Diagnosis of sub-performance due to respiratory disease.

Most equestrian disciplines rely on aerobic energy metabolism to power exercise. Oxygen delivery to the exercising muscles is therefore a critical determinant of athletic performance. Horses are considered to be elite athletes due to their high aerobic capacity, which exceeds that of other athletic species, and is largely achieved through the enhanced ability of the cardiovascular system. The respiratory response to exercise is achieved through an increase in both the breathing frequency and tidal volume, resulting in a dramatic increase in minute ventilation from approx. 100L/min at rest to over 2000L/min during strenuous exercise.¹ Yet it is widely acknowledged that the respiratory system is the limiting factor for athletic performance, with healthy horses exhibiting hypoxaemia and hypercapnia during strenuous exercise.¹ It is perhaps not surprising therefore that respiratory disorders, even in mild forms, are potentially deleterious to athletic performance.

Respiratory disease is common in equine athletes and may affect the upper airways, lower airways or a combination of both.² In some instances, the diagnosis is challenging because conditions can be subclinical at rest and become clinically relevant only during exercise. In such cases, an exercise test may be warranted in the evaluation of the patient, especially for definitive diagnosis of upper airway collapse. Additional diagnostic techniques such as bronchoalveolar lavage are also required for diagnosis of subclinical lower airway disorders that have the capacity to impair performance.

Horses are particularly prone to developing dynamic upper airway collapse because they are obligatory nasal breathers and cannot avoid the high pressures, associated with nasal breathing, during exercise. Modelling of the equine airways has identified that the most negative pressures and highest airflow turbulence occur at the floor of the rostral aspect of the nasopharynx and within the larynx³, and these are the most commonly affected regions. The nasopharyngeal region, which is not supported by osseous or cartilaginous structures and relies on muscular activity to maintain stability and patency, is especially prone to dynamic collapse resulting in palatal instability, dorsal displacement of the soft palate and pharyngeal wall collapse. Laryngeal collapse is also common, resulting in collapse of one or both vocal folds, arytenoid cartilages and / or aryepiglottal folds. Whilst some conditions may be evident on a resting examination, most forms of upper airway collapse can only be definitively diagnosed using exercising endoscopy.⁴

The prevalence of lower airway diseases, including mild to moderate equine asthma (mEA) and exercise induced pulmonary haemorrhage (EIPH) is high in equine athletes across a range of disciplines. Equine asthma is considered to be a disease of domestication, occurring as a result of exposure to particulate matter in the forage and stable environment.⁵ It affects all groups of horses, although the degree to which performance is impacted will be dependent on the intensity of exercise being performed, and the condition may often go undetected. EIPH is a unique condition resulting in bleeding into the airways during strenuous exercise, as a result of stress failure of the alveolar capillary walls in association with high transmural pressures. It occurs most commonly in racehorse populations, where up to 100% of horses are affected to a variable degree.⁶ The condition occurs less commonly in other disciplines and is largely dependent on exercise intensity. Despite the high prevalence of lower airway disorders, these conditions may easily be underestimated because most progress subclinically, and relevant diagnostic procedures are sometimes difficult to implement routinely in the field. Endoscopic examination of the tracheobronchial tree will enable visualisation of tracheal mucus and or blood, whilst bronchoalveolar lavage is required for a definitive diagnosis of lower airway inflammation and can also indicate prior EIPH through the identification of red blood cells and haemosiderophages, which may persist for several weeks following an episode of bleeding. Studies of the effects of respiratory disease on respiratory function and exercise capacity have yielded varying results. The impact of respiratory disease will depend not only on the nature and severity of the disease but also on the equestrian discipline performed.² In horses exercising at maximal aerobic capacity, even small changes in airflow and gas exchange may have an important impact on athletic performance, whereas horses exercising less strenuously may not exhibit exercise intolerance until respiratory impairment becomes more severe.