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

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.

Development of anesthetics

One of the first drugs used in equine surgery, starting about 1955, was succinylcholine, a depolarizing, neuromuscular blocking drug.

"Despite its lack of anesthetic or analgesic properties, and its potential to cause aortic rupture because of the hypertension produced, it was used for about 25 years because it allowed short surgical procedures to be performed, and its induction and recovery characteristics were as good or better than other techniques then available," Hubbell says.

Then came xylazine and ketamine, which virtually replaced succinyl- choline for short-term procedures.

"The adoption xylazine and ketamine for equine anesthesia is probably the most significant event in equine anesthesia in the last 50 years," Hubbell says.

Xylazine produces sedation, muscle relaxation and analgesia in a predictable manner. "Xylazine and ketamine were easily administered in two injections, produced 15 to 20 minutes of quality anesthesia with reasonable maintenance of cardiopulmonary function, and were associated with good recovery where the horse stood squarely within one hour from the start of the procedure," Hubbell says.

Various inhalants such as halothane — along with semi-closed circuit apparatus for their delivery to large animals — were developed during the 1960s and 1970s.

Today, isoflurane and sevoflurane are the primary inhalant anesthetics used in horses. Both produce more rapid induction and recovery than halothane. Cardiac output is better maintained with isoflurane and sevoflurane than it was with halothane.

But, because profound respiratory depression occurs with isoflurane and sevoflurane, horses anesthetized with them for 45 minutes or greater benefit from ventilatory support, and arterial blood pressure should be monitored whenever any inhalant anesthetics are used.

Horses anesthetized with isoflurane or sevoflurane may attempt to return to standing posture too rapidly, so small doses of sedatives often are administered during recovery to allow more time for anesthetic gases to be exhaled.

Importance of monitoring

The American College of Veterinary Anesthesiologists recommends assessment of circulation, oxygenation and ventilation every 5 minutes and recording every 10 minutes.

"Horses are specifically mentioned, with the suggestion that a continuous electrocardiogram and a non-invasive blood-flow or blood-pressure monitor and/or direct monitor of arterial blood pressure be employed in horses that are anesthetized for greater than 45 minutes and/or horses anesthetized with inhalant anesthetics," Hubbell says.

In addition to these indices, anesthetic depth should be regularly assessed and appropriate adjustments made as necessary, under ACVA guidelines.

Recognition of hypotension and ventilation abnormalities as significant factors in anesthetic complications occurred during the 1980s. Tying-up, or rhabdomyolysis, a significant complication during recovery, was shown to be linked to hypotension. In studies of hypotension and post-anesthetic myopathy, it was shown that horses with normal blood pressure during anesthesia were normal without myopathy during recovery.

"This recognition and adoption of strict monitoring are probably the second most significant event in equine anesthesia in the last 50 years," Hubbell believes.

Veterinarians are increasingly aware of the level of monitoring that is required for a good outcome — particularly for longer and more complex procedures.

"The level of monitoring required for a given anesthetic is dependent on the patient, the procedure and the planned duration of the event," Hubbell explains.

As for method, "Direct blood-pressure measurement is the way to go, which means putting a catheter directly into an artery. But there are a variety of ways to measure it once you have the catheter in place," Wagner says.

"If you're in a practice that does a lot of surgeries, it's probably worth buying an ECG (electrocardiogram) with a blood-pressure channel. But there are inexpensive ways to do it, too. If you're in a practice where you're doing gas anesthesia only once a month or so, you can buy a very inexpensive aneroid manometer at a pharmacy and that can give you a pretty good measurement of mean blood pressure," she adds.

Monitoring through recovery

The concern over the horse's standing up after surgery highlights the importance of monitoring through the recovery period, both experts say.

Various clinics differ in their recovery procedures. A swimming pool, for example, was used during Barbaro's recovery. At Ohio State, head and tail ropes are used during surgical recoveries. At Colorado State, Wagner notes that surgeons close the door and use a bit of sedation to keep the patient quiet during recovery.

"There are a lot of different ways of managing recovery, but I don't think people have found the perfect solution for every case. That is still an area that probably needs more investigation," she says.

"I think we do a pretty good job of getting the horse induced, anesthetized and monitored but once we get into recovery there is a lot of uncertainty. I've been to a few equine practices where once they get the horse into recovery, everybody just slams the door and walks away. They really didn't pay a lot of attention (to the recovering patient), and I think that's a real mistake. I would just encourage people to understand that the procedure is not over until the horse is standing, extubated and safe," Wagner says. "Recovery can be one of the most risky times.

"It would be great if we could come up with a drug you could give at the end of surgery that would magically make the horse lie quietly until fully awakened, then stand straight up with no problem, but I don't think we're there yet," she adds.

Help and documentation

"Another thing that is advisable is to have someone specifically assigned to monitor the anesthesia," Hubbell says. "Take a friend, so that the veterinarian is not trying to do the surgery and the anesthesia at the same time.

"All drugs administered — and the dose, time and route of administration — should be recorded regularly, along with other monitored variables, to complete the anesthetic record," Hubbell explains.

Keeping a good record is critical even from a legal aspect, the experts say. That means all details — the physical exam, what anesthetics were used, each response — should be well documented.

"As a profession, we're not as good at that as we need to be. We have to get better at documenting what we do," Hubbell says.

Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.