Cover Story | Surgery

Should Horses Be Fasted Before Elective Procedures?

By Marie Rosenthal MS

All animals are at risk for gastrointestinal dysfunction following general anesthesia. In horses, postanesthetic colic drastically increases the morbidity, mortality and expense associated with anesthesia for elective procedures. according to Charlotte K. Barton, BVetMed, MS, DACVS-LA, an equine surgery resident, now a postdoctoral fellow, at Colorado State University, the standard of care has been to fast horses between 6 to 12 hours prior to anesthetic induction. But should it be?

Colic makes up more than 65% of postoperative complications, and it has an overall incidence of between 3% and 11%. So, the thought behind fasting is to reduce the risk for anesthetic complications, such as postoperative colic, aspiration pneumonia, and impaired ventilation and oxygenation due to an overly full colon placing pressure against the diaphragm.

Although fasting has been recommended, adverse events can still occur, Dr. Barton said at the 70th AAEP Annual Convention, in Orlando, Fla. The contractile activity of the colon is reduced during a prolonged fast, which can lead to an increased transit time. This allows for impaction and gas buildup ultimately resulting in colic. Fasting has also been shown to concurrently decrease the water intake of these horses, which is another well-known risk factor for colic development, Dr. Barton explained.

In 2016, a retrospective study that assessed postoperative colic in a group of horses that had not been fasted found an overall incidence of 2.5%, while a 2013 study looking at fasted horses reported it to be 8.7%. Because the incidence of postoperative colic varies greatly depending upon the anesthetic protocol used, the management of horses and the geographical location, it’s impossible to make accurate comparisons among these studies that have looked at this issue.

“There is currently no prospective evidence-based research to support the practice of fasting and slow refeeding in horses, especially as it relates to the development of postoperative colic,” Dr. Barton said.

Therefore, under the expert guidance of her mentor Laurie Goodrich, DVM, PhD, DACVS, they set out to compare 3 perioperative feeding regimes in healthy horses with a prospective randomized controlled trial.
“Our first hypothesis was that horses that are fasted before and slowly refed after general anesthesia, will have a higher incidence of postanesthetic colic, will produce less manure, and will produce manure more slowly, than horses who are allowed access to hay,” Dr. Barton said.

“Our second hypothesis was that horses that are fasted prior to general anesthesia will have an equal incidence of anesthetic complications, such as hypoxemia or hypotension, as horses who were allowed access to hay,” she said.

There were 15 horses in each group. Enrolled horses were mares or geldings between the ages of 1 and 20 years, and they were all healthy, according to the results of a physical examination, a complete blood count and serum chemistry. None of the horses had a previous history of surgical colic or chronic medical colic.

The horses were all presented for elective orthopedic procedures to be performed under general anesthesia with a total anesthetic time of no less than 30 minutes and no more than 250 minutes. They were then separated into 1 of 3 groups:

1. Group 1 was not fasted. These horses were provided access to free-choice hay up until anesthesia and then returned to free-choice hay 30 minutes following recovery. They were not muzzled at any time.

2. Group 2 was fasted and muzzled. These horses had access to free-choice hay up until 10 hours before anesthetic induction, at which point the hay was removed, and the horses were muzzled. They were then slowly refed starting 2 hours after recovering from anesthesia, and they were given a quarter flake of hay every 3 hours for a total of 12 hours. Muzzles were placed before and between each feeding. After 12 hours, they were returned to full feed.

3. Group 3 was fasted but not muzzled. They also had their hay removed 10 hours before anesthetic induction. They were then slowly refed following the same protocol as group 2.

All horses were kept on straw bedding with access to free-choice water at all times, and their water intake was monitored.

A standardized anesthetic protocol was followed for all cases, and the horses were all mechanically ventilated. Prior to recovery, intramuscular morphine was administered. If the case involved a joint arthroscopy or a tendonoscopy, the morphine was also administered intra-synovially at the end of the procedure.

Comparisons among groups were made for anesthetic variables, manure output and colic incidence pre- and postoperatively. The total manure output was measured as the number of piles produced per hour, as well as the weight of the manure in kilograms per kilogram of the horse’s body weight per hour.

The time from returning to the stall to the first passage of manure was noted, as was the incidence of colic. A single observer also determined pain by using 2 pain scales—the composite orthopedic pain scale and the horse grimace scale. The total anesthesia time among the 3 groups was similar. Overall, horses had short durations of hypotension with no evidence of a difference between groups. There was also no difference in arterial oxygen tensions between the groups.

