Arterial and Mixed Venous Acid‐base Status and Strong Ion Difference During Repeated Sprints

SummarySeven horses performed six, 1 min sprints separated by 4 min intervals at a walk, followed by a 30 min recovery period. To evaluate changes in blood gases and strong ions, blood samples were taken from the carotid artery (A) and the right heart (mixed venous, V) at rest, after a submaximal warm‐up, during the last 15 s of sprints 5 and 6 and at 5 and 30 min of recovery. At both arterial and venous sites, plasma [H+], PCO2, PO2, albumin ([Alb]), strong ion concentrations ([Na+], [K+], [Cl−]), blood lactate ([Lac]) and haemoglobin concentrations ([Hb]) were measured. Strong ion difference ([SID]), bicarbonate concentration ([HCO3−]) and total weak acid ([Atot]) were calculated. Between sites (A vs. V) there were differences in [H+], PCO2, PO2, [HCO3−], [Cl−], [SID], [Na+], [K+] and [Lac−]. Arterial PO2 remained constant during exercise, was increased at 5 min recovery and returned to the resting level by 30 min recovery. The PvO2 decreased during exercise before returning to resting levels at 5 min of recovery. During exercise, [Cl−] increased at A and decreased at V, which is consistent with the chloride shift. The [H+], PCO2 and [HCO3−] decreased at A and increased at V. During exercise, [Na+], [K+] and [Lac] increased at both sites, with [Na+]V, [K+]V and [Lac−]A increasing to a greater extent. Plasma [SID]A decreased due to a greater increase in [Lac−]A compared to other strong ions and [SID]V increased due to increased [Na+]V and [K+]V and decreased [Cl−]V. Plasma [Alb] and blood [Hb] increased with exercise, with no site differences. Plasma [H+] increased at V and decreased at A before returning to pre‐exercise values at 30 min recovery. These changes reflected the patterns of change in PCO2, but not [SID]. In turn, the decrease in PaCO2 was probably associated with hyperventilation that maintained the PaO2 at a constant level during repeated sprints. Results show that SID, [Atot] and PCO2 have different effects on plasma [H+] and [HCO3−] at sites A and V and that the chloride shift is evident in the exercising horse.