Measurement of upper airway pressures in exercising horses with dorsal displacement of the soft palate.

To determine whether abnormal airway pressures have a role in development of dorsal displacement of the soft palate (DDSP), measurements of tracheal and pharyngeal pressures were correlated with nasopharyngeal morphology in exercising horses. Exercising videoendoscopy and measurement of tracheal and pharyngeal pressures were used in 14 clinically normal horses and 19 horses with intermittent DDSP. The pressure signals were superimposed on the videoendoscope image, and both images were saved simultaneously on a videocassette for slow motion analysis to determine the instant displacement occurred in the respiratory cycle. Horses were submitted to an escalating 8-minute high-speed test with a maximal speed of 14 m/s. Compared with clinically normal horses, horses with intermittent DDSP did not have excessively negative inspiratory pressures during exercise. Eight horses displaced the soft palate during inspiration, 4 horses displaced it during expiration, and 7 displaced it by swallowing. Some horses displaced the soft palate at the beginning of the exercise trial, before reaching maximal speed, some horses displaced it at the peak speed, and some horses displaced it when slowing down. Epiglottic size in horses with DDSP was within normal limits, ruling out epiglottic hypoplasia as a cause of DDSP during exercise. Airway pressures were significantly (P < 0.002) altered after DDSP. Pharyngeal and tracheal inspiratory pressures were less negative, whereas pharyngeal expiratory pressure became less positive and tracheal expiratory pressure became more positive after displacement, suggesting a decrease in airflow and an increase in expiratory resistance in the upper airway.