Blood-gas tensions and acid-base status in ponies during treadmill exercise

Blood-gas tensions and acid-base status were examined in 8 healthy grade ponies at rest (heart rate = 55 +/- 3 beats/min) and during moderate (fast trot; heart rate = 155 +/- 3 beats/min) and severe (gallop; heart rate = 218 +/- 7 beats/min) exercise performed on a treadmill. Arterial oxygen tension and hemoglobin-oxygen saturation of exercising ponies did not change from the resting values. Arterial oxygen content increased markedly during exercise, as a consequence of increased hemoglobin concentration. The total oxygen content, as well as the saturation of hemoglobin with oxygen in the mixed venous blood, decreased at each intensity of exercise. Arterial carbon dioxide tension decreased with moderate (16%) and severe (29%) exercise, indicating hyperventilation. In galloping ponies, during steady-state severe exercise marked metabolic acidosis developed, as indicated by a sharp increase in the arterial concentration of lactic acid (11.6 +/- 1.3 mM/L during severe exercise vs 0.6 +/- 0.3 mM/L at rest). This increase in lactate was accompanied by a decrease in arterial pH and bicarbonate concentration.