Equine Anesthesia Today: Both Injectables And Inhalants Continue To Have Their Place

New methods and drugs provide equine anesthesia with fewer complications.

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Emerging techniques, newer drugs and the continuing education of practitioners indicate that equine anesthesia is becoming less problematic.

Horses emerge from anesthesia faster, more safely and with fewer side effects. “The end point of anesthesia is typically to maintain the cardiovascular and respiratory functions as close as possible to normal physiological status,” said Bernd Driessen, DVM, Ph.D., associate professor of anaesthesia at the University of Pennsylvania College of Veterinary Medicine. “We know in horses that we cannot achieve that easily, but we are now able to stay at least within a 20percent –to 30 percent range."

Emerging Techniques and Drugs

More procedures are being done in the field with profound sedation than ever before.

“More and more students have come of age in a time where sedation became a routine thing,” said Cliff Swanson, DVM, MS, associate professor of anesthesia at North Carolina State University College of Veterinary Medicine.  “In the past, clients weren’t used to having their horses sedated for certain routine things [such as sheath cleaning and dental exams]. Now, it’s become more of a routine practice."

“This translates to better patient care because exams can be more thorough; there isn’t an overlay of anxiety with the owner; and the veterinarian can get on with assertive massaging of tissues,” he said.

This use of sedation also stems from better drugs, he said, in particular detomidine. Detomidine was first used in institutional practice and then recently translated into private practice.

“Detomidine has come into its own as the drug of choice among practicing vets,” Dr. Swanson said. “It is also a good analgesic, so you get the combined effect of sedation and pain control. Acepromazine was OK, but some horses can override the effect of the drug.”

Xylazine (same class as detomidine) is another popular sedative used in the field. It works with the same mechanism as detomidine but the behavioral modification and sedation is not as profound.

The No.1 inhalation anesthetic for years has been isoflurane. One idea put forward in 2002 as a scientific abstract by Neges K, et al. The isoflurane sparing effect of a medetomidine constant rate infusion in horses. Proceedings of the Association of Veterinary Anesthetists Autumn Meeting, Crete. September 2002, in Journal of Veterinary Anaesthesia and analgesia 2003, was to give a continuous infusion of medetomidine (same class as detomidine) as an adjunct to isoflurane.

“This technique was first reported as a means of supporting blood pressure and smoothing recovery from isoflurane anesthesia,” Dr. Swanson said. “We began using it here at North Carolina State University after talking with Dr. Conny Gunkel who finished an anesthesia residency at the University of Florida, and joined us here in July."

Medetomidine is also a good analgesic, something that is lacking in isoflurane and sevoflurane. And the analgesia lasts a long time after the drug has worn off.

“We’ve also noticed that the recovery has been superb. Horses are stronger and better co-ordinated when they get up then as if they’ve just been given gas alone."

Swanson and his colleagues have used this continuous infusion with sevoflurane several times and continue to investigate the technique as a potential means of obviating the emergence problems with ultra-fast recovery from sevoflurane.

Balanced Anesthesia

“In addition to using medetomidine, detomidine, xylazine, romifidine, constant rate infusions of lidocaine and/or ketamine during inhalant anaesthesia have become increasingly more popular during equine anesthesia,” Dr. Driessen said. “Specialists in anesthesia call the combined use of potent analgesics with inhalant anesthetics ‘balanced anesthesia,’ a concept widely used in human and small animal practice."

Private practitioners at equine surgical facilities can employ some of those same protocols with the advantages of balanced anesthesia and patient safety with the additional benefits of faster recoveries and decreased overall costs.

“Our practice uses sevoflurane for inhalant anesthesia in combination with a constant rate of infusion of lidocaine and ketamine,” said Christine Kessler, DVM at Pioneer Equine Hospital in Oakdale, Calif.

Sevoflurane has different chemical properties that allow for increased control over the depth of anesthesia and shorter recovery periods and ultimately more productive surgery days.

The combination of sevoflurane (very expensive) and lidocaine and ketamine (very inexpensive) can drastically reduce the overall cost of the sevoflurane.

