Some How-to’s Of Field Anesthesia

By Lynn M. Tiffany

The horse’s unique physiologic characteristics make field anesthesia a challenge.

Aside from its fight-or-flight mentality, which multiplies the risk of injury to patient and personnel, the lack of a sterile surgical environment and the possibility of prolonged recumbency extra medical precautions when anesthetizing a horse in the field.

Douglas O. Thal, DVM, Dipl. ABVP, would prefer to perform surgery in his equine hospital, but he says, “There are times a practioner just has to lay a horse down in the field.”

“More difficult and involved procedures are better in a hospital,” he says. “First consideration should always be transporting the horse to the nearest equine facility.”Dr. Thal owns Thal Equine LLC, a regional equine hospital in Santa Fe, N.M.

But there are unspoken standards as to what practitioners do in the field, Dr. Thal says.

“You’d never do emergency colic surgery in a field—you’d euthanize the horse if you couldn’t transport it to the hospital,” he says. “And though wound repair is certainly more diffcult in the field, it is probably the most common cause to use standing anesthesia in the field.”

Thal would use infiltration of lidocaine or nevidicaine to anesthetisize the wound area while suturing skin and structures.

He has performed epidurals to relieve pain for straining mares or horses with serious rectal or vaginal injury, he says, as well as desensitized the perineum for horses with severe pain in hind limbs.

He uses such sedatives as detomadine, xylazine and morphine to function as therapy as well as local anesthetics during procedures.

Thal says romifidine, one of the newest equine drugs on the market, with  butorphanol is one of the best drug combinations.

“When we use romifidine,” he says, “the horses are less ataxic, and they keep their heads up.”

A must for Thal is premedicating a horse before anesthetizing it.

“They calm down in five to seven minutes and then you can give the regular sedation,” he says.

“If you give a horse drugs while he is already excited, his brain’s receptors are already saturated in epinephrine,” he reminds practitioners. “The sedation won’t work. If you try to work on a horse without getting to that point, you’re inviting problems.”

Unexpected effects with a particular sedative—kicking, biting, excitability—can also happen. Using drug combinations and knowing what to expect is key to not having a problem.

“If you use xylazine without an opioid like butorphanol,” he warns, “the horse can fire so fast you won’t even see it.” He prefers to use a combination of xylazine and acepromazone.

Thal doesn’t like to anesthetize a horse in the field for more than a hour.

“If we have to extend anesthesia, I’d prefer to do it in a hospital setting.” he said.

Robert J. Brosnan, DVM, PhD, Dipl. ACVA, is associate professor of surgical and radiological sciences at the University of California, Davis, School of Veterinary Medicine.

“Standing chemical restraint has several advantages compared to general anesthesia,” Dr. Brosnan says. “First, horses remain standing, so muscle and nerve injury associated with recumbency is avoided. Gas exchange function in the lung is also better maintained when a horse is standing, so there is less risk of the patient developing low oxygen levels in the blood when breathing ambient air. Third, recovery from general anesthesia can be associated with excitement and/or weakness that can increase the risk of injury when an animal tries to stand. Standing chemical restraint avoids this problem, since the horse remains standing throughout the procedure.”

Administered as local or regional blocks, topically or by infusion, local anesthetics produce analgesic, antiarrhythmic, antishock, anesthetic, mild anti-inflammatory and gastrointestinal promotility effects, says Y. Lyon Lee, DVM, Dipl. ACVA, an associate professor of anestheisology at the College of Veterinary Medicine at Western University of Health Sciences in Pomona, Calif.

“Horses rarely regurgitate,” he says, “but still, leaving the endotracheal tube placed orotracheally or nasally as long as possible until full recovery is strongly recommended. Secure it with tape to the animal’s head.

“An open airway during the recovery can be a life-saving measure,” Dr. Lee continues, “particularly in horses with airway obstruction due to severe nasal congestion.”

Protective padding is often not readily available in the field, increasing the likelihood of a patient developing post-anesthetic myopathic syndrome, an adverse condition pretty much unique in equine anesthetic complication, Lee says.

“With limited means of cardiovascular and respiratory intensive monitoring, the anesthetized animal’s minute-to-minute physiologic changes are not as intensely monitored compared to those in the operating room,” he says. “Detrimental physiologic changes are less likely to be detected earlier.

“In some unfortunate cases, the animal may have to go through more challenging resuscitative efforts to normalize cardiopulmonary function, which also increases the inherent anesthetic risk in horses anesthetized in the field,” he continues.

Foals and Neonates

Lysa Pam Posner, DVM, Dipl. ACVA, notes that horses are one of the few species that can be anesthetized at less than 3 days old. Dr. Posner is a clinical associate professor of anesthesia in the College of Veterinary Medicine at North Carolina State University in Raleigh.

