A Quantitative Analysis of the Effects of Feeding and Daily Variation on Plasma Acid-Base Status in Resting Horses
A Thesis Presented to The Faculty of Graduate Studies of The University of Guelph by
KERRI JO SMITHURST, 2003
Daily variation in the measured blood constituents identified in this study was due to either feeding or dehydration. All values were within physiological limits, showing that any circadian rhythm evident in this study was within clinical reference ranges, with the exception of [TCO2]. Ninety percent of the horses had a [TCO2] above 35.0 mmol/L at some point during the study and 70% of the horses had a [TCO2] above 36.0 mmol/L and may have received a positive TCO2 test result in the Ontario racing industry.
Although no change in electrolytes over the 25-h DVT was found as compared to initial concentrations there was a tendency for the electrolytes to have a nocturnal variation, with decreased [Na+] and increased [Cl-] as shown by the change in [SID]. The only change in electrolytes when compared to initial concentrations was an increase in [Cl-] following the morning feeding. The small change in [Cl-] post-feed and the potentially small changes in [Na+] and [K+] found in this study negate the requirement of time dependent reference ranges for electrolytes for blood testing.
There was also increased [glucose] and [PP] as well as decreased PCO2 and pH found following the morning feed. The same variations were not present following the evening meal. The discrepancy between morning and evening feeding responses could not be determined to be due to a daily variation as it is possible that the continued digestion of hay over the day attenuated the evening response to feeding.
Theoretically, the independent variables account for all of the changes in [H+] and [HCO3-] (and thus [TCO2]) using the physicochemical approach to acid-base balance. In this study, however, contributions of the independent variables ([SID], [Atot] and PCO2) were not able to fully account for changes in either [H+] or [TCO2] as calculated by the physicochemical equation.