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It is a given that equine practitioners will have to use field anesthesia at some point—in treating an injured animal, during castration or when helping a mare with a foaling complication.
Conditions are usually less than ideal. Often there isn’t anyone around to handle the horse, the surgical “suite” may be a paddock or a stall floor and monitoring equipment is not always portable. Protecting the horse should be at the forefront of the practioner’s mind at all times.
Equine field anesthesia is a two-step process, says Rachael E. Carpenter, DVM.
First, a sedative such as the alpha-2 agonist xylazine, with or without butorphanol, is given.
“You want the horse nice and sedated before giving the induction drugs; head down and not really paying attention to his surroundings,” says Dr. Carpenter, an anesthesiologist with Ruffian Equine Medical Center in Elmont, N.Y.
The next injection is most commonly ketamine, with or without diazepam, to provide a smooth induction. Adding diazepam will prolong the anesthesia over using ketamine alone and will provide more muscle relaxation, she says, and guiafenasin can be used before giving ketamine or ketamine/diazepam and is useful in cases where a lesser xylazine dose is desired.
“Then, if you need to prolong the anesthesia a little,” she says, “you can give one-fourth to one-third your original xylazine and ketamine doses, or some additional ketamine and diazepam without adversely affecting field recoveries.”
Carpenter, a former professor of anesthesia and pain management at the University of Illinois, recommends the “triple drip” IV for field procedures lasting up to an hour. “You can dose it to effect—if he starts moving around, turn it up and give him a little more. When he’s at the desired plane of anesthesia, just turn it down or off.”
Nathan Voris, DVM, says the three common reasons a veterinarian uses short- and medium-term anesthesia in the field are castration, suturing lacerations and dystocia.
Dr. Voris, who serves on Pfizer Inc.’s Equine Veterinary Operations Team for the Eastern region, says xylazine has been used as an equine sedative since the late 1960s and in combination with ketamine for field anesthesia since the early 1980s.
He says the drug of choice in the field before that combination came about was succinylcholine, which paralyzed the horse’s skeletal muscles and diaphragm. Some horses stopped breathing and died. Succinylcholine is not a pain manager, “So the horses are awake, feeling the pain of the procedure, and they just can’t move.”
Carpenter says prolonged recumbency under anesthesia can cause ventilation problems and low oxygenation. Blood pressure needs to be maintained to ensure the blood continues to perfuse the organs and muscles. Without perfusion, the horse’s muscles can “tie up” and the animal will have trouble getting back up and walking, she says.
“The concern over hypoxia is one of the reasons horses should be given supplemental oxygen [most commonly in a hospital setting] if anesthesia is to last longer than about an hour,” she says. “In that setting, blood pressure and adequacy of oxygenation/ventilation can be monitored more closely as well.”
She cites a recent Rood and Riddle retrospective study that found lower mortality rates for general anesthesia than in previously published papers.
She says a horse “recovers smoothly from injectable anesthesia” most of the time.
“If a horse needs to be anesthetized for more than an hour, you can easily get in over your head for field anesthesia,” Voris says. “To minimize anesthetic complications, one should ideally go to a clinic setting, where you can monitor blood pressure, supplement oxygen and intravenous fluids, use a vaporizer and all the other equipment that is not portable or practical for field use.”
Carpenter agrees. In the hospital setting, she says, “a more consistent plane of anesthesia can be delivered by inhalent anesthesia.”
Inhalent anesthesia isn’t used in the field, in part because of the equipment required to use it safely.
Three Chief Goals
Jeff Ko, DVM, Dipl. ACVA, a professor of anesthesiology at Purdue University’s School of Veterinary Medicine, says complete anesthesia should create three conditions in the horse: a state of unconsciousness, muscle relaxation and analgesia.
A lot of field surgery can be completed using local anesthesia in combination with sedation because the horse can remain standing, Dr. Ko says. Local anesthetic drugs are used to block pain and induce muscle relaxation, which is necessary to repair injury or for a minor surgical procedure.
While equine anesthesia carries inherent risks, it can be performed safely by vigilant and skilled veterinarians, Ko says. Anesthetics are generally dangerous to horses because they depress the cardiorespiratory system, Ko says.
Ko reminds veterinarians that when a horse is in lateral or dorsal recumbency, there can be a lack of profusion to the muscles because of poor blood pressure maintainance, poor padding or poor positioning. When the horse comes out of anesthesia, nerve damage or myositis may occur if these three vital P’s are not watched.
“If they wake up in pain due to nerve damage or myositis and they can’t walk, it will probably be a long time before they are healed, if ever,” Ko says.
“Practitioners need to make sure they follow the anesthesia protocols,” Voris says. “The location, the condition and specific circumstances need to be considered carefully.”
Monitoring temperature, blood pressure, heart and respiratory rates and oxygenation are vital, Ko says. “When horses are under anesthesia, we must do all that for them to maintain their homeostasis,” he says.
Restraint is important, too. “When a horse is coming out of anesthesia, his brain tells him to run,” Ko says. “But the drugs are still in his system, resulting in muscle relaxation, sedation and grogginess, so his body’s not ready to support itself. This is why recovery can be so dangerous to the horse.
“If they land at a certain angle, they can undo what the verterinarian just repaired.”
Veterinarians and technicians can use head and tail ropes “as a reference point only to give the horse a lift and help him stand,” Ko says.
The location matters as well.
“Make sure you lay the horse down in an open spot,” Voris recommends. “When they come out of anesthesia, if a horse can find a water trough, a feeder, a fence to get tangled up in, he will.”
Maintaining a sterile environment is a challenge in the field, Ko notes. From flies and bacteria to dirt and manure, conditions are usually far from ideal. Care should be taken to provide the best location possible.
Voris likes to use a covered arena as for field surgery during bad weather. In good weather, an outside paddock is generally safe.
Watch the weather, though. If a horse has a low body temperature, the drug may metabolize slowly. “Veterinarians should keep in mind that recovery will be slow, too, especially in young foals,” Ko says.
Ideally, field assistance from veterinary technicians makes the equine practitioner’s job easier, but help is not always available. “Many times it is just the horse owner and the veterinarian,” Voris says.
Ko suggests that practitioners discuss with the horse owner any previous anethesia reactions. “Veterinarians can use a different drug, or a different combination, if the owner says, ‘Hey, my horse had a really rough time getting up last time,’ ” he says.
Bernd Driessen, DVM, Ph.D., a professor of anesthesiology at the Universtiy of Pennsylvania’s New Bolton Center, says there are so many equine hospitals today that “the majority of veterinarians take horses that need to be anesthetized to a controlled environment.”
He recommends that field anesthesia be performed only on systemically healthy animals.
In an emergency situation, a horse might be anesthetized and loaded into a trailer for transport to the hospital, Dr. Driessen says. Or a horse with a severe injury might be briefly anesthetized if absolutely necessary to splint or cast a limb.
“A veterinarian should never anesthetize a systemically sick animal,” he says. “They are not good candidates. Injured, yes. Diseased, no.”
He doesn’t recommend anesthetizing young foals in the field, either. “They need sophisticated monitoring that should be done in a hospital,” he says.
Mechanical ventilation is virtually impossible in the field without the use of heavy equipment, Driessen says. “Some instances, say for a military field hospital or field hospitals during civilian catatrophe situations, this equipment may be available,” he says. “But for the everyday field veterinarian, it is impractical to carry aboard their trucks.
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