If you're like most horse owners, the idea of surgery performed on your beloved animal is unpleasant at best, terrifying at worst. Those fears are not unfounded. Because of the species' size and weight, their physiological reactions to many anesthetic drugs, and the difficulties of recovery, horses are trickier to safely anesthetize than most other species. Yet through the years, veterinarians and researchers have devised strategies, pinpointed drugs, and developed equipment that can minimize the hazards and help your horse come through surgery with flying colors.
A Delicate Balance
Of course, anesthesia carries some risk for all species. "It is really controlled poison," explains Gene Steffey, VMD, professor of anesthesiology in the Department of Surgical and Radiological Sciences at the University of California, Davis, School of Veterinary Medicine. "And any drug in excess will kill. The job of the person doing the anesthesia is to give enough to provide pain relief and still maintain life. It's a delicate balance."
Statistics show that's especially true in the horse. According to some studies, says Steffey, "The reported incidences of life-threatening problems associated with general anesthesia are 0.07% in humans, 0.15% in small animals (dogs and cats), and 2.0% in horses. The populations of humans and small animals evaluated represented a broad range of physical conditions and degrees of healthiness, whereas the horse population was, in general, made up of systemically healthy individuals presented for elective surgery." That figure is open to debate, however.
For instance, John Hubbell, DVM, MS, Dipl. ACVA, professor of veterinary clinical sciences and associate dean of academic affairs at The Ohio State University Veterinary Teaching Hospital, quotes a lower number. "The reported incidence of death associated with anesthesia in 'normal,' elective-case horses is as high as 1%, or one out of every 100 horses, and most anesthesiologists believe that the actual risk of death is less than this--perhaps in the range of one in 1,000 or 2,000," he says. Still, he acknowledges, "Even at this level, the risk is significant."
What's Behind the Hazards
There are several reasons for the high risk. Consider, for instance, the recumbent position required for operations. While horses will naturally rest on their stomachs and sides sometimes, they typically don't hold this position for extended periods. And you're not likely to catch any horse relaxing on its back, a position used more and more frequently for equine surgery. And in any of these positions, says Steffey, "The horse breathes less efficiently and his circulation is less efficient."
Furthermore, laying in a position with one side down means all of the horse's considerable weight is pressing down on that side of the animal. "This can result in reduced blood flow to the underside muscles, which are not getting oxygen as they should," says Steffey. "Then there is pain when the horse tries to stand, or he simply cannot get up." It's somewhat analogous to your leg "falling asleep" if you keep it crossed for too long. Except, of course, you can shift position to alleviate the pressure and improve circulation, while a horse under anesthesia cannot. At worst, this can result in actual muscle damage, or myopathy.
Horses can also experience cardiorespiratory collapse due to exaggerated responses to the anesthetic drugs, says Hubbell. "This can manifest as low arterial blood pressure--inability to feel a pulse--or lack of respiration," he says.
Other risks horses face on the operating table are awakening from anesthesia during the procedure--potentially resulting in injury to the horse and/or handlers--and attacks of problems such as hyperkalemic periodic paralysis (HYPP) while under anesthesia, notes Hubbell.
Monitoring: A Modern Edge
The good news is that many of these hazards can be minimized with good monitoring, something that gives today's veterinary practitioners an edge over their brethren of the not-too-distant past. "Once we started monitoring on a regular basis, a lot of things started to make sense," says Steffey. "We recognize now that if blood pressure drops to a certain level for a certain length of time, then the incidence of myopathy increases substantially. But if we keep blood pressure above a certain value, then the incidence is much less."
Likewise, Hubbell notes that if monitoring shows the horse experiencing cardiorespiratory distress during an operation, drugs can be administered to increase blood pressure and assist ventilation. This gives the horse a chance to stabilize while surgeons continue the operation. But if the trouble is not detected and remains untreated, "Cardiovascular collapse can lead to death," he says.
Hubbell explains that there are primarily three body systems monitored during operations: Cardiovascular (heart rate and blood pressure), respiratory (respiratory rate, depth of respiration, and arterial blood gases), and central nervous systems (depth of anesthesia is evaluated in the latter using eye position and movement, and lack of movement of the horse).
Besides the monitoring equipment that is now standard in equine operating rooms, the surgical table has evolved over the years to better meet the horse's unique needs. In the old days, says Steffey, tables were padded sufficiently for small animals, but were not soft or thick enough for a horse. To illustrate the importance of ample padding, Steffey asks you to imagine sitting for an extended period on a hard chair vs. a soft one. The incidence of those "tingly" or numb feelings on your nether regions would be greater on the hard chair. Similarly, the horse is more likely to suffer the muscle problems discussed earlier if he's lying on a hard vs. soft operating table. And, because of his greater mass, the horse needs thicker padding to provide that cushion and prevent him from being affected by the hard table underneath.
