Large Animal - Equine Anesthesia
Equine veterinarians face a unique set of challenges when sedating their patients.
Equine veterinarians can and do sedate their patients during procedures. But, horses face a unique set of risks, so clinicians often restrict themselves to short-term procedures, which are relatively safer for the patient, according to John Hubbell, DVM, MS, ACVA, professor of veterinary medicine, The Ohio State University (Columbus, Ohio). And, even then, about half of them prefer having an assisting technician on board, he adds, although it varies as to which states require this. “Because short-term anesthesia is relatively safe, veterinarians often limit themselves to procedures they can accomplish in 30 minutes or less,” he says.
Horses under sedation are at risk for a number of things, he points out, including a higher mortality rate (as high as 1 percent, according to some experts) than is generally the case for small animals. For instance, they may develop high blood pressure, hypotension, hypoxemia or post-anesthetic myopathies and neuropathies. Certain equine breeds, such as the larger-set Clydesdales, Percherons and Belgians are more prone to these risks. And, horses that require emergency surgery for colic, an abdominal issue or another illness, are at higher risk while under sedation. And, temperament is always a factor in their recovery. “Horses [by nature] don’t lie down much,” says Hubbell. So, at the end of the procedure, they wake up and become restless, he explains. “They want to stand up, which requires a lot of [energy and] agility.”
Patient monitoring is an important part of any procedure. Veterinarians should pay attention to the horse’s hydration status and blood loss. Short-term procedures require the technician to monitor the horse’s pulse and respiration, as well as assess the position of its eye. “The position of the horse’s eye tells the technician if additional anesthesia is required or whether [he or she] is administering the correct amount,” says Hubbell. For longer procedures anesthesia is administered via inhalants and gas, both of which the technician must monitor, notes Hubbell. In addition, he or she must place a catheter to monitor the horse’s blood pressure, as well as administer electrocardiograms to monitor its heart activity.
Issues and controversy
Currently, only two drugs (guaifenesin and isoflurane) are marketed in the United States for use in horses for general anesthesia, according to Hubbell. He believes that general anesthesia cannot be safely produced if veterinarians are limited to these two drugs. Given the lack of approved drugs, equine veterinarians use such drugs as diazepam, ketamine, thiopental, camburn, tiletamine-zolazepam and sevoflurane, he says. While these extra-label uses and combinations are permissible in the United States if a valid veterinarian-client-patient relationship exists and the veterinarian is able to determine that no approved drug is effective, clinicians such as Hubbell feel this places the veterinarian in a compromising position. The veterinarian must “try to determine without resources which drugs or drug combinations are considered within the standard of care and treatment,” he says. (“The use of sedatives, analgesic and anaesthetic drugs in the horse: An electronic survey of members of the American Association of Equine Practitioners,” J.A.E. Hubbell, W.J.A. Saville and R.M. Bednarski)
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