John A.E. Hubbell, DVM, MS Dipl. ACVA, delivered the Milne State of the Art Lecture at the 2020 American Association of Equine Practitioners’ Convention, held virtually. Hubbell, the current chief of anesthesia at Rood & Riddle Equine Hospital in Lexington, Kentucky, has authored more than 70 scientific articles and 30 book chapters and co-authored two textbooks.
He shared his extensive knowledge of the paths taken to achieve our current understanding of equine anesthesia and how we should navigate the roads ahead to continue advancing this highly specialized field “to make things even better for future generations.”
Early in his presentation, Hubbell showed an old video of a surgery that involved anesthetizing a horse for an upper respiratory procedure. Many handlers forcibly lay the horse down and hobbled him while stuffing a wad of cloth soaked in chloroform in a muzzle to induce anesthesia. Monitoring involved simply standing around seeing if the horse kept breathing. At the end of the procedure, the handlers removed the chloroform cloth and helped the horse pop back up to a standing position.
That video, filmed around 1930, portrayed what anesthesia continued to look like until about the 1960s, Hubbell, said, and veterinarians would frequently mistake immobilization with loss of sensation.
Hubbell also showed a recent video of Tommie Turvey—a horseman and showman responsible for training horses for movies and television shows, including “The Walking Dead.” In that video Turvey had his horse lie down on command, both in lateral and dorsal recumbency while he walked over and around him, tugging on the horse’s tail and legs.
After pondering what the horse’s arterial blood oxygenation saturation was while the animal voluntarily lay in dorsal recumbency, Hubbell then wondered, “What if all the horses we anesthetized were trained at this level? It would stress them less.”
The contrast in the two videos mirrored Hubbell’s presentation: a view of where we were, where we are now, and the remaining challenges we have yet to overcome.
Looking Back: The History of Equine Anesthesia
Indeed, from the first anesthetic events in the early part of the 20th century to the first textbook published on “modern” veterinary anesthesia in 1941 and the first equine-specific text in 1991, much has changed. This is especially true when we consider that acepromazine didn’t reach the market until 1972, guaifenesin in 1968, xylazine in 1969, and finally butorphanol in the 1980s—all staples in the equine anesthesiologist’s toolbox. The use of a combination of xylazine and ketamine, first described by Muir in 1977, was “a game changer that now forms the basis of equine anesthetics worldwide, particularly for field anesthesia,” said Hubbell.
The addition of guaifenesin to the mix (“triple drip”) extended anesthesia times, allowing veterinarians to anesthetize horses for up to two hours with swift recoveries—only 15 to 30 minutes. Echoing the advances in field anesthesia, it also took years to determine that for longer surgical procedures conducted in a hospital setting, horses needed to be ventilated and their mean arterial blood pressures maintained above 60 mm Hg with the help of fluids and dobutamine.
Where We Are with Equine Anesthesia
Currently, Hubbell said, the most problematic part of anesthesia is that the recoveries are too quick and that stress is a “significant component of equine anesthesia.” Even taking away the surgery part of anesthesia, sources of stress include moving the horse, exposing him to new people, restricting feed, having the horse fall during induction, maintaining him in dorsal recumbency, and having him recover in a strange place, among others.
Hubbell likened the stress to the horse “being attacked by a lion,” and that the stress itself was linked to “higher morbidity and mortality compared with other species.”
Hubble summarized what we know we know about equine anesthesia:
- Anesthesia is stressful for horses, and “an excited horse should never be anesthetized.”
- Horses will attempt to stand as soon as they are conscious. “Horses need to be sedated prior to recovery.”
- Horses that cannot stand two hours after anesthesia are less likely to survive.
- Short anesthetic procedures lasting less than 60 minutes are safer, and procedures lasting more than three hours produce a greater risk.
- Finally, mean arterial blood pressures <60 mm Hg for significant durations in anesthetized horses are associated with complications such as rhabdomyolysis (tying-up, muscle injury).
Where We’re Going with Equine Anesthesia
Moving into the throes of the 21st century, Hubbell has five main goals to make veterinary anesthesiology better:
- Improve the availability of anesthetic drugs. Practitioners relies heavily on guaifenesin, ketamine, and xylazine (triple drip), yet guaifenesin is no longer commercially available. We need to be looking for viable alternatives for field anesthesia, he said.
- Improve pain management. Beyond non-steroidal anti-inflammatory drugs, analgesics are deficient. We need newer options and should also embrace improved pain scoring systems, Hubbell said.
- Improve the standard of care for anesthetic monitoring. Veterinary anesthesiologists need to be able to perform anesthesia even in the field, using proper equipment such as direct blood pressure monitoring and a blood gas analyzer.
- Improve the standard of care for management of recovery. The recovery period is an area of continued deficiency. “Some intraoperative problems don’t become apparent until recovery,” Hubbell said.
And while he acknowledged that there is currently no “best way” to recover horses, we do know that horses are at the greatest risk during this time. Again, horses must be sedated to recover slowly, and clinics must use nonslippery floors with quality footing.
Regardless of the chosen recovery system, Hubbell said, “Never be in a hurry to recover a horse.”
- Reduce the stress of anesthesia and surgery. Hubbell reminded viewers that horses are “fright or flight” animals whose mantra is “food, freedom, and friends.”
He again underlined that the consequences of stress from anesthesia can be severe, including premature attempts to stand during recovery, alterations in gastrointestinal function resulting in cecal impactions or diarrhea, and decreased immune function that could potentially lead to low-grade pneumonia [What are some suggestions for improvement the way we suggested ones for recovery?] He didn’t make specific recommendations just pointed out deficiencies that need to be addressed moving forward.
To encourage all parties involved in any aspect of equine anesthesia to strive toward helping achieve the five goals outlined above, Hubbell advised, “Everything we do, every drug we give, every decision we make must be done after considering what the effect that action has on the horse’s chance of a successful recovery.”