ISCUSSION
The results of this study are consistent with our hy- potheses that plasma tCO2 would increase in resting horses receiving furosemide and that there would be no difference between 250- and 500-mg doses of furosemide. However, the doses of furosemide used in this study mimic the prevailing veterinary practices at the racetracks in Illinois, and horses are given 250 or 500 mg furosemide rather than a standard weight in milligrams furosemide per kilogram body weight. The differences in weight-specific doses of furosemide be- tween horses in this study could be a source of variation that may have reduced the power of our tests of differ- ences between treatments. Another source of variation that may have reduced the power of our statistical tests was variable fitness among the horses used. Although all horses used were experienced racehorses, two of the animals had just finished retraining after layoffs and may not have been as fit as the rest of the horses.
In this study, the plasma tCO2 readings were ad- justed using a regression equation determined from NERL standard solutions. These standard solutions contained CO2 ranging from 10 to 40 mmol/L.This CO2 range was designed to represent the range of expected plasma tCO2 values to be measured in this study.
Documentation of linearity of measurements of CO2 across a representative range of expected plasma values for the Beckman Synchron EL-ISE instrument (model no. 446813), involves the measurement of three control samples.These three controls (Synchron Levels 1,2,and
3) contain approximately 10, 20, and 30 mmol/L CO2, respectively. Using these controls without documenting the upper range of CO2 measurement by the instrument may not be critical for making clinical evaluations in most cases. However, in the current study we wished to adjust the raw plasma tCO2 data based on a regression curve that provided a wider representation of the pre- dicted range of measurements because “positive” tests indicative of “milkshakes” occur well above 30 mmol/L plasma tCO2. Thus, the NERL standard solutions that included a higher CO2 standard solution containing 40 mmol/L were used to determine the slope-adjustment for all equine plasma samples. Ideally, because some positive tests for plasma tCO2 in racehorses do exceed 40 mmol/L, the use of standards slightly greater than 40 mmol/L would have been desirable, had they been available. Using the Beckman controls from 0 to 30 mmol/L CO2 gave us a straight-line regression with slope of 1 and Y-intercept of 0. The regression line de- veloped from measurement of the NERL standards should theoretically have the same slope and intercept as that from the regression line using the Beckman con- trols.The average slope for the NERL standard regres- sion lines in this study was 1.03208178, and our average Y-intercept was 0.55800722. The statistical variance be- tween our data and the theoretical was not significant in our opinion.
The allowable limit for plasma tCO2 for horses rac- ing in Illinois after treatment with furosemide is 39 mmol/L. Horses with greater than 39 mmol/L plasma tCO2 are presumed to have received an illegal alkalin- izing substance (milkshake). We found that some plasma samples from furosemide-treated horses in this study resulted in unadjusted tCO2 values in the “posi- tive” range for milkshakes. This observation shows the importance of adjusting blood gas results using a re- gression curve from as wide a range of tCO2 standards