Surgery

Atropine Not Linked to Colic In Some Surgical Settings

By Adam Marcus

The use of atropine to treat hypotension in horses undergoing surgery does not appear to be linked to abdominal discomfort in the animals—except in those being castrated, a new study found.

Although atropine is effective at boosting blood pressure in surgical settings, many equine veterinarians avoid using the drug out of concern the medication and related anticholinergic agents can increase postoperative abdominal pain.

However, the new study, a retrospective analysis of more than 220 horses, found no increased incidence of such discomfort in horses undergoing various operations, including arthroscopic procedures, airway surgery, neurectomy and fracture repair. However, the researchers did report a significant increase in abdominal discomfort in male horses undergoing castration that received atropine. In some cases, this effect might be linked to the drug rather than the surgery itself.

“Atropine is less dangerous than previously reported in the literature,” said Kelley Varner, DVM, DACVAA, an assistant professor of anesthesia and analgesia at the North Carolina State University College of Veterinary Medicine in Raleigh, who led the study. “If practitioners use isoflurane to anesthetize horses—not field anesthesia—I believe it is safe to use atropine to increase the heart rate in the face of low blood pressure. I have included this treatment option in my practice with success.”

Dr. Varner added she does not recommend the approach during castration or in critically ill animals.

For the study, Dr. Varner and her colleagues looked at the medical records of 111 horses that received atropine during surgery under general anesthesia with isoflurane and 111 case-matched controls that underwent the same operations without the anticholinergic drug.

The researchers found that atropine use was not associated with abdominal discomfort in any animals except those undergoing castration, among which 18 (8.1%) had apparent symptoms of the problem. However, looking only at those horses that underwent castration, the use of atropine was linked to a roughly 3-fold increase in the risk for abdominal discomfort (odds ratio=3.143, 95% CI [1.082, 9.132]; P=0.04).

“Because colic is a general term for abdominal discomfort, it is impossible for us to tell if these signs are secondary to abdominal pain post-castration or atropine-induced ileus leading to discomfort,” Varner said.

The colts did not receive locoregional anesthesia and, therefore, could have experienced significant discomfort after surgery. “It is also vital to note that none of these colts required treatment beyond the addition of analgesia, which leads me to believe this discomfort was secondary to surgery,” she said. “This is supported by the fact that not a single case that underwent anesthesia for an orthopedic or airway surgery demonstrated signs of postoperative abdominal discomfort despite receiving atropine.”

Dr. Varner noted that the results prompted her practice to provide locoregional anesthesia routinely during castration procedures. “At the time of this study, testicular blocks were not being used for colts undergoing castration,” she said. “Following this study, we changed our practice to include locoregional anesthesia. We subjectively noticed a decline in the number of postoperative castrations who require additional analgesia. Having another study where this confounding factor was removed would help clarify the answer to this question.” MeV

Related: Should Horses Be Fasted Before Elective Procedures? looks at research on feeding hay before and after anesthesia and the risk of postanesthetic colic.

For more information:
Varner KM, Curtiss AL, Hogan PM, et al. Retrospective evaluation of the impact of atropine administration on incidence of post-operative colic in healthy, isoflurane-anaesthetised horses. Equine Vet J. 2024 Oct 29. https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.14428