Factors contributing to plasma TCO2 and acid-base state in Ontario Standardbred racehorses

Standardbred and Thoroughbred racehorses around the world are tested for performance enhancing substances. Among these are blood alkalising substances that raise plasma pH and total carbon dioxide (TCO(2)) concentration. However, many horses have an elevated TCO(2) due to dietary, environmental and health concerns without having been administered an alkalising substance.

The purposes of this study were to determine the acid-base profile of a cross section of Standardbred horses in racing/race training in Ontario and the main independent variables that contributed to acid-base state.

On nonracing days, blood from 211 horses at rest, from 9 training facilities, was analysed within 30 min for plasma pH (7.406 ± 0.039; mean ± s.e.), PCO(2) (50.0 ± 3.4 mmHg), from which [HCO(3)(-)] (31.2 ± 2.8 mmol/l) and [TCO(2)] (33.1 ± 2.9 mmol/l; range 25.66-42.9) were calculated. From these, a subset of 161 horses had full data sets for plasma protein and strong ion concentrations. These data were further analysed by facility and level of TCO(2). Data on nutrition, training, racing and medications were also collected.

There were significant differences amongst facilities with respect to plasma pH, TCO(2), strong ion difference ([SID]), PCO(2) and total weak acid concentration ([A(tot)]). Horses having the highest TCO(2) (37.0-42.9 mmol/l, n = 16) had significantly higher [SID] (52.9 ± 0.8 mEq/l) and PCO(2) (52.5 ± 0.7 mmHg) and relatively low [A(tot)] (14.9 ± 0.7 mEq/l) compared to average TCO(2) (32.1.0-34.9 mmol/l) horses (n = 75). In horses with the lowest TCO(2) (n = 11) the greatest contributor was elevated [A(tot) ] (21.0 ± 0.7 mEq/l) and unmeasured (acetate, citrate, proprionate, butyrate) weak acids (7.0 ± 0.2 mEq/l) while [SID] (49.6 ± 0.8 mEq/l) and PCO(2) (47.8 ± 1.0 mmHg) were similar to average TCO(2) horses. Thirty-two horses had a TCO(2) ranging from 35.0-36.9 mmol/l).

There is a wide range of acid-base state and factors contributing to acid-base state amongst Standardbred race horses in Ontario. Dietary, environmental and handling practices and health concerns, that elevate plasma [SID], lower [A(tot)] and lower the concentration of unmeasured weak acids are the primary contributors to alkalosis and elevated TCO(2).