Ventilatory responses of horses to prolonged submaximal exercise


College of Veterinary Medicine, Department of Veterinary Clinical Sciences, Washington State University, Pullman, Washington 99164–6610, USA


While prolonged submaximal exercise is associated with an increase in core body temperature, the role of ventilation in the dissipation of this heat load is unclear. We examined airway mechanics in 5 horses exercising on a treadmill at 40% V̇O2max for 60 min or until fatigued, in order to study the ventilatory response to prolonged low intensity exercise. Respiratory tract mechanics, pH and arterial blood gases were recorded at rest and at regular intervals during exercise. Mean ± s.d. run time was 51.7 ± 5.9 min. No changes in ventilatory mechanics were seen in the first 12–15 min of exercise. Mean breathing frequency (fb) did not change during the test (109 ± 11), however, tidal volume (VT) and fb varied considerably in individual horses over the course of the run. Some animals freely interchanged their breathing pattern between shallow tachypnoea and regular tidal breathing with fb ranging from 76 to 171. Minute ventilation was not affected by breathing pattern but increased from 935 ± 119 l/min after 12–15 min to 1365 ± 343 l/min at the end of exercise. This was associated with a rise in pulmonary arterial blood temperature (BT) from 39.2 ± 0.2°C to 41.8 ± 0.4°C and an increase in work of breathing (Wrm) from 214 ± 30 to 475 ± 122 kg/cm. Wrm/min/l and kg·min increased 1.98-, 1.28- and 1.87-fold, respectively, in the same period. VT and each expression of Wrm were logarithmically correlated with BT. PCO2 decreased from 41.2 ± 1.7 Torr after 12–15 min to 30.6 ± 1.3 Torr after 57–60 min, while pH increased from 7.43 ± 0.03 to 7.56 ± 0.03 in the same period. We conclude that while the thermoregulatory role of the equine respiratory system may be unclear, horses mount a concerted hyperventilatory response during prolonged submaximal exercise. This response may be associated with rises in core temperature and intermittent tachypnoea, is characterised by increases in Wrm and may be physiologically costly and inefficient.