Wind Machine: The Equine Respiratory System

The horse’s respiratory system is designed to move a huge quantity of air into and out of the lungs to meet the demands of intense exercise. Oxygen must be provided and carbon dioxide, a waste product, must be removed continuously as the horse works. Some surprising facts:

• A horse exercising at top speed can breathe more than twice per second, taking in up to 12 liters (2.7 gallons) of air in each inhalation.

• Maximal oxygen uptake during exercise can be 40 times as great as when the horse is at rest. In comparison, maximal oxygen uptake in exercising elite human athletes is only about six to eight times higher than when the human is at rest.

The horse’s soft palate is much longer than the corresponding structure in humans. The soft palate extends all the way to the base of the epiglottis, blocking airflow from the mouth to the lungs and requiring the horse to breathe only through its nostrils. Dorsal displacement of the soft palate, a condition in which the soft palate becomes displaced to a position where it restricts movement of air, limits performance in an estimated 10-20% of equine athletes. This displacement may happen intermittently, making diagnosis difficult.

Another common performance-limiting respiratory problem is laryngeal hemiplegia. In horses with this condition, cartilage in the larynx is not pulled back to open the airway completely as the horse inhales. As the horse struggles to take a deep breath, a roaring sound is produced, leading to the term “roarers” to describe affected horses. Surgical correction of this defect, known as tieback surgery, can provide a permanent cure in many horses.

Endoscopic examination of the resting horse’s airway may fail to show the cause of breathing-related limitation of performance because some conditions are seen only when the horse is exercising at a high intensity. Endoscopes have been developed that can be used while the horse is galloping on a treadmill or track. With these instruments, veterinarians can watch the changes in respiratory tract structures in exercising horses, allowing diagnosis and possible treatment of performance-limiting conditions.