New procedures in equine anesthesiology have led to fewer problems and reduced mortality, according to two veterinary anesthesiologists.
William Muir, DVM, a consultant for Woodland Run Equine Facility in Grove City, Ohio, is seeing more standing anesthesia for abdominal, head, sinus, tooth and orthopedic procedures.
“We’re not knocking as many horses down,” he says, using the term to describe full-out anesthesia that makes a horse recumbent.
Dr. Muir, Ph.D., ACVA, Dipl. ACVECC, VPPCS, is the co-author of “Handbook of Veterinary Anesthesia” (December 2006) and “Equine Anesthesia” (December 2008). He served as an anesthesiology professor at Ohio State University for 37 years and now works as a consultant to the animal pharmaceutical industry and several veterinary referral centers.
He says many surgical procedures can be completed in standing horses “still heavily sedated, with sedatives and analgesics.”
Doing it this way “reduces the cardiorespiratory depression associated with general anesthesia and the problems that can occur during recovery,” Muir says.
An attendant can have trouble controlling a horse’s behavior and cardiorespiratory function during general anesthesia because of the animal’s anatomy and very nature. Most problems occur during the induction and recovery phases.
“Horses usually protect themselves by trying to escape,” Muir says. “Physiologically, anatomically and behaviorally, horses are very different from cats, dogs and humans. Not only do they have a flight mentality, their response to drugs is different.
“You need to have a good attendant to help prevent the horse from getting hurt,” he says. “Knocking horses down does facilitate more involved surgical procedures.”
|Proper padding of a recumbent horse is essential. (Photo by Bob Langrish)|
Thomas J. Doherty, MVB, MSc, Dipl. ACVA, is a professor of anesthesiology in the Department of Large Animal Clinical Sciences at the University of Tennessee’s College of Veterinary Medicine. He is the co-author of “Manual of Equine Anesthesia and Analgesia” (August 2006).
“For the past half dozen years,” says Dr. Doherty, “anesthesiologists have begun to swing away from total dependence on inhalant anesthesia to intravenous mixtures that provide analgesia or to a combination of inhalants and injectable anesthetics.
“There is evidence that total intravenous anesthesia (TIVA) or partial intravenous anesthesia (PIVA) are associated with improved outcome.”
TIVA involves a “triple drip of drugs,” he says. Alpha-2 agonists (e.g. xylazine), ketamine and guaifenesin are used most commonly, he says, generally with good results.
Evidence suggests that TIVA is better than inhalational anesthesia for maintaining cardiovascular function, Doherty says. And TIVA can be used satisfactorily for procedures of up to two hours’ duration. Advantages include better analgesia and blood pressure and improved recoveries. Injectable drugs have a wider therapeutic index, he says, compared to volatile agents.
Anesthetic drug combinations, such as xylazine-diazepam-ketamine or xylazine-guaifenesin-ketamine are commonly used, Muir says. The analgesic effects of these drug combinations supplement the unconsciousness.
The muscle relaxants are important, too. Intravenous administration of these adjuncts aims to reduce the amount of inhalant anesthetic required, thereby producing fewer cardiovascular depression effects.
Total intravenous anesthesia “doesn’t require a lot of fancy equipment,” Muir says. “Veterinarians can tailor the anesthesia protocol to the horse, considering the ideal qualities specific to patient needs.”
Risks involved with anesthesia, Muir says, include excessive respiratory and cardiovascular depression. Hypotension, or low arterial blood pressure, can lead to myopathy, which can be a reason to euthanize some horses.
Less common risks include muscle soreness or stiffness, and secondary gastrointestinal problems. “Horses seem to be predisposed to GI problems,” Muir said. “Anesthesia doesn’t help.”
Using intravenous lidocaine intraoperatively provides analgesia and may improve post-op bowel function, Doherty says, a welcome benefit as the species is prone to colic, and colic surgeries carry a high mortality rate.
“Horses get up poorly from anesthesia,” Doherty says, “and this may be exacerbated by muscle weakness resulting from poor muscle perfusion secondary to cardiovascular depression during anesthesia. Using drugs such as lidocaine, xylazine and ketamine intraoperatively reduces the need for volatile agents and improves cardiovascular function. With TIVA and PIVA, the horses generally experience a better outcome. Delaying the time until the horse makes its first attempt to stand has been shown to improve recoveries, and in our clinic we administer a mixture of romifidine and ketamine once the horse is in recovery.”
Because injectable drugs are not excreted by the lungs, TIVA causes less pollution of the surgical area. Doherty recommends that inhalant gases be vented to the outside of the building or that charcoal filters be used. Pregnant women are especially vulnerable to the inhalant gases, though Doherty says care should be taken to reduce the exposure of all employees.
PIVA uses less inhalant anesthetic; however, the inclusion of a low concentration of inhalant in the drug regimen improves muscle relaxation, Doherty says. Anesthesiologists add lidocaine, ketamine, an alpha-2 agonist or combinations of these drugs to the inhalant anesthetic. Drug combinations reduce cardiovascular depression and the horse is much less likely to move in response to surgical stimulation.
Volatile agents are safe but need to be properly administered and monitored, Muir says. “There are no safe drugs or procedures, only safe anesthetists.
“A lot of equine practitioners aren’t specifically trained to administer anesthesia drugs,” Muir says. “But they do it anyway, and the results aren’t always successful. More veterinarians are referring their patients to surgical facilities and specialty hospitals, with better results.
“The field of equine anesthesia has become specialized,” he says. “Many veterinary practitioners were not prepared for the complications associated with anesthesia, and we lost horses. Now it seems as though they have come to respect their limitations.
“The injectable anesthetic drugs that are available today produce relatively predictable effects, but we can’t always depend upon them to produce the desired response.”
A horse can die if it stays recumbent and can’t be brought to a standing position, but Muir says there are many ways to get a horse up after surgery.
“Some suggest that horses be allowed to stand unassisted from a dirt floor,” Muir said. “Recovery from anesthesia could also occur on water beds, air mattresses, or a foam pad. Recovering horses in a pool is another idea. Some equine centers use people in a stall to lift and guide the horse to its feet through human power. Some use a sling. Or ropes.”
Muir prefers using a foam bed or a waterbed with head and tail ropes and experienced assistants.
Doherty notes that veterinarians have learned to use better padding and are getting good results from properly padding the horse during and after surgery. The incidence of muscle problems is reduced considerably and this can be attributed to better maintenance of blood pressure and adequate padding.
“No one method has emerged as the one to use,” he says. “Every horse and every procedure is different.”
Doherty and Muir emphasize the importance of experienced help when monitoring anesthesia in field surgery. “You have to monitor breathing, heart rate, mucous membranes, capillary refill time, peripheral arterial pulse,” Muir says.
Equipment designed specifically for horses “fits the need much better than the equipment adapted or borrowed from small-animal clinics and human hospitals,” Muir says.
Machines that administer and monitor anesthesia in dogs and cats, and even humans, are hard to use with horses, he says.
Most equine facilities have fancy monitoring equipment, Muir says, “designed for equine use. ECG machines, blood pressure measuring devices, non-invasive pulse oximeters and gas analyzers are common. So when necessary, horses should be referred to a specialty hospital.”
New, upgraded mechanical ventilators have become common. “Surgeons like to do surgery when the horse is supine (on his back),” he says.“ The horse can’t expand his lungs as effectively as when standing when in this position, so the mechanical ventilators are extremely helpful.”