A few important points before we begin this discussion:
Anesthesia involves more than simply looking at a chart and pulling up drugs. Patient behavior, breed, age, the environment, available supples, skill of the surgeon and anesthetist, position of the patient, and expected length of the procedure will all go into your decision making process when determining the protocol for each patient.
Our protocols may be different than what you are used to or what you were taught in school. There are many ways to perform field anesthesia in a horse. We have developed these protocols over years of experience working with untrained/minimally handled horses with limited supplies and availability for immediate follow-up at times.
Jugular venipuncture in healthy, well-trained and/or anesthetized horses is generally not difficult. Therefore, we do not routinely use catheters in horses, but find them useful in donkeys, minis, and horses where venipuncture is more challenging. Otherwise, we use 18g x 1.5″ needles for administering all IV drugs in both awake and anesthetized horses.
Note for student volunteers: IV injections are to be supervised by a veterinarian.
Steps for anesthesia of routine castration or other short procedures
Evaluate your patient
If possible, perform a brief PE. If a “hands-on” exam is not possible or is likely to excite/frighten/stress the patient, then consider skipping it. Do not compromise the patient’s anesthesia or get hurt to perform a PE.
We often perform anesthesia on horses/donkeys/mules that have had minimal handling. These animals generally require higher doses of drugs than trained animals and are often more challenging to maintain under anesthesia.
We perform visual PEs on animals that we are unable to handle. We ask ourselves is the hair coat healthy, are the eyes bright, is the energy level normal for the level of excitement. We also consider whether or not the patient is likely to be castrated without anesthesia if we are not able to castrate it. Other reasons we may anesthetize a horse that we are unable to handle is to perform a closer exam and/or treat a sick or injured horse.
Calculate/adjust premed and induction doses
Calculated doses are based on the weight of the animal, which you will often estimate or can use a weight tape if possible. A starting dose can be calculated based on the weight of the animal, but the following 10 factors should be considered for dose adjustment.
1. Temperament. Excitable, fearful, stressed animals will require more drugs than calm animals. This amount can be dramatically more (double to triple the amounts of Xylazine and ketamine in some cases). This factor can play a huge role. See table 2. below for more details in adjusting based on temperament.
2. Species/breed. Burros and mules clear anesthetic drugs at a higher rate. Draft breeds and foals can be harder to read and may require more drugs than anticipated.
3. Anesthetist and surgeon skill level. You may want to err on keeping your patient deeper if you are an inexperienced anesthetist. Inexperienced surgeons will take longer to perform the procedure and the patient will likely require more drugs.
4. Patient positioning. Patients in dorsal recumbency burn through drugs more quickly than patients in lateral recumbency.
5. Use of local blocks. Local blocks should decrease anesthetic requirement by decreasing stimulation during the procedure.
6. Type of procedure. This will determine the length of the surgery and amount of stimulation during the surgery. The longer the procedure and the more stimulating the procedure, the more drugs your patient will require.
7 . Surrounding environment. Load noises can stimulate the patient, even when they are anesthetized and can increase anesthetic requirement. You may also want to keep the patient on the deeper side if you have limited space to work and there are people in close proximity to the animal.
8. Available equipment or personnel. Available equipment may determine how the procedure is performed and the length of the procedure. If there are not enough people available to hold the patients legs, you may have to tie the legs. You may want to err on the side of keeping the patient deeper if you are short staffed and having to tie the patients legs.
