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

Objective-To determine components of the increase in oxygen consumption ((V) over dot O-2) and evaluate determinants of hemoglobin saturation (So(2)) 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 Po-2, hemoglobin concentration ([Hb]), So(2), and determinants of acid-base stale (protein, electrolytes, and Pco(2)). Results-Increased (V) over dot o(2) during exercise was facilitated by significant increases in cardiac output (CO), [Hb], and widening of the arteriovenous difference in O-2 Arterial and venous pH, Pao(2), and Pvo(2) decreased during exercise. Arterial Pco(2), bicarbonate ([HCO3-])(a), and [HCO3-](v) decreased significantly, whereas Pvco(2) 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 O-2 extraction contributed equally to increased (V) over dot o(2) during exercise. Higher Pco(2) did not provide an independent contribution to shift in the oxyhemoglobin dissociation curve (OCD) in venous blood. However, lower Paco(2) shifted the curve leftward, facilitating O-2 loading. The shift of ODC resulted in minimal effect on O-2 extraction because of convergence of the ODC at lower values of Po-2 Decreased pH appeared responsible for the rightward shift of the ODC, which may be necessary to allow maximal O-2 extraction at high blood flows achieved during exercise.