Work of Breathing in Exercising Ponies

This paper attempts to evaluate the changes in the mechanical work of breathing induced by the increase of ventilation in ponies exercising on a treadmill. Airflow, tidal volume (VT) and oesophageal pressure were simultaneously recorded in eight ponies (four to six years old and weighing 258 +/- 11 kg) before, during and after standardised exercise. Respiratory frequency, VT and minute volume (Ve) for each phase of the experimental protocol were calculated from the collected data. The pressure-volume diagrams were traced and the work per cycle (Wrm) was estimated by measuring the area enclosed in the loop. The work per minute (Wrm) and the work per litre of ventilation (Wrm litre-1) were also calculated. From rest to fast trot Wrm litre-1, Wrm and Wrm had increased 8.1, 13.0 and 55.6 times, respectively. The relationships between Ve and Wrm litre-1 was linear and that between Ve and Wrm curvilinear. Results suggested that the mechanical cost of the work of breathing could be a limiting or at least a constraining factor of the increase of ventilation during strenuous exercise in ponies