Anesthesia: Keeping close watch on blood pressure, ventilation vital during surgery

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Nov 01, 2008

In 1915, Sir Frederick Hobday, a British veterinary surgeon, noted that "it is of no avail to have done any operation, however clever, if the patient succumbs to the anesthetic."

That statement is no less true today.

So what is the best thing modern practitioners can do to ensure that their use of anesthesia regularly leads to good surgical outcomes?

Two of the nation's leading experts agree that it's effective monitoring — keeping close watch on the horse's blood pressure, correcting it when necessary — and doing so, not only during surgery, but during recovery as well.

"I would say the most critical thing folks can do to improve their equine anesthetic protocols would be to directly monitor arterial blood pressure," says John Hubbell, DVM, MS, Dipl. ACVA, professor of anesthesia and interim dean in the Department of Veterinary Clinical Sciences, College of Veterinary Medicine at The Ohio State University, who has 25 years' experience in anesthetizing horses.

"The second most important thing they can do is to correct arterial hypo- tension if it's present. That reduces the incidence of complications pretty dramatically," Hubbell says.

"It's also important to assure adequacy of ventilation, but to me that's secondary to adequacy of perfusion. The measures to support blood pressure when it is decreased, or to support ventilation when it's not adequate, I think are the biggest things and where the most progress can be made (in this field).

"I tell people the best thing they can do is place an arterial line and, if there is hypotension present, to correct it with fluids and vasoactive substances (e.g., dobutamine, ephedrine)," Hubbell says.

Concurring with him on the importance of monitoring is Ann Wagner, DVM, MS, Dipl. ACVA, Dipl. ACVP, professor of anesthesia at Colorado State University's College of Veterinary Medicine and Biomedical Sciences.

"Horses as a species seem to have a lot more problems with low blood pressure, hypotension, during anesthesia," she says.

"We see hypotension in about 25 to 30 percent of dogs, cats and other species, but horses — even normal, healthy young horses that are having a (simple) arthroscopy or whatever — can be extremely hypotensive during anesthesia. So if people are going to do gas anesthesia of horses, they really must monitor blood pressure and be prepared to treat low blood pressure because it's going to happen," Wagner explains.

"I'm guessing probably 80 percent of horses have some problems with hypotension and need some kind of inotropic support during (gas) anesthesia.

"If people are doing surgery with field anesthesia — using xylazine, ketamine or triple drip — then that's not nearly the same, it's a different situation. But with gas anesthesia, the blood pressure is a real concern," Wagner says.

"If you don't carefully monitor and they get really hypotensive, then they'll get post-anesthetic myopathy. They won't stand up, and that's a problem."

Current anesthesia-related mortality in normal horses is reported to be about 1 percent, believed to be due in part to cardiopulmonary changes that occur with anesthesia, the horse's temperament and its primordial need to regain its feet rapidly after anesthesia.

"I think we do a better job monitoring than we probably did 20 years ago, but I don't know if there is any one drug (or procedure) that has made things especially better," Wagner says.

"I wouldn't say that there has been any breakthrough (in recent years) in terms of equine anesthesia (or anesthetics)," Hubbell states.

"If there has been any one breakthrough, it's been the realization of how difficult it is and how much attention has to be paid to it in order to have a successful outcome," he explains.