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.