Cardiorespiratory Causes of Poor Equine Performance

Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.


A number of body systems must run properly to ensure a horse performs at the top of his game; a malfunction in just one system can decrease performance and cause potentially serious health risks to the horse.

During a recent veterinary meeting, one researcher discussed how the cardiac and respiratory systems impact equine performance, and when malfunctions in any of these areas can affect a horse's performance.

"In the field of exercise physiology horses demonstrate exceptional athletic capability in comparison to many mammalians studied," said Emmanuelle van Erck-Westergren, DVM, PhD, Dipl. ECEIM, during a presentation at the 2012 American College Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.

Van Erck-Westergren, of Equine Sports Medicine Practice in Belgium, discussed a few key elements to remember about horses' cardio-respiratory systems function before delving into related causes of poor performance.

Keys to Consider

The respiratory system provides oxygen to facilitate metabolism, and the cardiovascular or circulatory system delivers the oxygen around the body; in short, the cardiorespiratory system provides a horse's body with the oxygen it needs to function properly.

"The soundness and intrinsic capacity of each of these systems plays a substantial role in determining the performance potential of a horse, whatever the equestrian discipline he is involved in," van Erck-Westergren explained. "Conversely, any limitation in one of the chain links will limit the efficiency of the entire chain and may result in poor performance."

Van Erck-Westergren made note of several important points to know about the equine cardiorespiratory system:

  • Horses have a "remarkably high muscle mass" that requires more oxygen uptake and carbon dioxide production than the respiratory system's ventilation capacity can handle;
  • The configuration of the horse's airway and the fact that he can only breathe through his nose results in increase airflow resistance, she said; and
  • The respiratory system may be the ultimate limiting factor to high intensity exercise in horses.

Next, van Erck-Westergren discussed problems of both the respiratory tract and the cardiovascular system that could limit equine performance.

Respiratory Problems

"Respiratory diseases can affect both exercise capacity and athletic performance," van Erck-Westergren said. "Because diseases of the respiratory system are highly prevalent in horses, ensuring respiratory health should be a priority in equine athletes."

Van Erck-Westergren relayed that there are a number of risk factors that could predispose a horse to respiratory disease, including the animal's environment, management, and training practices.

"Housing in a dusty environment, feeding dry food, training, transportation, and mingling during competition are all factors that favor the development of infectious, inflammatory, and allergic disease," she said. Taking steps to minimize these factors in an athletic horse's environment can help reduce the likelihood of respiratory disease developing.

Within the umbrella of respiratory problems, van Erck-Westergren said that there are specific issues that affect both the upper and lower airways; she described some of these conditions in detail:

Upper Respiratory Conditions: "To provide the lowest resistance due to interference, friction, and turbulence, the horse's upper airway should ideally have been short, large, straight, and rigid," van Erck-Westergren said. "However, they are long, narrow, curved, and collapsible." For this reason alone, issues impacting the amount of oxygen the horse can take in should almost be expected.

Dynamic upper-airway obstructions--such as dorsal displacement of the soft palate, dynamic pharyngeal collapse, and idiopathic laryngeal hemiplegia--are common causes of respiratory noise (which can have little to no impact on performance potential in horses working at submaximal levels of exercise such as dressage or show jumping) and poor performance during exercise, she said.

Van Erck-Westergren added that clinical examinations of affected horses should include a respiratory pattern observation, larynx palpation, nasal airflow evaluation, head conformation, and an endoscopic examination, ideally an "overground" one that allows a veterinarian to observe the horse's upper respiratory tract during normal exercise.

Suggested causes of dynamic upper-airway obstruction include upper airway inflammation and pharyngeal instability, however more research is needed to determine definitive causes.

Lower Airway Conditions: Van Erck-Westergren said that the main lower-airway diseases that impact athletic performance are inflammatory airway disease, recurrent airway obstruction (or heaves), exercise-induced pulmonary hemorrhage, and viral or bacterial infections.

Proper environmental management is key to helping affected horses remain healthy; however van Erck-Westergren noted that competitions the horse attends could exacerbate the problem. "Competition may trigger metabolic and hormonal alterations that disturb immunity," she explained. "Depression of the immune system and the occurrence of repeated inflammatory and oxidative processes during training and competition seem to create favorable conditions to the outbreak of infectious respiratory diseases."

Van Erck-Westergren noted that supportive testing--such as tracheal endoscopy, tracheal or trans-tracheal wash, or bronchoalveolar lavage--should be used in conjunction with clinical signs when diagnosing lower-airway diseases.

"A dust-free environment and hygienic environment both at home and on shows limits the risks of lower airway inflammation, she added.

Cardiovascular Problems

Switching gears, van Erck-Westergren said that while cardiac problems are relatively rare in sport horses, they can have "significant repercussions" when they do occur. She discussed two cardiac problems in detail:

Exercising Arrhythmias: Very simply put, cardiac arrhythmias are irregular heartbeats. "Cardiac arrhythmias are more frequently seen in athletic horses, both at rest and during post-effort recovery," van Erck-Westergren said.

The most frequently diagnosed arrhythmias are supraventricular or ventricular premature beats (meaning the heart beats earlier than expected in the atria or ventricles) and atrial fibrillation (disorganized electrical conduction and pumping of the atria), she said, and it's not uncommon for these arrhythmias to occur "extensively" during exercise.

Van Erck-Westergren said that arrhythmias can originate from a cardiac disease or as a result of an acid-base or electrolyte imbalance; if they develop for the former reason, she recommended carefully addressing the underlying problem. These arrhythmias can be diagnosed via an electrocardiogram performed during exercise.

Atrial fibrillation, she said, is "probably the most frequent arrhythmia associated with poor performance." In addition to poor performance, exercise intolerance, and premature fatigue, atrial fibrillation can predispose horses to fatal ventricular tachycardia (rapid heartbeat); thus, careful heart rate monitoring is crucial for affected horses.

Cardiac Murmurs: Finally, van Erck-Westergren discussed cardiac murmurs. She explained that intense training can cause changes in the heart's morphology (physical characteristics) and function; "athlete's heart" is characterized by hypertrophy (enlargement), she said.

Most commonly found in athletic and older horses, murmurs are diagnosed via auscultation (listening with a stethoscope) and the underlying cause by echocardiography. Once a diagnosis is made, the veterinarian can advise as to whether the horse can kept in work, she said. The horse's prognosis for continued work will be impacted by the type of murmur he has, as some tend to cause more clinical signs and ill effects than others.

Additionally, "assessing the rate of progression of cardiac disorders by regular echocardiographic follow-up is important if the horse is to continue work and competition," she added.

Take-Home Message

In order for a performance horse to reach his potential, his cardiovascular and respiratory systems must function properly. Understanding what in these areas can cause decreased performance can help owners reduce the risk of their horses developing cardiorespiratory problems.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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