Have you ever observed or seen photos of a horse in a hospital undergoing surgery that requires general anesthesia? After he's anesthetized in a padded room he's hoisted onto a table where, while the surgeon takes care of the procedure at hand, numerous assistants monitor the patient's vital signs, ready to spring into action if any problems occur.
But when a horse undergoes a surgical procedure requiring general anesthesia in the field, the set-up is much less elaborate. At the 2013 Western Veterinary Conference, held Feb. 17-21 in Las Vegas, Nev., Robert Brosnan, DVM, PhD, an associate professor in the Department of Surgical and Radiological Sciences at the University of California, Davis, School of Veterinary Medicine, reviewed some points equine veterinarians must consider before implementing field anesthesia.
Pros and Cons
According to Brosnan, total intravenous anesthesia—a general anesthesia technique that uses a combination of agents given solely through a vein—is commonly used for field surgeries such as castrations. Benefits of intravenous anesthesia, he said, include that it requires minimal equipment, it avoids the need for patient transportation, it effectively renders a horse immobile and unconscious, and most currently used drugs allow horses to maintain a consistent blood pressure and recover quicker than from inhaled anesthesia.
But it's not without drawbacks, he said. Horses become more likely to experience poor or prolonged recoveries with increasing length of anesthesia time. Most drugs used in field anesthesia are dependent on kidney and liver metabolism and excretion and, thus, are unsuitable for patients with renal or hepatic insufficiency, he explained. Techniques are typically associated with reduced cardiac output, and hypoxemia (abnormally low blood oxygen levels) is common unless supplemental oxygen is administered, he said.
Brosnan reviewed the different drug combinations veterinarians can use to induce anesthesia during his presentation.
When to Avoid Field Anesthesia
Not every horse or situation is a candidate for field anesthesia. Brosnan said horses should be healthy with, at most, a mild systemic disease. Candidates should not have substantial cardiorespiratory problems or be anemic and should have normal kidney and liver function, he said. And finally, because recovery assistance will be limited, patients shouldn't have severe neurologic or musculoskeletal problems, he said. Essentially, horses should be in overall good health before undergoing field anesthesia.
Procedures carried out under field anesthesia should be relatively short in duration (less than an hour), be associated with a low morbidity (illness) risk, and be uncomplicated, Brosnan said.
It's also important to ensure there's an acceptable location to induce and maintain anesthesia and carry out the procedure, he cautioned. A soft, flat surface—such as a grass field—is ideal, he said. Brosnan cautioned that the horse should be positioned away from hazards, including trees, rocks or stones, mud, water sources, hills, and other animals.
Finally, Brosnan recommended veterinarians take weather conditions into account before using field anesthesia. Especially in summer and winter, extreme weather conditions could place horses under anesthesia at risk for hyperthermia or hypothermia.
What to Watch For
Brosnan said that once a horse has been anesthetized, there are several things veterinarians must watch for to ensure a good outcome.
Hypoxemia—As previously mentioned, hypoxemia is a concern with horses anesthetized without supplemental oxygen. Brosnan noted that patients in high altitudes will "invariably become hypoxemic unless inspired oxygen is supplemented." Brosnan said proper patient and procedure selection can help minimize the risk of hypoxemia. Additionally, he said, if supplemental oxygen is required, veterinarians can use intranasal or endotracheal oxygen insufflation or ventilation using a product called a Hudson demand valve.
Bradycardia—Brosnan said hypertension (high blood pressure) is a common side effect of some anesthetic techniques. While short spans of hypertension aren't generally a major problem, he said hypertension can cause bradycardia (an abnormally slow heart rate) leading to a significantly decreased cardiac output. Severe bradycardia (which Brosnan described as less than 20 beats per minute) requires emergency intervention to prevent the horse's heart from stopping (called asystole). In the event of severe bradycardia, he said veterinarians can use certain drugs to increase heart rate. He cautioned that some of these medications can increase a horse's risk for impaction colic, so patients receiving them should be monitored closely for colic signs for about eight hours.
Recovery—While horses generally recover better from intravenous anesthesia than inhaled anesthesia, recovery isn't without risk. Brosnan said potential complications include myopathy (muscle disorders), neuropathy (disorders of the peripheral nervous system), dysphoria (feeling generally unwell), and trauma; attending veterinarians should monitor the animal until the horse is on his feet.
Pain—Brosnan said veterinarians can optimize analgesia during surgical procedures by employing local and regional techniques as appropriate. Good analgesia will also decrease patient responsiveness during anesthesia, he said.
Take-Home Message
Field anesthesia is a useful technique when surgical procedures are carried out away from a clinic. Veterinarians should ensure appropriate patient and procedure selection to reduce the risks of an unfavorable outcome.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.