Lidocaine potentiates the effects of sevoflurane by reducing the minimum alveolar concentration value, which relates into thousands of dollars of savings a year.

“Our practice began utilizing sevoflurane one year ago and we solely use sevoflurane now with great success,” Dr. Kessler said.

Specialists say although the adjustable virtue in inhalation anesthesia is renowned, there are still certain subsets of anesthetic patients that benefit from injectable anesthesia only, given either in the form of one drug or combinations of drugs.

Traditionally, combinations of xylazine or detomidine with ketamine and guaifenesin, also known as “triple drip,” were used in horses to induce and maintain anesthesia for short procedures under field practice conditions.

Since the injectable drug propofol, the top injectable anesthetic in human medicine, came on board for veterinarians a few years back, veterinary surgeons, particularly small animal, have changed to an even more outpatient style practice. Because rapid and clear-headed recovery is one of the great advantages of propofol, veterinary anesthesiologists have begun to evaluate its use in horses.

While propofol, unlike inhalant anesthetics, promises to ensure excellent quality of recovery from anesthesia after long surgeries, allowing horses to go back to their stall or even home soon after the procedure, anesthetic induction quality with propofol is somewhat unpredictable, unless horses are very well sedated beforehand, Driessen said.

Propofol is more popular for use in small horses, ponies and foals than in large horses, partially because behavioral and cardiopulmonary effects of propofol are less variable in smaller animals.

“The problem also is that the formulation comes as a 1 percent solution, which would mean 10 milligrams per milliliter,“ Driessen said. “If you have to dose 2 to 6 milligrams per kilogram, in an adult horse you would have to have several big syringes in your hand to administer the drug.

“Once the solution has changed to 10 percent and its costs come down it will become easier to use it in adult horses, as well.”  

Another advantage is that propofol depresses central nervous activity more gradually from a state of sedation to hypnosis to anesthesia, in comparison to other potent injectable anesthetics.

Therefore propofol can be titrated to effect much easier and be used as a sedative or hypnotic, as well.

During the World Congress of Veterinary Anaesthesia held last year in Knoxville, Tenn., Eugene Steffey, VMD, Ph.D., Dipl. ACVA & ECVA, and his colleagues from the University of California at Davis and Colorado State University (Kursheed Mama, B.V.Sc., DVM, Dipl. ACVA, Peter Hellyer, DVM, MS, Dipl. ACVA, A. Wagner, DVM, MS, Dipl. ACVA &ACVP) reported encouraging results suggesting that infusion of low-dose propofol at the end of inhalant anesthesia substantially improves the quality of equine recovery.

The Anesthesia Record

The anesthesia record is standard institutional practice and a requirement by the American Veterinary College of Anesthesia. The record, which was modeled after human hospitals, serves as a record for the practitioner to track patients’ changing vitals’ patterns under anesthesia and if medication needs to be adjusted.

Charts should also note drugs and dosages administered, and which catheters are used. Blood gas analyses should also be recorded.

“We make the standard readings of heart rate, blood pressure and respiratory rate every five minutes,” said Cliff Swanson, DVM, MS, associate professor of anesthesia at North Carolina State University College of Veterinary Medicine.

“The anesthesia record is essentially a grid with time points on it. We record these objective numbers every five minutes and after awhile we are able to tell whether the dots and dashes are forming a trend or moving,” he said.

Since anesthesiologists tailor things as they go, they annotate each update. This is for efficiency as well as for legality.

“We want to be able to say with absolute certainty that our decisions were bonafide, rational and within the realm of legal practice. It’s our record of how we performed,” Dr. Swanson said.

“Human and veterinary anesthesia societies encourage people to monitor and make informed decisions as much as they can, keep records and question everything. Therefore, if something goes wrong you don’t have to remember what was happening five minutes ago. You have a record in real time.”

This record then becomes part of the individual’s permanent medical history and anesthesiologists will refer to them if the individual is to be anesthetised at their practice again.

A more pragmatic reason for the charts is to help anesthesiologists calculate their bill.

Sharon Biggs, a frequent contributor to Veterinary Practice News, lives in England.

This article first appeared in the August 2004 issue of Veterinary Practice News.

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