Heads or Tails? Anesthesiologist Finds New Route to Sedating Horses
Detomidine, a commonly used sedative, was formulated as a gel so veterinarians could conveniently give it to their patients sublingually—beneath the tongue–where horses absorb it into their mucosa. Some horses, however, refuse to cooperate with this pain-free protocol. They ungraciously spit it out, swallow it whole, or evade the oral dosing syringe altogether.

So much for achieving the desired calming effects without resorting to a sharp needle.

Reza Seddighi, DVM, MS, PhD, Dipl. ACVAA, an associate professor specializing in anesthesiology and pain management in the University of Tennessee’s Department of Large Animal Clinical Sciences,  in Knoxville, reasoned there might be another route to sedation—at least when patients are mares.

At the 65th Annual American Association of Equine Practitioners Convention, held Dec. 7-11, in Denver, Colorado, Seddighi shared the results of a study  in which he and colleagues compared the efficacy of administering detomidine gel intravaginally, to intravenously, (IV) or sublingually. Their findings included data from two previous studies.

Seddighi explained that vaginal tissue is thin, permeable, and highly vascularized, like the mucosal membranes in the horse’s mouth. It absorbs and moves detomidine rapidly into the bloodstream. An added benefit of intravaginal administration is that the drug bypasses the gastrointestinal tract, thereby eliminating any potential enzymatic degradation or first-pass metabolism effects in the liver.

During two separate treatment rounds at the University of Tennessee, Seddighi and his team sedated and observed six study mares for four hours, scoring them every 15 minutes for ataxia (incoordination), behavioral changes, appearance, muzzle-to-floor distance, and heart rate. They also sent blood samples to the University of California, Davis, for analysis to better correlate and characterize the drug’s pharmacokinetics (what the body does to a drug), Seddighi said.

The researchers confirmed that the drug takes effect most rapidly when given intravenously. A 10 microgram/kilogram IV dose of detomidine hydrochloride went to work in approximately three minutes and, then, provided approximately two hours of clinical sedation, he reported. In contrast, detomidine gel administered intravaginally as a 40 microgram/kilogram dose took approximately 16 minutes to achieve full sedation. Notably, said Seddighi, this is twice as fast as how long the same gel dose given to mares sublingually (30 minutes). Clinical sedation lasted for approximately three hours in the horses that received the drug in gel form.

He also reported that sedation was deeper and longer-lasting in the intravaginally administered gel group than in the IV group. Heart rates dropped in all the mares receiving detomidine, but the number of beats per minute was not low enough to be a cause for concern, he said.

From a practical standpoint, this study reveals that in certain situations, approaching sedation from the tail end—with attention to safety of the person administering the drug, of course—could be an easier route rather than facing a challenging horse head-on.