OklahomaHorses Magazine September 2021

24 OklahomaHorses • September/October 2021 When Horses Roar: Evaluating the Equine Upper Respiratory Tract By Kelsey Jurek, DVM H orses are exceptional athletes that perform a wide variety of disciplines. During performance, the muscles of equine athletes use a large amount of energy, which is produced largely through a process called aerobic metabolism. The process uses oxygen to convert carbohy- drates and fats to adenosine triphosphate (ATP), the energy source used by muscle cells. Adequate supply of oxygen to muscle cells is critical. This supply begins at the horse’s upper respiratory tract (URT), which includes the nostrils, nasal passages, pharynx, and lar- ynx. Dysfunction of the URT is common in horses, manifesting as abnormal respira- tory noise, poor or decreased performance, dysphagia (an inability to swallow correct- ly), and involuntary aerophagia (“wind sucking”). Airway dynamics play a significant role in equine URT disorders. Horses are obligate nasal breathers, which means they breathe exclusively through the nose. Unlike people, they cannot breathe through the mouth very efficiently. Breathing only through the nasal passage creates substan- tial resistance to airflow, generating signif- icant negative pressure in the URT during inspiration. During maximal exercise, airflow and inspiratory negative pressure increase approximately 20 times compared to that experienced at rest, often causing URT dysfunction. Evaluation of a horse’s URT involves a series of diagnostic steps to pinpoint an un- derlying cause. Gathering a thorough his- tory, which includes a detailed discussion of the horse’s clinical signs, is incredibly helpful. A thorough physical examination is important to evaluate the horse for other causes of the clinical signs, such as a heart Mike Schoonover, DVM (center), adjusts the position of the endoscope to produce an optimal image on the wireless display monitor held by Hugh Duddy, DVM (left). Kelsey Jurek, DVM (mounted), and Cherlyn Simpson, registered veterinary technician (right), prepare to exercise the horse during a dynamic endoscopy exam.