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

Abstract

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fitted with indwelling carotid and pulmonary artery (PA) catheters, had PA temperature measured, and had blood samples withdrawn for immediate analysis of plasma ion and gas concentrations.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.

Tracing Acid-Base Variables in Exercising Horses: Effects of Pre-Loading Oral Electrolytes

“…The dependent acid-base parameters (pH, [H + ], TCO 2 , [HCO 3 − ]) were calculated using AcidBasics II software (©2003, PD Watson, Columbia, SC, USA) as described previously [1,23]. The contributions of each of the independent variables ([SID], [Atot] and pCO 2 ) to the dependent variables ([H + ] and TCO 2 ) were determined by maintaining 2 of the 3 independent variables constant and calculating [H + ] and TCO 2 in response to changes in the third independent variable.…”

“…Performance horses frequently undertake exercise bouts that result in a disturbance of acid-base balance [2,3,[23][24][25][26][27][28][29]. The disturbances in acid-base balance can be traced to alterations in the concentrations of strong and weak ions as a result of fluid and electrolyte shifts between body fluid compartments, as well as sweating and renal losses.…”

“…In addition, increases in the production of metabolic products of increased muscle activity, mainly CO 2 and lactate -, also contributed to the acid-base disturbance. For horses performing prolonged exercise, including transport, it is recommended that electrolyte supplements be provided orally 1-2 h prior to the activity [17,18,23]. Oral electrolyte supplementation may, however, influence acid-base state because of the provision of large amounts of water and electrolytes.…”

Oral Electrolyte and Water Supplementation in Horses

“…The effects of prolonged sweating on blood acid-base balance are small, but measurable and are a reflection of the sweat losses of ions. Prolonged moderate intensity exercise in horses typically results in a mild systemic hypochloremic alkalosis (low plasma [Cl − ] with raised plasma pH; [38,39]. The hypochloremia results from the sweat loss of Cl − to required to provide electrical charge balance for the total cation loss [5,38,40,41].…”

“…Prolonged moderate intensity exercise in horses typically results in a mild systemic hypochloremic alkalosis (low plasma [Cl − ] with raised plasma pH; [38,39]. The hypochloremia results from the sweat loss of Cl − to required to provide electrical charge balance for the total cation loss [5,38,40,41]. The electrolyte imbalance in the body fluids may compromise neuromuscular function, performance and health.…”