Anesthetic Drug Dosages
Drug
Basic Protocol
Dose Range
Xylazine
1 mg/kg
0.3 to 1 mg/kg
Butorphanol
0.02 mg/kg
0.02 to 0.04 mg/kg
Midazolam
0.03 mg/kg
0.01 to 0.1 mg/kg
Ketamine
2 mg/kg
1.5 to 2.75 mg/kg
Dosing Chart for Basic Equine Anesthetic Protocol
Weight
(lbs)
Weight
(Kgs)
Xylazine (ml)
Butorphanol (ml)
Midazolam (ml)
Ketamine (ml)
1mg/kg or
0.5mg/lb
0.02mg/kg or
0.01mg/lb
0.03mg/kg or
0.015mg/lb
2mg/kg
or
1mg/lb
110
50
0.5
0.2
0.3
1.0
220
100
1.0
0.2
0.4
2.0
330
150
1.5
0.3
0.5
3.0
440
200
2.0
0.4
1.0
4.0
550
250
2.5
0.5
1.5
5.0
660
300
3.0
0.5
2.0
6.0
770
350
3.5
0.5
2.5
7.0
880
400
4.0
0.5
3.0
8.0
990
450
4.5
0.5
3.5
9.0
1100
500
5.0
0.5
4.0
10.0
1210
550
5.5
0.5
4.5
11.0
1320
600
6.0
0.5
5.0
12.0
Adjusting Premed to Fit Your Patient
Premed
Drug
Standard Dose
Very quiet patient
No butorphanol available
Slight excited/excitable
Moderately excited/excitable
Very excited/excitable
Intractable/unable to obtain IV access*
Xylazine dosing adjustment
1 mg/kg
Decrease by up to 50%
Increase 10%
Increase 20%-25%
Increase 50%
Increase 80% – 100%
Xylazine dose
1 mg/kg
0.75 mg/kg
1.1 mg/kg
1.2 mg/kg
1.5 mg/kg
1.8 mg/kg to
2 mg/kg
Butorphanol dose adjustment
0.02 mg/kg
0.02 mg/kg
——-
Use standard dose
Increase 50% – 100%
Increase 100%
Butorphanol dose
0.02 mg/kg
0.02 mg/kg
——-
0.02 mg/kg
0.03-0.04 mg/kg
0.04 mg/kg
*In select situations where patient temperament or facilities present a danger to the patient or handlers we may use IM sedation. This will be done using either dart gone or pole syringe. Following sedation the anesthesia process will proceed as with an un-sedated patient. This is because patients requiring IM sedation will be roused when approached and will be difficult to anesthetize. They should be approached very quietly and only with the help of skilled staff. We generally try to avoid the use of long-acting IM alpha 2 agonists such as detomidine due to increased difficulty of recovery. Improving the quality of these recoveries in intractable patients and difficult anesthesias will be covered later.
See dart and/or pole syringe IM sedation protocol
Sedate your patient with the premed
Every effort should be made to sedate the patient quietly and avoid stressing/exciting the patient.
Administer calculated/adjusted doses of xylazine + butorphanol IV. Mix these drugs in a single syringe (3-12ml, most commonly 6ml). Use an 18g x 1.5″ needle. We typically leave the needle in the jugular vein until after the induction is given.
Evaluate the degree of sedation achieved following the premed. Within 2-5 minutes the patient’s head should drop below the withers. It may be necessary to administer additional xylazine if patient does not become sedate with the initial premed.
Induce the patient
Once the patient is adequately sedate (head below the withers), induce the patient. You may not want to let the patient become overly sedate if inducing in the chute.
Ensure the needle in the vein is still in place/patent if you are using the same needle you used to administer the premed.
Administer ketamine and midazolam (+/- additional xylazine) combined in a single syringe (6-20ml, most commonly 12ml). Remove the needle from the neck after giving the induction.
If inducing a halter-broke patient, have the person who will be “dropping the hose” take the halter and lead rope into their hands in front of the horse immediately after induction Is administered. Have all people and equipment at least 20-30ft away from the horse. The patient may stagger and fall in any direction. Give the patient plenty of space!
If inducing the horse in the chute, open the chute immediately after giving induction and encourage the patient to move out of the chute by waving arms and pulling on the neck rope.
The patient should fall within 1 minute after giving the induction. Once the patient becomes laterally recumbent, ensure the patient stops paddling and is fully anesthetized before getting close to the patient. The anesthetist is in charge of giving the go-ahead for placing the leg rope.
