Avoiding Respiratory Problems in Anesthetized Horses (AAEP 2010)

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). He summarized these as follows:

  • During short procedures veterinarians should monitor anesthetized horses' ventilation and oxygenation by looking at their respiratory rate and the "pinkness" of their gums. During longer procedures, the best way to assess the adequacy of ventilation is to take a sample of arterial blood to measure pH and the blood gases (carbon dioxide and oxygen). Portable, "Stall-side" units are allow veterinarians to assess ventilation even in remote locations. Arterial blood-gas analysis is the "gold standard" method of monitoring ventilation and oxygenation;
  • Veterinarians should enhance oxygenation by delivering oxygen ("insufflating") via a compressed gas source through a delivery tube to the pharynx or trachea; and
  • Veterinarians should support ventilation and oxygenation either manually (by compressing an air-filled bag that delivers the air to the horse) or with a commercial ventilator. Larger breaths and lower respiratory rates are better. Excessively fast respiratory rates should be avoided because the distribution of ventilation is less optimal when they are used.

Any anesthetic procedure has the potential to significantly impair the respiration and oxygenation in the horse, Hubbell added. Anesthesia of a short duration is relatively well tolerated but horses anesthetized for longer procedures benefit from increased inspired oxygen levels and assisted ventilation.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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