She conducted a session on “Anesthesia for the Equine Foal or Neonate” at the North American Veterinary Conference in Orlando, Fla., in January.

Foals and neonates suffering ruptured bladders, limb injuries and other complications of birth may need to be anesthetized for analgesia or surgery at the farm.

Foals have less body fat and less glycogen stored in their livers, so they can be adversely affected by metabolism of the drugs. They should never be fasted and their blood glucose should be assessed before administering any drugs, Posner says.

“Consider fat vs. water-soluble drugs,” she says.

The neonatal myocardium is not fully developed, so foals’ hearts should be monitored during anesthesia. Their blood pressure is lower–MAP is 40-50, rather than 60 for an adult horse.

Neonates can be oxygen-deficient and with their higher metabolism, she advises that it is easier for them to become hypoxemic. They will need ventilation during anesthesia. Particular care should be taken with their lungs, which may have only been open a few days.

Thermoregulation should also be monitored.

“They are not efficient at maintaining their body temperature, with their meager fat stores and hair coat,” she says.

The drugs can cause vasodilation and shivering metabolically. Instances of hypothermia can increase morbidity and mortality, so keep them warm, she says. Use adequate amounts of glucose and oxygen because healing and recovery use more.

Pain management using anesthesia drugs is important, Posner says. “A horse’s neural circuitry is intact from birth. What they experience as neonates can alter their pain perception for life.”

She notes that profound sedatives and analgesics, such as xylazine, detomidine and romifidine, “can cause significant side effects in foals, such as peripheral vasoconstriction and reflex bradycardia.”

She prefers to use xylazine at half the adult dose, administered intravenously.

For sedation or as a muscle relaxant, benzodiazepines are moderately successful in treating foals, Posner says. Since they bind to the GABA receptors, they work by inducing a state of relaxation.

Ketamine, Posner says, is a dissociate anesthetic that works as an analgesic and amnesiac, but is a poor muscle relaxant. Administered IV or IM, it does have a large therapeutic effect, she says, though it can increase heart rate, blood pressure and cardiac output.

Propofol, a GABA agonist, gives a nice induction on foals, Posner says. It is not cost-prohibitive in small patients, requiring about 10 mls per dose. However, it does have significant extrahepatic metabolism uptake. It can cause respiratory depression so practitioners must be able to intubate.

Posner notes good results with an induction of guanefensin-ketamine-xylazine by IV and maintained throughout surgery lasting less than an hour. She cautions against prolonged infusions, citing muscle weakness, a slowed metabolism and a prolonged effect of ketamine.

She favors injectable techniques for foals and neonates over gas inhalants using bag masks and naso-tracheal intubation because dosage is difficult to measure and monitor. Inhalants also cause irritation of mucous membranes. And horses wake up faster with gas inhalants than with IV or IM sedatives, which can be a disadvantage.

Topically, some practitioners use fentanyl patches applied to the inside of a clipped limb. Posner says she has heard of good results with lowering fevers and sometimes sedation. The down side is that the patches are toxic to humans, and children have been known to remove them from horses. She says horse owners ask for them because they offer pain relief without a needle – ideal for both owners and horses who dislike injections.

Hui-Chu Lin, DVM, MS, Dipl. ACVA, is a professor and chief of the Equine Section in the Department of Clinical Sciences, College of Veterinary Medicine at Auburn University, Auburn, Ala.

Dr. Lin says the newest drug for mild but potent standing sedation is sublingual detomidine gel. Administered orally in a single-use prescription, it is used for minor procedures like grooming, hoof trimming, floating teeth or placing stomach tubes. Because the gel is topical rather than oral, drug degradation in the stomach is avoided.

“The key for horses recovering from anesthesia is allowing time (30 to 40 minutes) for them to regain enough muscle strength,” Lin says. “So after a longer duration of anesthesia, make the horses stay down for 30 to 40 minutes, so when they stand up, they stand up first try, without assistance, and are able to remain standing.

“Usually if xylazine and ketamine are used for short-term anesthesia, like for castration, then horses usually stand up smoothly, first try,” Lin says. “If needed, a low dose of xylazine can be administered to prolong duration of recumbency. Cover their eyes with a towel to allow time for horses to gain muscle strength before trying to get up.”

Lin suggests that the current economy is probably the reason that standing sedation or anesthesia is used more frequently these days.

“General anesthesia in horses is always a risk,” she says. “If the surgery can be performed under standing anesthesia and good analgesia is provided so animals do not feel pain, it is an acceptable technique.” 

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