General Anesthetics in Horses: Inhalants and Injectables (AAEP 2009)

The basic strategies for general anesthesia in horses--gas (inhalant) versus injectable (intravenous) medication--might soon be sharing and/or trading places on the popularity scale, according to Ann Wagner, DVM, MS, Dipl. ACVA, ACVP, professor of veterinary clinical sciences at Colorado State University. She discussed balancing general and intravenous anesthetics in horses at the 2009 American Association of Equine Practitioners Convention, held Dec. 5-9 in Las Vegas, Nev.

Before the 1970s, injectables were "often all we had, and often associated with prolonged and/or violent recoveries," she recalled. "Equine anesthesia was pretty scary in those days. In the 1960s, halothane (inhalant) became popular. In the 1970s, xylazine and ketamine revolutionized equine anesthesia. Today, halothane has given way to isoflurane and other gases, but because of the cardiovascular consequences of inhalants plus the unpredictable or rough recoveries, there is interest again in using intravenous anesthetics to supplement or replace inhalants."

A few factors affect the decision to use inhalant vs. injectable anesthetic, she noted. These include:

  • The equipment needed (anesthesia machine required for inhalant anesthetic);
  • Muscle relaxation required for the procedure (gas gives better relaxation);
  • Duration of the procedure (most veterinarians are not comfortable with using injectables for more than an hour, she explained);
  • Cardiopulmonary effects (inhalants tend to cause more severe depressive effects, which can lead to muscle damage);
  • Recovery quality (inhalants tend to have rougher recoveries);
  • Stress on the horse (inhalants tend to cause more stress), and
  • Higher risk of death with inhalant (almost 1% of cases in one study, and roughly three times greater risk than with injectables).

Wagner went on to compare the usage and benefits of several intravenous anesthetic medications for different situations, specifically short-duration anesthesia (20 minutes or less) and longer duration (more than 20 minutes). For the first case, she reported that xylazine premedication followed by ketamine anesthesia induction, possibly with diazepam for muscle relaxation, is the most common technique used in the United States. This yields about 16 minutes of general anesthesia, with horses standing afterward in about a half-hour.

When anesthesia is needed for more than 30 minutes, Wagner recommended a continuous rate infusion to maintain a more stable level of anesthesia compared to intermittent doses. It's important to monitor the horse closely during anesthesia, in particular the horse's breathing and oxygenation.

"Any animal can become hypoxemic when lying flat out under anesthesia, particularly horses with their large size," she noted. "With a low respiration rate they can get very low blood oxygen levels, particularly at higher altitudes. Consider providing oxygen for foals, horses with chronic obstructive pulmonary disease or heart murmurs, or for any horse undergoing a prolonged procedure."

She also noted that in quiet settings, horses tend to require less anesthetic compared to busy settings. The same phenomenon has been observed with standing sedation.

For procedures more than an hour, Wagner recommended inhalant anesthesia under hospital conditions. She cautioned that blood pressure tends to drop quite a bit in horses on inhalants, noting that more than 80% of their horses on inhalants receive some type of support for their blood pressure.

The choice of anesthetic doesn't have to be one or the other, however. Wagner described "balanced anesthesia" (also called partial intravenous anesthesia) where both inhalants and injectables are used with the following goals:

  • Less cardiovascular depression than inhalants alone;
  • Decreased stress response compared to inhalants alone;
  • Improved anesthesia quality compared to either type alone, and
  • Improved quality of recovery.

Wagner went on to discuss in detail the various injectable and inhalant anesthetic drugs available for use in horses (including the propofol that was implicated in Michael Jackson's death). Dosages, timing, techniques, and comparisons of the quality of induction, anesthesia, and recovery for various scenarios were covered in-depth.

"To date, there are numerous drug combinations that can be used to produce general anesthesia in horses," she summarized. "Currently, there is no single perfect method that is uniformly reliable and safe, particularly for long-term anesthesia. However, the development of newer injectable drugs and continuing studies into their use for total or partial intravenous anesthesia promise further improvements in the near future."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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