The Sedative Advantage
It's not uncommon for a horse to become excited when exposed to the unfamiliar sights and sounds of the hospital and the operating area. An excited horse, says Hubbell, should not be placed under general anesthesia. So, he continues, it's common practice to sedate the horse before inducing anesthesia. Not only does this help alleviate the horse's anxiety, but it also creates a safer environment for the people who are working in a confined space around the animal, says Steffey.
Certain sedatives also have an analgesic (pain-relieving) effect, which creates yet another benefit. "The combination of a calm patient with analgesia allows for a reduction in the drugs that are used to produce the anesthesia," explains Hubbell. "Generally, this is beneficial because the drugs used to produce anesthesia cause more deleterious cardiovascular side effects than the drugs used to produce sedation."
In short, sedation means less anesthetic, and that makes for a safer experience for your horse.
Steffey notes that there are really only two categories of sedatives used for this procedure--tranquilizers, such as acetylpromazine (probably best known by the brand name Acepromazine), and alpha-2 agonists such as xylazine (Rompun) and detomidine (Dormosedan). Deciding which to use depends on several variables, including the practitioner's familiarity with the drug and the type of surgery to be performed. Even the distance the horse needs to travel between sedation and the operating table can have an impact. "Acepromazine, for instance, doesn't prevent mobility," says Steffey. "But a significant dose of one of the alpha-2 drugs makes the horse very reluctant to move."
When it comes to the anesthesia, there are intravenous (or injectable) agents and inhalant agents, with surgery often involving the use of both types, says Hubbell.
In nearly all cases, an injectable anesthetic is used to initially induce anesthesia, causing sleep and recumbency. What the vet uses for the duration of the operation, says Steffey, "is based on the status of the animal, the technology available, and other factors."
A major deciding factor at the start is simply how long the veterinarian expects to keep the horse under anesthesia, he adds. For procedures lasting 30 minutes or less, such as castration, veterinarians often opt to simply continue with the same intravenous agent they started with.
For longer procedures, particularly when you reach the hour-or-longer mark, the surgeon will often decide to transition to inhalation anesthesia. "The technology is more difficult and more involved," says Steffey, "but it gives you better control of the animal (physiologically) for longer periods."
Another advantage is that inhalant anesthetics are delivered in oxygen, often in concentrations near 100%, says Hubbell. This added oxygenation helps counter the low oxygen tensions in the horse brought on by recumbency (laying down), he adds.
Drugs used for inhalation anesthesia in horses include halothane, isoflurane, and sevoflurane, says Hubbell. For intravenous anesthesia, combinations of drugs including xylazine, ketamine, and diazepam (Valium), or xylazine, guaifensein, and thiopental sodium can be used, he says.
Risks of Recovery
The last major obstacle to overcome with equine anesthesia is recovery from it. This is risky, says Steffey, because the horse recovers from anesthesia with a cloudy mind, unlike awakening from a natural sleep. Worse yet, in this fuzzy-minded state, "The horse suddenly finds himself in a strange environment, maybe with strange noises or even flashing lights," says Steffey. "He may revert to his primitive nature and feel the need to get out, and not in a thoughtful manner."
In conjunction with this, says Hubbell, "The horse may try to rise too early, before it has recovered sufficient coordination and strength to do so." If the horse then falls in his efforts to stand, he can easily injure himself or nearby humans." And, notes Steffey, "Just the momentum of that large body--the possibility of trauma is so much greater than with a smaller body."
Unfortunately, he continues, no one has yet devised a single solution to eliminate recovery hazards. However, veterinarians have devised a variety of ways to minimize the dangers. "Here at UC Davis, we have six different techniques to recover a horse," says Steffey. "We gauge which one or which combination to use by assessing the nature of the horse, what's been done, how long he's been down, and the physical status of the horse, and then apply an appropriate recovery technique."
Some techniques used at various equine hospitals, says Hubbell, include:
- Sedating the horse as it enters the recovery stall, so it won't try to get up too soon.
- Using head and tail ropes to help steady and support the horse when he rises.
- Placing the horse on a raft in a swimming pool, then sedating the horse to remove him from the pool (primarily used for fracture patients).
