Effects of training on the development of exercise-induced arterial hypoxemia in horses

Objectives To compare the development of exercise-induced arterial hypoxemia in horses before and after training, and to determine whether increases in maximum oxygen uptake (V˙O2max) following training results in a greater degree of exercise-induced arterial hypoxemia. Animals 13 three- to five-year-old Standardbred geldings without clinical signs of respiratory or cardiovascular disorders. Procedure Horses were rested for 4 months prior to commencing a 16-week training program. Arterial blood was collected from the transverse facial artery during standardized exercise tests performed before and after 8 and 16 weeks of training. Variables measured during exercise tests included arterial blood gas tensions and V˙O2max. Training and testing was performed on a treadmill set at a 10% slope. Results Minimum arterial partial pressures of oxygen (PaO2) during exercise decreased from 83.3 ± 1.5 mm of Hg before training to 77.8 ± 1.0 mm of Hg after 16 weeks of training. Maximum arterial oxygen content increased from 239.2 ± 3.1 to 257.9 ± 3.8 ml/L, which resulted from an increase in hemoglobin concentration. The V˙O2max increased by 19% following training. Minimum values of PaO2 and arterial oxygen saturation were significantly correlated with V˙O2max when data from each stage of training were pooled. Calculated values for minimum alveolar oxygen tension decreased after 8 weeks of training, and alveolar ventilation increased at the end of training. Alveolar-arterial oxygen tension difference increased by 4 mm of Hg following training. Conclusions In trained horses, an increase V˙O2max is associated with a decrease in minimum PaO2 during intense exercise and is mostly attributable to an increase in the alveolar-arterial oxygen tension difference. Therefore, interpretation of blood gas data from exercising horses should take into consideration V˙O2max. (Am J Vet Res 1997;58:653–657)