Temporal pattern of pulmonary gas exchange during exercise in ponies

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Abstract
The temporal patterns of pulmonary O2 consumption (.ovrhdot.VO2) and CO2 excretion (.ovrhdot.VCO2) in exercising ponies were characterized. The cardiac, ventilatory, and hematologic contributions to the .ovrhdot.VO2-.ovrhdot.VCO2 changes during exercise were assessed. At five moderate treadmill work loads in normal ponies, .ovrhdot.VO2 and .ovrhdot.VCO2 generally increased progressively throughout the 1st min reaching, respectively, .apprx. 95 and 80% of steady-state by 45-60 s. Between 1 and 1.5 min, .ovrhdot.VO2 and .ovrhdot.VCO2 were either stable or they decreased 300-500 ml, but then each increased slightly, reaching steady-state levels between 3 and 5 min. The increases to steady-state .ovrhdot.VO2 and .ovrhdot.VCO2 were clearly not monoexponential. Arterial O2 content (CaO2) increased during exercise, presumably because of mobilization of stored splenic erythrocytes. For example, after 1 and 6 min at 6 mph 3% grade, CaO2 had increased above rest by 3.2 and 4.9 vol%, respectively (P < 0.05). Hyperventilation during exercise decreased arterial CO2 content (CaCO2) below rest. For example, after 1 and 6 min at 6 mph 3% grade, CaCO2 had decreased below rest by 2.8 and 1 vol%, respectively (P < 0.05). At all work loads, heart rate (HR) and presumably cardiac output increased rapidly during the 1st min, reaching a maximum by .apprx. 45 s. HR then decreased (P < 0.05), reaching a steady-state by 3 min of exercise. During the first 1.5 min of exercise in carotid body-denervated (CBD) ponies, the changes in .ovrhdot.VO2, HR, and CaO2 were temporally delayed relative to the normal; the changes in .ovrhdot.VCO2 and CaCO2 were relatively augmented. Relating the .ovrhdot.VO2 and .ovrhdot.VCO2 changes to the cardiac, ventilatory, and hematologic changes for normal and CBD ponies emphasizes that .ovrhdot.VO2 kinetics during exercise are effected relatively more by the cardiac and hematologic responses; .ovrhdot.VCO2 kinetics are affected relatively more by alveolar ventilation kinetics.