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):
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 surgery and horses 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.
As with other species, anesthesia involves
- sedation – xylazine, romifidine, detomidine
- 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 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.
Injectable field anesthesia, 2011 ACVS
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.
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
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.
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
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.
How to Get 15 minutes more Anesthesia , 2018 AAEP Proceedings. nice summary of maintenance options
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
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)
Substances in this schedule have a potential for abuse less than substances in Schedules I or II and abuse may lead to moderate or low physical dependence or high psychological dependence.
An anesthetic that distorts sensory perceptions and causes feelings of detachment from reality.