Equine Drugs

Equine General Anesthesia Protocols and Procedures

Anesthetizing horses is quite different from anesthetizing dogs and cats.

Watch this video for a general overview. FWIW I have never had someone put a tarp over me and I tend to be less hands on but this is pretty typical for the induction and recovery (the castration bit has been removed):

Preoperative care

It is always important to do a complete physical exam. The death rate for healthy horses that undergo procedures using anesthesia is 0.9% and this is increased to 1.9% in sick horses. Elective procedures should be performed only in healthy horses.

Food should be withheld for 6-12 hours prior to general anesthesia. Horses are allowed free access to water. The longer food is withheld, the more likely GI upsets are to occur. The normal stomach empties in about 1-2 hours.

Tetanus vaccination should be up to date (within 6 months).

Preoperative NSAIDs are almost always indicated! Surgery causes inflammation and the effect of the drugs is much better if given prior to windup. Preoperative antibiotics should be given for GI, urogenital and/or respiratory surgeries and with any known infections.  See the perioperative medications chapter.

Anesthetizing horses

As with other species, anesthesia involves

  1. sedation – xylazine, romifidine, detomidine
    1. optional additional sedatives or analgesics –

butorphanol or acepromazine (given with sedatives)

diazepam or midazolam (given at induction)

2. induction – ketamine, telazol, guaifenesin (GG)

3. maintenance – repeated doses of sedatives and induction agents or infusion of drugs and drug combinations – infusions of drugs iv, repeated doses of induction agents, intubation and inhalants

This narrated powerpoint works more slowly through the steps of anesthesia for castration of a pony, including some short videos. Watch through 3:50. You will see this again in the castration section.

Horse are generally not intubated or maintained on inhalant anesthetic agents except in specialty or referral settings. Horses are easy to intubate but intubation requires a deeper plane of anesthesia than does the surgery. Camelids are NOT easy to intubate. In both situations, it isn’t common to carry oxygen cylinders in trucks (explosion hazard) and LA anesthesia machines are not portable.  Intubation holds the airway open but does not bring the same advantages in the field setting as it does in a hospital where it is conduit for oxygen and inhalant anesthetics.

Monitoring is typically through vital signs (pulse quality and rate, respiratory quality and rate, ocular reflexes and positioning). Pulse oximeters are portable and easy to use. Doppler blood pressure monitoring is useful, particularly in foals.

Common induction and maintenance drugs

Ketamine

Schedule III

Ketamine is a dissociative anesthetic agent used to induce recumbency in horses after sedation. It is commonly combined with diazepam to prolong the anesthetic duration and to enhance relaxation.

Ketamine is a relatively safe drug with minimial cardiovascular or respiratory depressant effects. “It is hard to kill with ketamine.” However, horses will pass through an excitatory phase so it should be given iv after strong sedation and given rapidly. If redosed, ensure horses are still sedated (eg with xylazine) prior to recovery. They should not recover on ketamine alone.

Initial dose is typically 1cc/100# (100mg/ml). Eg a 1000# horse gets 10cc ketamine. A redose is typically 1/2 the original dose.

Ketamine is also used as an iv infusion and epidurally for pain relief to awake horses.

 

Telazol

Schedule III

Telazol is a combination of tiletamine (dissociative agent like ketamine) and zolazepam (benzodiazepene like diazepam). It lasts longer than ketamine/diazepam but is more expensive.

The drug needs to be reconstituted prior to use and then has a limited shelf life.

It seems to work better when paired with detomidine vs xylazine.

Ketamine, telazol, xylazine and detomidine. A comparative anesthetic drug combination study in ponies. Acta Vet Scand 1992

Triple drip

Triple drip is a combination of xylazine, ketamine and guaifenesin used to maintain anesthesia.

Xylazine (500 mg), ketamine (1000 mg or 1 g), in a liter of 5% guaifenesin (50mg/ml), given at 1ml/lb/hr.

Guaifenesin (GG) is a muscle relaxant that helps counteract the rigidity associated with ketamine.

Injectable GG is only available through compounding pharmacies at this time.

Side effects: Very irritating perivascularly. Use a catheter. Overdoses of GG can lead to increased muscle tone, making the horse appear too light vs too deep.

Equine anesthesia: triple drip. Int J Pharm Cmpd, 2008

http://www.r-vets.org/Anesthesia_Basics.pdf

Other

Other drugs and drug combinations are possible and used. Newer ones are being developed all the time.

Currently thiobarbiturates are not available in the US. Propofol has been used but is costly.

