Many procedures are performed in horses using standing sedation and local blocks. This avoids the risk of general anesthesia and can make surgery easier due to the position, height and decreased blood loss (lower flow to the head and back). However, it can be more dangerous for the veterinarian and any assistants; patient selection is important.
General anesthesia in horses can be challenging due to horse’s fight or flight response. Horses tend to react rather than think; this can lead to dangerous situations particularly when they are waking up from anesthesia. Large horses can also develop myopathies and neuropathies due to the pressure of the large body weight on bony prominences. Finally horses tend to not ventilate well and hence have poor oxygenation under anesthesia.
Xylazine is used daily in the equine world as a sedative + analgesic. Xylazine can cause significant bradycardia and arrhythmias. To minimize these effects, we want to use low doses. We can use low doses if we add other drugs. Combining xylazine with butorphanol potentiates xylazine sedation and analgesia, meaning we don’t need as much xylazine.
Remember, there are times when xylazine is a bad idea:
- Neonatal animals – cardiac output is HR x stroke volume. If you use xylazine, you slow HR. However, neonatal animals can’t adjust stroke volume. So you end up with minimal cardiac output. Not good. You can use butorphanol alone in this age group – they get sedated vs excited. We also use the benzodiazepines – diazepam and midazolam for calming.
- Hot days –xylazine interferes with thermoregulation and animals can become dangerously overheated.
- Horses with xylazine aggression –every once in awhile a horse will become a biter when given xylazine. It is rare but if you see it, make a big note in the record and only use detomidine.
- Heart conditions and arrhythmias – probably not a good idea if the horse already has an abnormal heart rate. Always listen first.
Xylazine + butorphanol
While excitatory on its own, butorphanol potentiates the sedation and analgesia of xylazine. Adding butorphanol to xylazine also helps with human safety by creating a better balanced stance and less risk to anyone standing behind the horse.
Remember, butorphanol is a partial opioid agonist/antagonist so can actually reverse the effects of other narcotics. Do not combine it with other narcotics.
Detomidine and romifidine
Xylazine is also fairly short acting. It lasts about 15-20 min. If you are doing surgery, it can be challenging to stop every 20 min and re-dose. That is when we change to detomidine. Detomidine is another alpha-2 agent and lasts much longer. It also provides a better balanced stance so sometimes we use it instead of the xylazine/butorphanol combination for hind end work. Detomidine can last 30-60 minutes. For surgeries, we often combine it with butorphanol to prolong the duration and add more analgesia.
Romifidine is another alpha-2 agent. Equine dentists like romifidine as the horses are less ataxic (wobbly) with romifidine than with xylazine. Horses often sway with xylazine, probably due to the weight shift. You can add butorphanol to minimize the swaying but some horses get tics or muscle twitches with narcotics and that can be annoying if you are doing stuff around the head (dental surgery, eye surgery).
Typical protocols involve an alpha-2 agent and a narcotic, either given as repeated injections or as an infusion.
Example 1. Xylazine 1.5 ml and butorphanol 0.5 ml (“150 & 5”)
Example 2. Detomidine 0.5 ml and butorphanol 0.5 ml iv (“5 & 5”)
Example 3. Butorphanol 1ml + detomidine drip- bolus of 0.01 mg/kg iv followed by an infusion of 0.01 to 0.04 mg/kg/h
We say “1.5/5” to mean 150 mg xylazine and 5 mg butorphanol. Or “5/5” to mean 1/2 ml detomidine and 1/2 ml (5 mg) butorphanol. Agreed – it is terribly inconsistent in terms of mgs and mls.
Acepromazine is another drug used in standing procedures. Ace is a tranquilizer. It calms rather than sedates. It also causes severe hypotension so is used in healthy animals only. Ace can be added to the sedation protocols, as well. Horses that are super excited do not respond well to alpha-2s. They can override the sedative effect. Ace can help take the edge off.
Really misbehaving horses can get RAT –rompun (xylazine), ace, and torbugesic (butorphanol).
Butorphanol is rarely used as a sole agent in horses. Narcotics cause excitement in horses and cats. Generally avoid giving butorphanol or other narcotics to these species unless they are already sedated. However, butorphanol is a sedative in neonatal foals.
For general sedation and for front end procedures such as skull rads or forelimb surgery, we can sedate with an alpha 2 drug – xylazine, romifidine or detomidine. To prolong the sedation and better balance the horse’s stance, we add butorphanol. These drugs can be re-dosed or be administered as an infusion for longer procedures.
Dr. Branson’s constant rate infusions- sedation and/or analgesia
All are for a 450kg horse at the low end of the range and all should be preceded by a loading dose.
|Xylazine||0.55mg/kg/hr||250 mg/hr for 450kg horse|
|Detomidine||0.01-0.04 mg/kg/hr||5 mg/hr*|
|Romifidine||0.03 mg/kg/hr||14 mg/hr*|
|Dexmedetomidine||0.0025-0.005 mg/kg/hr||1.1 mg/hr*|
|Butorphanol||0.01-0.025 mg/kg/hr||5 mg/hr*|
|Morphine||0.025-0.05 mg/kg/hr||11 mg/hr*|
|Ketamine||0.4-0.8 mg/kg/hr||180 mg/hr*|
|Lidocaine||3 mg/kg/hr||1350 mg/hr* (1.3 mg/kg loading dose)|
Anesthesia and analgesia for standing equine surgery, Vet Clin N Amer 2014
How to maximize standing chemical restraint, AAEP 2013- by our own Dr. Guedes
Ch 24 Sedation and Anaesthesia, Equine Medicine, Surgery & Reproduction 2nd edition