Determinants of oxygen delivery and hemoglobin saturation during incremental exercise in horses

Objective - To determine components of the increase in oxygen consumption (Vo₂) and evaluate determinants of hemoglobin saturation (So₂) during incremental treadmill exercise in unfit horses. Animals - 7 unfit adult mares. Procedures - Horses performed 1 preliminary exercise test (EXT) and 2 experimental EXT. Arterial and mixed venous blood samples and hemodynamic measurements were taken during the last 30 seconds of each step of the GXT to measure Po₂, hemoglobin concentration ([Hb]), So₂, and determinants of acid-base state (protein, electrolytes, and Pco₂). Results - Increased Vo₂ during exercise was facilitated by significant increases in cardiac output (CO), [Hb], and widening of the arteriovenous difference in O₂. Arterial and venous pH, Pao₂, and Pvo₂ decreased during exercise. Arterial Pco₂, bicarbonate ([HCO₃^-]_a, and [HCO₃]_v decreased significantly, whereas Pvco₂ and increased. Arterial and venous sodium concentration, potassium concentration, strong ion difference, and venous lactate concentration all increased significantly during exercise. Conclusions and Clinical Relevance - Increases in CO, [Hb], and O₂ extraction contributed equally to increased Vo₂ during exercise. Higher Pco₂ did not provide an independent contribution to shift in the oxyhemoglobin dissociation curve (OCD) in venous blood. However, lower Paco₂ shifted the curve leftward, facilitating O₂ loading. The shift of ODC resulted in minimal effect on O₂ extraction because of convergence of the ODC at lower values of Po₂. Decreased pH appeared responsible for the rightward shift of the ODC, which may be necessary to allow maximal O₂ extraction at high blood flows achieved during exercise.