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