Inspiratory and expiratory tracheal pressures during high-intensity exercise in harness racehorses

Exercise-related upper respiratory tract (URT) disorders are common in racehorses. Objective assessment of URT mechanics is essential to quantify degrees of obstruction caused by URT disorders identified upon dynamic endoscopy.

To establish reference values for inspiratory and expiratory tracheal pressures (cmH₂O) during high-speed treadmill endoscopy in harness racehorses with clinically normal URTs.

Prospective observational study.

Tracheal pressures were measured in harness racehorses (Standardbreds and Norwegian-Swedish coldblooded trotters) in which no URT abnormalities were detected. Peak inspiratory and expiratory tracheal pressures were determined for each minute (phase) of a standardised treadmill test, which alternated between trotting with free head carriage (phases 1, 3, 5 and 7) and with poll flexion (phases 2, 4 and 6). Linear mixed-effects models assessed changes in tracheal pressures across the exercise test, and effects of breed, racing experience, respiratory rate and head-neck position (free vs. poll flexion).

Seventy-six horses were included. Mean (SD) peak inspiratory tracheal pressures ranged from -21.8 (5.0) cmH₂O in phase 1 to -34.9 (5.3) and -34.3 (5.7) cmH₂O in phases 6 and 7. Inspiratory pressures became significantly more negative across phases (p < 0.001) and were -3.5 cmH₂O (95% CI: -4.0 to -3.0, p < 0.001) lower during poll flexion versus free head carriage. Expiratory tracheal pressures remained stable across exercise phases (11.5 [2.8] to 12.5 [2.6] cmH₂O) with no significant changes. There were no significant differences between the two breeds. Respiratory rate ranged from 79.8 (12.6) to 90.8 (15.0) breaths/min and remained stable between 89.6 and 90.8 breaths/min through phases 3-7.

Simultaneous airflow measurements were not performed.

Inspiratory pressures became more negative with exercise progression and were significantly lower during poll flexion versus free head carriage. Reference values for tracheal pressures from clinically normal horses provide an objective tool for evaluating URT function during exercise.