July 2013 | Down for the Count: The Art and Science of General Anesthesia in the Horse
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Down for the Count:
The Art and Science of General Anesthesia in the Horse
July 2013 - Mariana Crumley, DVM, MS, Lecturer in Anesthesia at Penn Vet’s New Bolton Center

Horses everywhere require surgery on a daily basis. Along with surgery comes general anesthesia.  General anesthesia induces the animal into a state that results in unconsciousness, amnesia, and lack of movement and pain. Typical procedures performed under general anesthesia include, but are not limited to, orthopedic procedures such as joint arthroscopies and fracture repairs, soft tissue procedures such as throat surgeries and hernia repairs, and emergency procedures such as colic surgeries, C-sections, and extensive laceration repairs. Many owners are skeptical of general anesthesia because of concerns about associated risks. This fear is well-warranted given the fine balance between finding the appropriate depth under general anesthesia to allow the surgeon to perform the desired procedure and avoiding a terminal outcome caused by cardiovascular or respiratory collapse. Safety in anesthesia has increased significantly in the recent past, with the development of new drugs, better monitoring of the patient, and an increase in the number of highly trained anesthesiologists. With the risk of complications lower, so should the fear of general anesthesia be less, as long as the animal is in expert, well-trained hands. 

Sedation
Equine general anesthesia follows a sequence of steps. It starts with the sedation period, followed by induction, maintenance of unconsciousness, and then, finally, the recovery period. The sedation period is crucial to ensure that the animal is calm and relaxed; this assures a safe environment for the horse and personnel, reducing the stress and risks of injury on both sides. After the animal is sedated, induction of anesthesia follows. Induction of anesthesia (the moment the animal becomes unconscious and “goes down”) is most often achieved in equine medicine with a drug called ketamine.  Since the horse will fall down the moment it becomes unconscious, methods have been developed to make that “fall” more controlled and safer. One method uses ropes attached to the horse’s head and tail, which along with the assistance of a well-trained team of extra personnel, allows the horse to slowly descend into a sternal position. Another common method of induction makes use of a swing gate, where a well-padded gate is closed toward a horse standing by a wall. The gate helps control the fall by allowing the animal to slowly slide down while its head is supported by a rope. Once the animal is in sternal position, the gate swings open and the animal lies on its side.  A less common method, reserved for special cases such as fractures and ataxic horses, is the sling induction method, during which a sling is wrapped around the body of the horse with rings that attach to a hoist. With this method, the animal never falls down; instead, as soon as it loses consciousness, the sling lifts the patient off the ground and it is then laid on the surgery table.

Once general anesthesia is induced, the horse is intubated with an endotracheal or a nasotracheal tube. The endotracheal tube allows for mechanical ventilation (in case the animal does not breathe well under anesthesia) and it also protects the lungs from any possible contamination originating from the stomach or esophagus.  Once intubated, the animal is hoisted and moved into the surgery room.  Proper positioning of the horse on the table is essential to avoid any damage to the muscles or nerves (myopathy or neuropathy).

General anesthesia may need to be maintained for a short period of time (castration or laceration repairs lasting less than one hour) or for prolonged periods of time (colic surgery or fracture repairs lasting more than one hour). Anesthesia may be maintained with only intravenous drugs if the procedure is short or with an inhalant anesthetic if the procedure is expected to last more than one hour.  If an inhalant anesthetic is used to maintain general anesthesia, an anesthesia machine needs to be available.  Inhalant anesthetics result in lack of movement and unconsciousness by affecting the brain and spinal cord. They can be titrated to the animal’s needs and turned off when the procedure is finished.  While anesthetized, analgesia drugs are also given to help alleviate the pain associated with the procedure being performed. Commonly used drugs include NSAIDS such as phenylbutazone and flunixin meglumine (banamine), opioids such as butorphanol and morphine, and alpha-2 agonists such as xylaxine, detomidine, and dexmedetomidine. Other methods to alleviate pain include the use of local anesthetics such as lidocaine for local infusions as well as for constant rate infusions (small amounts of the drug given continuously through a vein).

Recovery
The recovery period is critical during equine anesthesia. The quality of the recovery depends on many variables, including the length and type of procedure being performed, drugs used, and the animal itself (breed, temperament, and disposition). If the animal has been on inhalant anesthesia, it is important to allow it enough time to exhale the gas before trying to stand. If the animal tries to stand too soon, excitation and uncoordinated attempts to stand may result, which can hurt the animal. Sedation is often administered to the patient to encourage it to lie down until it is more aware, which therefore increases its chance of succeeding when attempting to stand. 

There are many ways to recover a horse. The most common methods include free recovery, head and tail rope recovery, and rapidly inflating-deflating pillow recovery. Additional special methods include sling and pool recoveries. The recovery method of choice depends on the facility, preference of the anesthetist, temperament of the horse, and procedure being performed. Free recoveries are commonly used throughout the country. They consist of placing the animal in a well-padded recovery stall, sedating it, and then allowing the horse to stand by itself. During head and tail rope-assisted recoveries, one head rope is attached to a flat halter and another to the tail of the animal. The animal is sedated and allowed to lie down until ready to stand. As the animal stands, the head and tail ropes provide some support, helping the animal balance and stay standing. The rapidly inflating-deflating pillow allows for a free or rope-assisted recovery. The pillow is inflated as soon as the animal is placed in the recovery stall. While the pillow is inflated, the animal has a difficult time achieving sternal recumbency, forcing it to lie down until fully awake and ready to stand. Once the animal is ready to stand, the pillow is rapidly deflated, providing a flat surface for standing. The other two recovery methods (sling and pool recoveries) are reserved for special circumstances due to the labor and cost involved. They are mostly used to recover horses with recently repaired fractures. New Bolton Center pioneered the first equine pool recovery system. The main advantage of pool recovery is that the animal is not allowed to try to stand on its own. Instead, it is kept in a raft placed in the water until fully awake. Once the animal is awake, is it moved into a recovery stall and placed directly into a standing position with the help of a sling.

General anesthesia has many steps and each of them should be taken as seriously as any other procedure being performed on the patient.  Hundreds of horses are anesthetized at the George D. Widener hospital for large animals at Penn Vet’s New Bolton Center every year.  Each patient anesthetized here has a well-trained anesthetist assigned to the case who will not only choose the best drugs for that individual patient, but also provide constant intense monitoring while the patient is under general anesthesia.  The main advantage of having anesthesiologists on site and available 24 hours a day, 7 days a week is the level of care provided to each individual patient. Anesthesiologists are specially trained to understand the physiology of the patient, the pharmacology of the drug, and how the two interact. New Bolton Center offers state-of-the-art monitoring equipment, a wide variety of intravenous as well as inhalant anesthetic drugs, and several well-padded induction and recovery stalls. The large, well-equipped facility also allows for several different styles of induction and recovery methods that better suit each patient based on the animal and procedure being performed.  When combining the knowledge and equipment available, we are proud to say we take general anesthesia very seriously and do our best at all times to make it as safe as possible.