Letting 'sleeping horses lie' may not be the best thing when they're anesthetized, according to John A. E. Hubbell, DVM, MS, Dipl. ACVA (anesthesiology), professor of veterinary anesthesiology in the Department of Veterinary Clinical Sciences at The Ohio State University's College of Veterinary Medicine. In fact, the horse might not get enough oxygen, so some kind of ventilation is preferred, particularly when anesthesia extends for 60 minutes or longer. Hubbell reviewed support ventilation techniques for the anesthetized horse at the 2010 American Association of Equine Practitioner's Convention, held Dec. 4-8 in Baltimore, Md.

Hypoventilation occurs when a horse's lung ventilation is reduced or deficient. Hubbell explained, "Hypoventilation in anesthetized horses is undesirable because reductions in breathing cause changes in the amount of oxygen that is delivered to the body and changes in metabolism by altering the cellular environment of various organs, including the heart, liver, and muscles.

"Even healthy horses hypoventilate under general anesthesia," he continued. "This decrease in respiration and subsequent decrease in oxygenation of their tissues is caused by three main things: the drugs they are administered, being laid down on their sides or back, and the length of time they are anesthetized." Most anesthetic procedures in the field last less than an hour, and horses kept under for that amount of time usually tolerate the hypoventilation well in most instances. Regardless, all anesthetized horses benefit from the administration of oxygen while they are recumbent, Hubbell noted.

A number of veterinary researchers have looked at ways veterinarians can minimize the degree of this "respiratory embarrassment" (i.e., respiratory depression, hypoventilation, and decreased oxygen delivery)