Overview

If I want to anesthetize a large animal species, I start with a (mental) base of xylazine +ketamine and adjust from there.  For general anesthesia, medetomidine is gaining some use as another alpha-2 agent that can replace xylazine. Detomidine (0.02 mg/kg) or romifidine (0.1 mg/kg) could also replace the xylazine if needed. Basically any sedative that would relax the horse sufficiently to have its nose on the ground works. As with standing sedation, using more types of drugs generally decreases the amount of each drug needed and can increase the safety and duration.
To prolong the surgery time, we can add butorphanol or benzodiazepines. Butorphanol potentiates the sedation and analgesia and decreases the amount of xylazine needed. Butorphanol (0.01-0.02 mg/kg iv) can also be helpful if the horse is excited and not responding well to the xylazine. The benzodiazepines also prolong the effects of the xylazine/ketamine. Adding midazolam or diazepam (0.03 mg/kg) to the protocol not only prolongs the duration of surgical anesthesia but also decreases the dose of xylazine required and smooths out the process. These drugs also relax muscles which is handy because ketamine tenses muscles and can make some procedures harder to perform (eg castration due to cremaster tone). Too much benzodiazpene and relaxation can hinder recovery.
Acepromazine can be used for healthy agitated animals and can help smooth their recovery. Ace does take at least 20 min to kick in and can last for hours so give prior to anesthesia.
For small animals, the sedatives are often midazolam + butorphanol or acepromazine + butorphanol. Those can work in pigs but aren’t enough for most of our healthy large animal patients. 
Ketamine is an induction agent and “induces” general anesthesia. In the “old days” when I was a new grad, we gave xylazine to create heavy sedation, then gave ketamine. Longer procedures got smaller doses of both as needed until we were done.
Ketamine is a very safe drug as it has minimal cardiovascular or respiratory effects.  There are rare times we won’t use ketamine. It can be assumed safe as long as appropriate sedation is present. However it causes excitement during equine induction and recovery.  It is important to ensure the horse is very sedate prior to injecting ketamine and that a sedative is still acting for recovery. E.g. don’t give a top off dose of ketamine near the end of your procedure without also giving a sedative. There is no reversal agent for ketamine at this time.
The other common induction drug is guaifenesin, otherwise known as GG. Yes, as in the cough syrup. GG is an infusion and is given iv to effect. It leads to muscle relaxation and the infusion makes it easy to adjust anesthetic planes. It is very useful for agitated animals, donkeys and stallions that tend to require frequent redosing,  as well as for longer procedures.
For small animals, the induction drug is often propofol. Propofol works in large animals but is expensive and leads to poor recoveries in horses. Thiopental and thiamylal are similar drugs that are no longer available but will still appear in the literature. Anesthesiologists are also starting to play with alfaxolone in horses but so far not too successfully.
To maintain anesthesia, we can continue the GG infusion. If GG isn’t available, we usually give repeated 1/2 doses (1/2 of the original sedation/induction doses) of the sedative and ketamine. The benzodiazepines, acepromazine and narcotics tend to have long enough half lives that redosing isn’t needed. You can create an infusion of xylazine and ketamine, as well. Recently, medetomidine/ketamine/midazolam infusions were shown to be similar to medetomidine/ketamine/GG infusions. Medetomidine is primarily in use in referral centers as it lasts longer than xylazine but shorter than detomidine.
For most large draft breeds, decrease doses by ~10%. Propofol has gotten cheaper and can be useful for induction of donkeys or minis.

If you need an all intramuscular protocol for a wild child

  • detomidine 0.04 mg/kg im
  • dissolve a bottle of telazol powder in 2.5 mls of ketamine
  • either give detomidine first or all together (eg dart or pole syringe)
  • let them be for 15 min
For small animals, maintenance of anesthesia is commonly managed by moving them to an inhalant (isoflurane etc). 
Inhalants (isoflurane and sevoflurane) just aren’t used in the field. The machinery is too expensive and cumbersome. For that reason, any procedures anticipated to last longer than an hour should be referred. Referral is also indicated for sick animals that will need oxygen support.
Field anesthesia is typically safe for up to 60 minutes. After that time, complications are more likely.

Recovery

Watch this video about the recovery after castration:

While field recoveries are generally safe, they are not as safe as for other species. Horses are flight animals and, when faced with pain or strange situations, attempt to run away. This can be problematic if the animal can’t stand well yet. The environment is also important. It can be hard to control a horse that is awake, much less one that is fuzzy and light headed. Make sure the horse isn’t on an incline or near a ditch, pond, cars, or fences. Finally, many are really big animals. Don’t get underneath them or think you can do much other than point them in the right direction. You can use their smaller brains to your advantage. Keep it dark and quiet and they may keep sleeping. Push on them and they will push back. Talk quietly and softly and they will often calm down. Another horse whinnies at them and they wake up [keep other horses away unless you want them to wake up!]

Common complications include facial and radial nerve paralysis as well as traumatic injuries (bitten tongue, lacerations). Prolonged field anesthesia (more than 60 minutes) should be avoided as the risk of complications increases greatly. These complications include hypoxia, myopathies and death.

Resources

How to Get 15 minutes more Anesthesia , 2018 AAEP Proceedings. nice summary of maintenance options, audio

Total intravenous anesthesia in horses, The Veterinary clinics of North America. , 2013, Vol.29(1), p.123-129

R-vets anesthesia basics- for more challenging cases- these guys work with wild ones

Field anesthesia in the equine, Clin Tech Eq Pract 2007- provides step by step instructions

FOAL RESOURCES

Foal physiology and special considerations during anesthesia, Vetlearn 2010 (written for vet techs)

Eq Surgery anesthesia analgesia foals highlighted, Auer & Stick  (use the study questions in the ruptured bladder section to work through this)

Plumb’s Veterinary Drug Handbook and app (lots of versions out there in different formats and prices)

 

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Large Animal Surgery - Supplemental Notes Copyright © by Erin Malone, DVM, PhD is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.