“Since morphine has been shown to reduce gastrointestinal motility and could also potentially increase the risk for colic, we analyzed the total morphine that was administered to each horse. Nine horses received only an intramuscular dose of morphine—2 were from the not fasted group, 3 from the fasted-muzzled group, and 4 from the fasted not-muzzled group. The remaining 36 horses received both intramuscular and intrasynovial morphine, and the mean intra-synovial dose of morphine was similar among the groups,” Dr. Barton said.

Significantly reduced manure output was seen in the the fasted-muzzled and not-muzzled groups compared with the fasted group postoperatively. The mean reduction in manure weight for the not-fasted group was 39%, compared with 81% in the fasted-muzzled horses, and 70% in the fasted not-muzzled horses.

“All groups showed a significant decrease in manure output between pre- and postoperative variables,” she said. “Both the fasted-muzzled and fasted not-muzzled groups had a significantly reduced output in the postoperative period as compared with the not-fasted group.”

The fasted group had a mean manure reduction of 22% compared with 63% in the fasted and muzzled group, and 55% in the fasted and not-muzzled group.

The mean time until the first passing of manure for the not-fasted group was 238 minutes versus 507 minutes for the fasted-muzzled group, and 444 minutes for the fasted not-muzzled group.

“There was a statistically significant difference between the not-fasted group and both the muzzled and not-muzzled groups,” Dr. Barton said.

Overall, 4 horses displayed signs of colic—2 were in the fasted-muzzled group, and 2 were in the fasted not-muzzled group. Three horses developed a large colon impaction, and 1 horse developed a cecal impaction. Two of the horses had undergone a stifle arthroscopy; 1 horse underwent a tarsocrural joint arthroscopy, and 1 horse underwent a carpal arthroscopy.

However, “there was no significant difference among these groups regarding colic incidence, and all of these colics resolved with medical management but did require additional days of hospitalization,” she said.

Significant predictors of manure output regardless of the feeding protocol included the preoperative manure output and the horses’ body weight. “Horses that had higher preoperative manure tended to pass more manure postoperatively, and horses that had a higher body weight tended to be a bit slower to pass manure following anesthesia,” Dr. Barton explained.

Pain scores were all low pre- and postoperatively on both pain scales, and they were not associated with manure output or colic incidence. “Since we know that orthopedic pain is a significant risk factor for the development of colic, these findings may not accurately reflect these more painful procedures,” she said.

The study had limitations when comparing the results with previous studies. Much of the current literature is retrospective and uses various definitions of colic and GI dysfunction. Also, the anesthesia and surgical protocols, horse management and geographical locations were different.

“Because we did not use an objective measure of gastrointestinal motility or transit time, such as ultrasound or non-digestible bead recovery, we do not know the exact mechanism by which these fed horses passed more manure,” she said.

And finally, this study could not be blinded for logistical and practical reasons, and therefore, could introduce bias, especially into the scoring variables.

However, their data show that horses that have access to free-choice hay before and after general anesthesia tended to pass more manure and pass manure more quickly following recovery compared with fasted horses. While there were 2 horses in each of the fasted groups that developed signs of colic, they did not find a statistically significant difference among the groups in colic incidence.

“Horses that are not fasted also have no difference in intraoperative variables with no evidence of increased risk of hypoxemia,” Dr. Barton said.

She hopes the study will provide evidence-based recommendations for pre- and postanesthetic feeding of horses for elective cases. In addition, the data have changed the preanesthetic feeding protocols for many surgeries in her hospital, she said. “We currently allow all horses to have access to free-choice hay up until the time that they’re taken from their stall to go to anesthesia,” she said. “And then whenever they come back from recovery, about 30 minutes later, they have access to free-choice hay again.”

The process is slightly changed for horses undergoing more painful procedures, such as fracture repair. Those horses are slowly refed, particularly if they are painful. MeV

For more information:
Barton CK, Hector RC, Hendrickson DA, Kawcak CE, Nelson BB, Goodrich LR. Fasting horses perioperatively decreases manure production and increases time to manure output postoperatively: a controlled randomized trial. J Am Vet Med Assoc. 2024;262(9):1201-1208.
https://avmajournals.avma.org/view/journals/javma/262/9/javma.24.04.0235.xml