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