Equine Anesthesia Deaths “Remain High,” Risk Factors Identified
Horses undergoing general anesthesia die about a thousand times more often than humans do, at a rate of around 1 to 1.5 deaths per 100 anesthetized horses. While human rates have dropped significantly over the decades, equine deaths related to anesthesia seem to remain stable across the years and across hospitals, according to a study by French researchers.

“It’s important for owners to be aware that anesthesia-related risks in horses are non-negligible, even though every effort is made to reduce risks within the limits of our knowledge about those risks,” said Karine Portier, PhD, DVM, CertVA, Dipl. ECVAA, DScV, HDR, MRCVS, professor in the anesthesiology department of the University of Lyon.

Some risk factors are clear, Portier said. For example, she found that in her teaching hospital, horses undergoing colic surgery had a 3% anesthesia-related mortality rate, more than three times the 0.9% rate of anesthesia-related deaths in noncolic cases.

Weight is another critical factor, said Portier. Heavier horses are more likely to die from general anesthesia, probably in part because increased weight affects oxygenation in the blood and lungs, making horses more likely to suffer from blood oxygen deprivation. Age is a factor, as well, with older horses incurring greater risk of mortality from anesthesia. Surgeries requiring the presence of a more senior veterinarian (hence, more complex surgeries like fracture repair) also increased the risk, she said. In general, orthopedic surgery carried a 10-fold greater risk of anesthesia-related mortality—probably due to the length of surgery, she explained. Additionally, emergency surgeries (mainly colic and orthopedic trauma) carried higher risks.

One “Controllable” Risk Factor: Time

The only risk factor that veterinarians could actually control, according to Portier’s study, was the duration of the anesthesia. But the time it takes to operate on a horse isn’t all that easy to control, she said.

“We can try to minimize how long a horse is anesthetized, but you can’t just wake up a horse in the middle of a laparotomy,” she said. “What we can do is try to prepare the surgical site before surgery so as to minimize the time on the table, but some steps just can’t be reduced. Of course, the faster the surgeon goes, the less time it takes to operate, but some surgeries are complex and require a lot of time—like a fracture repair which can last more than seven hours. It’s physiologically not good to keep a horse lying down for seven hours but unfortunately it’s often difficult to go any faster.”

Documenting and Analyzing Mortality Data

Portier and her fellow researchers analyzed data from more than 1,000 general anesthesia cases in their equine teaching hospital in Lyon from 2012 to 2016. They looked at all cases of death clearly related to anesthesia (not surgical complications) and a variety of possible factors that might be related to mortality. While their statistics lined up more or less with those from other equine hospitals in different parts of the world that have published mortality data, it’s “very difficult” to compare rates because of different study protocols. Ideally international researchers should team up to carry out a single universal study on anesthesia-related deaths in horses (as has already been done in humans) to have comparable data, but so far such a project has been cost-prohibitive. The benefits of such a study would be primarily to clearly identify risk factors that can be changed and to provide owners with clear information about tangible risks related to anesthesia, she said.

“It’s really important to follow this data over time,” she said, adding that it’s “surprising” to see that mortality rates in horses aren’t improving.

“We have to continue to work to reduce anesthetic risks in horses, which remain too high,” Portier said. “And we have to continue to identify and communicate about risk factors and apply that knowledge to changes in best practices.”