The Costs and Demands of Respiratory Function in the Equine Athlete

At rest, an average horse takes in approximately five liters of air with each breath. It takes 12 breaths per minute and has a heart rate of approximately 32 beats per minute. Therefore at rest, our average horse will inhale and exhale approximately 60 liters of air per minute. However, at the racetrack, our horse named "Average Thoroughbred" will increase the volume of air per breath to between 12 and 15 liters taking over 150 breaths per minute. The lungs now have to move over 2,250 liters of air per minute, with less than half a second for each breath.

The breathing patterns of Thoroughbreds are closely synchronized with locomotion. The cantering and galloping horse has respiratory and locomotor cycles locked into a 1:1 phase. The galloping horse takes over 150 breaths per minute. This is in contrast to a Standardbred which takes upward of 70 breaths per minute with between 20 and 25 litres of air per breath. Thus, the galloping horse takes shallower and more frequent breaths than the Standardbred. However, both athletes have to move large volumes of air efficiently to compete successfully.

The equine athlete has to move large volumes of air in a very short period of time. Even a small increase in the amounts of mucous in the airways, and the minor degrees of airway spasm or thickening of the lining of the airways will adversely affect performance.

The demands of athletic exertion are not without their costs on the horse's lungs. Training, strenuous exercise, and the stress of transport affect one of the lungs' main defense mechanisms, i.e. macrophage(clean-up cells). Such changes are believed to be important in explaining the increased incidence of respiratory diseases in horses exposed to these conditions. Horses involved in most forms of athletic exertion suffer exercised-induced pulmonary hemorrhage (EIPH).

While only a small percentage of horses show blood at the nostrils, a much higher percentage will have a small amount present in the airways after their competition. This hemorrhage originates from the upper back corner of the horse's lung. Previously. it was believed that underlying airway disease led to EIPH. However, it has also been shown that blood in the lung causes inflammation. This new knowledge will change our approaches to tackling EIPH.

Although the cause is not clear, on a day-to-day management basis we must remember that this hemorrhage occurs commonly and the site of hemorrhage makes an ideal location for secondary infections.