Research has been invaluable in achieving the advances made thus far in reducing the risk of equine anesthesia. "And there is a continual need to move forward and be developing new ideas, new techniques," says Steffey. Currently, he sees studies focusing on these areas:
- Examining new drugs in human medicine for equine surgical applications.
- Seeing how equipment now used for other situations might be useful in the surgical arena. For instance, the slings used for emergency rescue procedures are being studied as a tool for anesthesia recovery.
- Studying the relationship of anesthesia to head position during surgery, as it affects such things as blood flow and recovery.
Hubbell notes that there has been considerable work done in recent years comparing the effects of various drugs and drug combinations, the relevance of recent exercise on the surgical candidate, and the overall mortality risks of anesthesia.
What You Can Do
As a horse owner, unfortunately, there isn't much you can do to reduce the risk of anesthesia. The one area you can control is selecting a preferred equine hospital in advance, just in case your horse needs surgery one day. Look for a modern facility with up-to-date monitoring equipment and well-padded tables. Ask about pre-op sedation and recovery procedures.
Just as important, get the scoop not only on the surgeons, but also on the anesthesiologists. As Hubbell says, quoting physician anesthesiologist Robert Smith, MD, "There are no safe anesthetic drugs. There are no safe anesthetic techniques. There are only safe anesthetists." If you select one who fills you with confidence and works in a reputable hospital, then you can at least feel assured that your horse will be in good hands and thus stand the best chance of coming through any surgery safe and sound.
WHEN THERE'S A CHOICE: General or Local?
Not all surgical procedures require general anesthesia. In some cases, a local anesthetic will do the job quite nicely. So how does a veterinarian decide which to use? According to John Hubbell, DVM, MS, Dipl. ACVA, professor of veterinary clinical sciences and associate dean of academic affairs at The Ohio State University Veterinary Teaching Hospital, and Gene Steffey, VMD, professor of anesthesiology in the Department of Surgical and Radiological Sciences at the University of California, Davis, School of Veterinary Medicine, the procedure and the patient are the primary considerations.
"If it is imperative that the horse not move during the procedure, general anesthesia is usually used," says Hubbell. Other factors related to the procedure include:
Accessibility--If the surgeon can easily access the necessary body part while the horse is in a standing position, then local anesthesia might suffice, says Steffey.
Surgical area--If the surgery involves a large area, then general anesthesia is the common choice, because the amount of local anesthetic required could cause motor dysfunction in nearby muscles, explains Hubbell.
Time involved--Longer procedures typically call for general anesthesia, if for no other reason than the difficulty of keeping a horse standing still for long periods while he's awake.
"Some horses are amenable to being approached while sedated, some are not," says Hubbell. If a horse can safely be approached, local anesthetic is often the choice for minor procedures, such as stitching a laceration.--Sushil Dulai Wenholz
THE BOTTOM LINE: The Cost Factor
General anesthesia is not an inexpensive part of the overall cost of surgery. Part of the cost comes from the capital investment that the hospital must make in equipment. Anesthetic machines, required for inhaled anesthetics, cost anywhere from $5,000-$20,000, says John Hubbell, DVM, MS, Dipl. ACVA, professor of veterinary clinical sciences and associate dean of academic affairs at The Ohio State University Veterinary Teaching Hospital. Blood pressure monitoring equipment can run another $5,000-$10,000, and a portion of these costs are rolled into the price of every operation. In addition, you'll pay for the pre-op sedative and the anesthetic agent.
It's hard to put an actual price tag on general anesthesia, though, because traditionally these expenses have simply been rolled into the surgery fee, not pulled out and itemized on your bill. That practice is starting to change, says Gene Steffey, VMD, professor of anesthesiology in the Department of Surgical and Radiological Sciences at the University of California, Davis, School of Veterinary Medicine, especially at universities and other large hospitals. "Now people can start to understand why the cost is what it is: The cost of the monitoring equipment, the equipment to administer the inhalation agent, and the cost of the different drugs," he says.
This itemization will also allow your veterinarian to present you with a better picture of the financial side of treatment options. For instance, in a given situation, the veterinarian can more easily explain the cost of using general anesthesia to give the optimal treatment vs. the cost of using a local anesthetic to do minimal treatment. "Like it or not, that's a reality in veterinary medicine," says Steffey.--Sushil Dulai Wenholz
About the Author
Sushil Dulai Wenholz is a free-lance writer based in Lakewood, Colo. Her work appears in a number of leading equine publications, and she has earned awards from the American Horse Publications and the Western Fairs Association.
POLL: University Equine Hospitals