Adjusting Induction Dose to Fit Your Patient or Environment
Induction
Drug
Standard Dose
Routine environ-ment or castration
Crypt or hernia surgery
Excitable/high energy horse
Induction in which patient will be chased away from chute before they drop*
Midazolam
0.03 mg/kg
0.03
mg/kg
0.05
mg/kg
0.03-0.04
mg/kg
0.03-0.04 mg/kg
Ketamine
1.5-2.75mg/kg
2.0-2.2 mg/kg
2.2-2.75 mg/kg
2.2-2.75 mg/kg
2.2-2.75 mg/kg
Xylazine
Standard dose
1.5-2.0
mg/kg
~20-30% of original xylazine dose added to induction dose
*To move patient further away from chute, or site of induction, do not allow patient to become very sedate after premed. Administer ketamine, midazolam, and xylazine together and chase the patient away from the chute, only until adequate forward movement has begun. Allow the patient to fall unassisted.
Maintain anesthesia
As soon as the patient becomes lateral, the anesthesia team should place a pillow under the patient’s cheek/neck and a blue towel over the patient’s eyes upper and lower.
Check the patient’s respiratory rate, eye for nystagmus, ears for movement, stiffness in the legs (all hooves should be touching the ground). If patient is adequately deep, give the go ahead for placing the leg rope and prepping the patient. Place an 18g x 1.5″ needle in the jugular vein and administer flunixin. Leave the empty syringe attacked.
If the patient is light, administer a topper immediately (1.5-6ml depending on patient size and depth of anesthesia). Wait 30sec-2min for the topper to take effect before proceeding.
Throughout the procedure, continue to monitor the patient for changes in respiratory rate/depth, tearing, nystagmus, blinking, other movement (ear, tail, limb), etc. Consider topping the patient if they are showing signs of being light. Other things to consider before topping a patient include time remaining in the procedure, position of the surgeon and other personnel, etc. Remember, it will take at least 30 seconds for the topper to take effect and the surgeon may need to pause the procedure to wait for the patient to become deeper.
Keep in mind, if a patient is premeded properly, induced with adequate amounts of ketamine and midazolam, and the surgeon is able to perform the castration in less than 6-7 minutes, no toppers should be required.
Signs of Anesthetic Depth
Light depth
Adequate or deep depth
Nystagmus present
No nystagmus
Eye position rostral
Eye position central
Lacrimation (“tear formation”)
Minimal or absent lacrimation
Muscle rigidity
Relaxed muscle tone
Vocalization
No vocalization
Movement (esp. tail, ear, leg)
No movement
Strong palpebral reflex
Reduced palpebral reflex
Frequent spontaneous blink
No or occasional spontaneous blink
“Heavy”/deep, rapid breathing
Slow, steady breathing
Change in respiratory pattern, rate, and or depth indicates a change in plane and is considered by some to be the most reliable sign.
Recover the patient
Pull the down leg forward following the procedure.
If the patient received a large volume of anesthetics (3 or more toppers, repeat induction doses, etc), then administer an additional 20% of initial xylazine dose (usually ~1ml) at the end of the procedure. This will help the patient stay down and sleep off the ketamine before trying to stand.
The towel should be left in place over the horses eyes. It will fall off when the horse sits up/stands.
Once standing, if the horse is wearing a halter and is comfortable with a handler, someone can stand and hold the end of the lead rope, without encouraging the patient to move. If the patient is wanting to move away from people, put the lead rope down and give the patient more space. You can try picking up the lead rope again when the patient is more steady on their feet.
Occasionally a patient will try to rise prematurely when they still have significant nystagmus. In these cases, we may try and hold the horse down in lateral recumbency by placing a knee of the patient’s neck and holding the patient’s nose up. Do not attempt this if you are not familiar with this technique.
A horse should not be loaded and hauled until it is steady on its feet. This is usually another 30-45 minutes after the patient rises.
If a horse has been laterally recumbent for 30-45 minutes following a procedure, consider rolling the patient. If the patient remains recumbent for an additional 20-30 minutes after rolling, consider reversal.
Tips for gaining access to the intractable patient
Use of a quiet, well broke horse to pin a horse for sedation.
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