lasting alpha2 agonist
• *can mix drugs OR give drugs separately
• *want to use sedative with opioids as they can cause excitement

Question 19

Alpha2 agonists with opioids and acepromazine

Answer 19

• COMMON=Detomidine/butorphanol/acepromazine
• Detomidine/acepromazine is good for MARES

Question 20

What happens with romifidine mixed with acepromazine?

Answer 20

• Precipitation=do NOT administer

Question 21

Induction of anesthesia: 6 steps

Answer 21

1. Pre-anesthetic preparation
2. Assess facilities/environment
3. Place an IV catheter
4. Check halter and ropes (thicker banded are better)
5. Protect facial nerve if leaving halter on head
6. Select site: brief the help on safety and potential problems

Question 22

Pre-anesthetic preparation

Answer 22

• Fast minimum 6 hours; water until 1 hr before
• Medical history
• Physical examination
• Clinical haematology as necessary

Question 23

What are the common anesthetic field induction protocols?

Answer 23

• Xylazine (1mg/kg IV) OR detomidine (10-20 micrograms/kg IV)
• *wait a few minutes for peak sedation
• Mix: diazepam and ketamine
  o Diazepam: optional but does ‘smooth’ induction
• Can add butorphanol
• Consider local anesthetic techniques

Question 24

What do you need to do ensure before using Ketamine to induce?

Answer 24

• ‘nose to knees’ sedation!
  o If poor=top up sedation with alpha 2 agonist
• Check HR
  o If extremely low=wait until it comes back up (2-3mins)

Question 25

Things to consider with recumbency?

Answer 25

- Don’t allow head to smack ground - Check vital signs and anesthetic depth - Position for procedure - Protect eyes - Proteins cranial nerves: remove halter

Question 26

What are the 2 options for maintenance of anesthesia?

Answer 26

1. Incremental top-up IV bolus 2. Continuous IV infusions

Question 27

What is a ‘top-up’ for maintenance of anesthesia?

Answer 27

- Ketamine + xylazine o Mix 1/3 to1/2 the pre-medication and induction doses o May provide 2-3 ‘top ups’ o Given every 10-15 mins depending on depth of anesthesia - *ketamine is slightly cumulative o After 3rd ‘top up’ dosing interval should be increased OR subsequent doses decreased

Question 28

When would you use ‘triple-drip’ in maintenance of anesthesia?

Answer 28

- When need a little longer duration of anesthetic time - Max anesthesia time=~90mins

Question 29

How do you prepare and use a ‘triple drip’?

Answer 29

- In 1L 5% guaifenesin add o 500mg Xylazine o 1000 mg Ketamine - Rate of 2ml/kg/hr - Good for procedures up to 60mins o Ideally with Oxygen and good positioning - Cardiopulmonary stability - Good analgesia - Easy to use - Good recovery

Question 30

What are the ways the check the depth of anesthesia?

Answer 30

- Pulse quality, mucous membrane colour, CRT, HR - RR and depth - Check eye reflexes: palpebral reflex is ‘brisk’ - *do NOT use corneal reflexes=CAUSES TRAUMA TO EYE - Horses eyes stay central

Question 31

What do you see in a horse that is in a ‘LIGHT’ plane of anesthesia?

Answer 31

- Spontaneous blinking - Tearing - Nystagmus: *good indicator they are deep enough and also that they are going to wake up

Question 32

What are some things used for monitoring in the field?

Answer 32

- Portable equipment - Measure BP with doppler and sphygmomanometer (ex. on tail) - Use pulse oximeter

Question 33

What is some life support examples?

Answer 33

- If longer procedure, consider o Oxygen o Fluids - *refer long procedures

Question 34

Recovery

Answer 34

- Usually don’t use reversal - Control recovery: ‘hold’ tail to help them be more stabilized - Keep safety of people in mind - Avoid noise - Keep eyes covered - Consider sedation (xylazine) if recent ‘top-up’ with ketamine alone in previous 10mins - *if longer procedure: consider placing a urinary catheter and emptying the bladder

Question 35

What are the 3 drug categories used in horses for analgesia?

Answer 35

- NSAIDs - Opioids - Local anesthetic block

Question 36

NSAIDs for analgesia

Answer 36

- Used pre-emptively to minimize surgical/inflammation pain

Question 37

Opioids for analgesia

Answer 37

- Provides good analgesia and INCREASE sedation o Butorphanol: 60-90mins o Morphine: 2-4 hrs o Buprenorphine: 12 hrs, but 30-45min onset

Question 38

Local anesthetic blocks for analgesia

Answer 38

- Useful part of balanced analgesia - Ex. Intra-testicular for castration