Parker County Today November 2017 - Page 78

our expertise: VET COLUMN Exercise Intolerance: Part 1 The Upper Airway Tom Hutchins, DVM, DABVP T 76 he respiratory system is second only to the musculoskeletal system as a cause of lost training in racehorses. Multiple body systems may be responsible for exercise intolerance. Many horses have more than one body system affected, making determination of the relative contribution of each system difficult. Upper respiratory sources of exercise intolerance are common in all breeds of horses. The frequency of specific conditions, the degree to which each condition limits performance, and the method of treatment and corresponding success vary with the breed, age, and intended use of the horse. As a general rule, horses engaged in racing have the highest requirements for airflow and show exercise intolerance with lesions that may be inapparent during examinations performed at rest. Horses engaged in less strenuous activities require more airway obstruction before performance limitation is apparent. These lesions are usually apparent at rest. The upper airway accounts for the majority or total respiratory resistance during exercise. Resistance to airflow is inversely proportional to the radius of the airway to the fourth power. Therefore, small reductions in airway size lead to large increases in airway resistance. Increases in airway resistance require increased respiratory work to generate the same airflow. Airway obstruction leads to the generation of airway noise as a result of tissue vibration and turbulent airflow. Obstructions of the upper airway are accompanied by noise in all horses, although not all horses with airway noise have exercise intolerance. The presence of airway noise increases the likelihood that airway obstruction is responsible for the exercise intolerance compared to horses that show poor performance without respiratory noise. For many causes of upper airway obstruction, the noise Dr. Tom Hutchins is characteristic. Trainers and owners are valuable sources for information and careful questioning can lead to a strong suspicion for a certain condition before examining the horse. Endoscopy using a flexible fiber endoscope or videoendoscope is essential for diagnosis. This allows the veterinarian to determine the location of the problem and the severity of the inciting cause. Treadmill examination can be helpful in some instances. Treadmill exercise evaluations allow for simultaneous eval uation of the musculoskeletal, cardiac, and respiratory systems. Videoendoscopy allows for direct visualization of the airway during exercise. Treadmill examination offers the additional advantage of providing objective measurements of fitness levels to differentiate poor conditioning as a cause of exercise intolerance. Exercising under tack or harness to simulate conditions under which the owner or trainer reports the noise to occur can be helpful as well. Numerous examples exist of intermittent airway obstructions that occur only during exercise and that are demonstrable only during videoendoscopy on a treadmill. These cases are few in proportion to all the horses examined for airway obstruction. The majority of upper airway obstructions can be diagnosed using history, signalment, endoscopy, and other techniques available to your veterinarian. Treadmill evaluation is indicated for those horses showing airflow limitation or airway noise that cannot be identified by endoscopy at rest or after exercise. Endoscopic examination should be performed in the unsedated horse in a systematic manner such that