Anesthesia in horses is always a risk, and the older the horse, the more the risk. Dr. Lori Bidwell, DVM, DACVAA, CVA of East-West Equine Sports Medicine is an anesthesiologist, and in the publication "The Horse", Dr. Bidwell says that anesthesia of horses is a higher risk than most domestic species. Due to their large body mass and the recovery period that requires them to stand quickly after anesthesia, a horse's ability to breathe under anesthesia can be compromised. Your horse’s flight instinct prevents them from staying down until completely awake and they sometimes injure themselves trying to rise too quickly. Any anesthetic procedure longer than three hours increases the risk of complications, and this is not unusual for colic or orthopedic surgery. 

Field anesthesia is generally safe as it is used for shorter procedures such as castration, wound repair, or bandage changes. These horses generally recover well. Most field anesthesia cases are performed on a grassy area with plenty of room for recovery, but it is not a sterile environment. After field surgery, horses usually stand on their own while horses under longer-term anesthesia in a hospital usually require some help in standing. 

Horses under longer-term general anesthesia in the hospital are induced by similar drugs used in the field but since longer-acting medications are needed for longer surgery, recovery can be a problem. Most hospitals have a rope system to aid horses in standing after surgery and have a padded recovery room to prevent injury. Some have a pool filled with water for the recovery of horses to prevent injury. Monitoring anesthesia in the hospital is certainly easier than on the farm as parameters like blood pressure and oxygen administration are available in the hospital, which is more important with older horses.