Physicochemical Analysis of Mixed Venous and Arterial Blood Acid-Base State in Horses at Core Temperature during and after Moderate-Intensity Exercise

The present study determined the independent contributions of temperature, strong ion difference ([SID]), total weak acid concentration ([Aₜₒₜ]) and PCO₂ to changes in arterial and mixed venous [H⁺] and total carbon dioxide concentration ([TCO₂]) during 37 min of moderate intensity exercise (~50% of heart rate max) and the first 60 min of recovery. Six horses were fitted with indwelling carotid and pulmonary artery (PA) catheters, had PA temperature measured, and had blood samples withdrawn for immediate analysis of plasma ion and gas concentrations. The increase in core temperature during exercise (+4.5 °C; p < 0.001) significantly (p < 0.05) increased PO₂, PCO₂, and [H⁺], but without a significant effect on [TCO₂] (p > 0.01). The physicochemical acid-base approach was used to determine contributions of independent variables (except temperature) to the changes in [H⁺] and [TCO₂]. In both arterial and venous blood, there was no acidosis during exercise and recovery despite significant (p < 0.05) increases in [lactate] and in venous PCO₂. In arterial blood plasma, a mild alkalosis with exercise was due to primarily to a decrease in PCO₂ (p < 0.05) and an increase in [SID] (p < 0.1). In venous blood plasma, a near absence of change in [H⁺] was due to the acidifying effects of increased PCO₂ (p < 0.01) being offset by the alkalizing effects of increased [SID] (p < 0.05). The effect of temperature on PO₂ (p < 0.001) resulted in an increased arterio-venous PO₂ difference (p < 0.001) that would facilitate O₂ transfer to contracting muscle. The simultaneous changes in the PCO₂ and the concentrations of the other independent acid-base variables (contributions from individual strong and weak ions as manifest in [SID] and [Aₜₒₜ]) show complex, multilevel control of acid-base states in horses performing even moderate intensity exercise. Correction of acid-base variables to core body temperature presents a markedly different physiological response to exercise than that provided by variables measured and presented at an instrument temperature of 37 °C.