High-speed treadmill endoscopy protocol
Horses were exercised using a previously standardised HSTE protocol
(Fjordbakk et al. 2008; Strand et al. 2012) and driven on the treadmill
by an experienced driver with full racing tack including bit, bridle,
harness, conventional overcheck and long reins. Horses were equipped
with a heart rate monitor and following a warm-up period of
approximately 2500 m, the horses were exercised in 1-minute phases,
alternating between free head carriage (phases 1, 3, 5) and poll flexion
(phases 2, 4 ± 6). During phases with poll flexion, horses were driven
“onto the bit” by applying even tension on the long reins as if they
were being driven on a racetrack. During “free head carriage”, tension
on the reins and bit were released. Standardbreds (STBs) were trotted at
9 m/s and Norwegian-Swedish Coldblooded Trotters (NSCTs) at 8.5 m/s, all
on a 1.5-degree treadmill incline. Exercising racehorses typically
achieve a heart rate over 200 bpm during phase 1 and reach exhaustion at
the trotting gait by phases 5 or 6. Videoendoscopic evaluation of the
larynx was carried out through the length of the exercise protocol, with
the endoscope being passed through the right ventral meatus to the level
of the caudal nasopharynx in all cases. Videoendoscopic recordings were
evaluated in real time and slow motion retrospectively for horses
included in the study. Exercise-induced compression of the base of the
epiglottis was identified as unilateral or bilateral if present. Other
concurrent URT abnormalities that developed subsequent to compression of
the base of the epiglottis were recorded if present.