our expertise: VET COLUMN
Exercise Intolerance: Part 1
The Upper Airway
Tom Hutchins, DVM, DABVP
T
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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