28 – Equine Field Anesthesia Flashcards
What are 5 considerations for field anesthesia?
- Procedural
- Environmental
- Patient
- Equipment
- Personnel
Procedural considerations
- 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
What are the optimal environmental conditions?
- Soft ground
- Flat or gentle gradient
What should be avoided for environmental conditions?
- Man made or natural hazards
o Barbed wires fences
o Farm equipment
o Ponds
o Streams
o Ravines
What are the 4 points for patient considerations?
- Thorough history and physical examination
- Stabilization
- Weight estimation
- Owner consent
Thorough history and physical examination (patient considerations)
- CV and respiratory systems
- Gut motility
- Pain
Stabilization (patient considerations)
- Hemorrhage
- Fasting (minimum 6 hrs)
Weight estimation (patient considerations)
- *Drug dosages require an accurate weight
- Tape
- Calculation
What is the MINIMUM anesthesia equipment?
- IV catheters
- sufficient amount of drugs
- soft ropes with an appropriate halter
- small towels: covering/protecting eyes
- Oxygen supplementation
Oxygen supplementation
- Horses very prone to hypoxemia during GA (V/Q MISMATCH!)
- E-tank with nasal insufflation
o Full E-tank (2200psi) holds 625L of O2
Personnel considerations
- You are responsible for safety of the horse, but ALSO safety of owner and every other individual!
- *wide range
What are some sedation/premedication’s for horses?
- Acepromazine
- Alpha2 agonists
Acepromazine (sedation/premedication)
- 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
Alpha2 agonists (sedation/premedication)
- 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
What are the 3 alpha2 agonists used in horses?
- Xylazine (20 mins)
- Detomidine (40 mins)
- Romifidine (40 mins)
Standing sedation vs pre-anesthetic medication doses of alpha2 agonists
- Standing: low dose
- Pre-anesthetic medication: high dose
Alpha2 agonists have CV effects with IV administration
- 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
Alpha2 agonists with opioids
- *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
Alpha2 agonists with opioids and acepromazine
- COMMON=Detomidine/butorphanol/acepromazine
- Detomidine/acepromazine is good for MARES
What happens with romifidine mixed with acepromazine?
- Precipitation=do NOT administer
Induction of anesthesia: 6 steps
- Pre-anesthetic preparation
- Assess facilities/environment
- Place an IV catheter
- Check halter and ropes (thicker banded are better)
- Protect facial nerve if leaving halter on head
- Select site: brief the help on safety and potential problems
Pre-anesthetic preparation
- Fast minimum 6 hours; water until 1 hr before
- Medical history
- Physical examination
- Clinical haematology as necessary
What are the common anesthetic field induction protocols?
- 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
What do you need to do ensure before using Ketamine to induce?
- ‘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)
Things to consider with recumbency?
- Don’t allow head to smack ground
- Check vital signs and anesthetic depth
- Position for procedure
- Protect eyes
- Proteins cranial nerves: remove halter
What are the 2 options for maintenance of anesthesia?
- Incremental top-up IV bolus
- Continuous IV infusions
What is a ‘top-up’ for maintenance of anesthesia?
- 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
When would you use ‘triple-drip’ in maintenance of anesthesia?
- When need a little longer duration of anesthetic time
- Max anesthesia time=~90mins
How do you prepare and use a ‘triple drip’?
- 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
What are the ways the check the depth of anesthesia?
- 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
What do you see in a horse that is in a ‘LIGHT’ plane of anesthesia?
- Spontaneous blinking
- Tearing
- Nystagmus: *good indicator they are deep enough and also that they are going to wake up
What are some things used for monitoring in the field?
- Portable equipment
- Measure BP with doppler and sphygmomanometer (ex. on tail)
- Use pulse oximeter
What is some life support examples?
- If longer procedure, consider
o Oxygen
o Fluids - *refer long procedures
Recovery
- 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
What are the 3 drug categories used in horses for analgesia?
- NSAIDs
- Opioids
- Local anesthetic block
NSAIDs for analgesia
- Used pre-emptively to minimize surgical/inflammation pain
Opioids for analgesia
- Provides good analgesia and INCREASE sedation
o Butorphanol: 60-90mins
o Morphine: 2-4 hrs
o Buprenorphine: 12 hrs, but 30-45min onset
Local anesthetic blocks for analgesia
- Useful part of balanced analgesia
- Ex. Intra-testicular for castration
